includes/libs/objectcache/wancache/WANObjectCache.php
<?php
/**
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
* @ingroup Cache
*/
use Liuggio\StatsdClient\Factory\StatsdDataFactoryInterface;
use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\LightweightObjectStore\ExpirationAwareness;
use Wikimedia\LightweightObjectStore\StorageAwareness;
/**
* Multi-datacenter aware caching interface
*
* ### Using WANObjectCache
*
* This class intends to boost performance of code paths by using cache-aside logic
* for data potentially derived from source databases subject to replication lag. Callers
* will generally make use of the getWithSetCallback() method.
*
* All operations go to the local datacenter cache, except for delete(), touchCheckKey(),
* and resetCheckKey(), which are also broadcasted to caches in all datacenters.
*
* To ensure consumers of the cache see new values in a timely manner, you need to
* follow either the validation strategy, or the purge strategy.
*
* The validation strategy refers to the natural avoidance of stale data
* by one of the following means:
*
* - A) The cached value is immutable.
* If the consumer has access to an identifier that uniquely describes a value,
* cached value need not change. Instead, the key can change. This also allows
* all servers to access their perceived current version. This is important
* in context of multiple deployed versions of your application and/or cross-dc
* database replication, to ensure deterministic values without oscillation.
* - B) Validity is checked against the source after get().
* This is the inverse of A. The unique identifier is embedded inside the value
* and validated after on retrieval. If outdated, the value is recomputed.
* - C) The value is cached with a modest TTL (without validation).
* If value recomputation is reasonably performant, and the value is allowed to
* be stale, one should consider using TTL only – using the value's age as
* method of validation.
*
* The purge strategy refers to the approach whereby your application knows that source
* data has changed and can react by purging the relevant cache keys. Since purges are
* expensive, this strategy should be avoided if possible. The simplest purge method is
* delete().
*
* In any case, callers must not assume that updates and purges are immediately visible
* to all application servers. It should be treated like a replica database in this regard.
* If such semantics are required, then solutions must be sought outside WANObjectCache.
*
* Note that write operations, such as set() and delete(), are allowed to return true as
* soon as the command could be sent or buffered via an open socket to the relevant cache
* backend server. If that server is a proxy, then it is responsible for detecting
* and tracking downed servers.
*
* @anchor wanobjectcache-deployment
* ### Deploying WANObjectCache
*
* There are two supported ways to set up broadcasted operations:
*
* - A) Set up mcrouter as the cache backend, with a memcached BagOStuff class for the 'cache'
* parameter, and a wildcard routing prefix for the 'broadcastRoutingPrefix' parameter.
* Configure mcrouter as follows:
* - Define a "<datacenter>" pool of memcached servers for each datacenter.
* - Define a "<datacenter>/wan" route to each datacenter, using "AllSyncRoute" for the
* routes that go to the local datacenter pool and "AllAsyncRoute" for the routes that
* go to remote datacenter pools. The child routes should use "HashRoute|<datacenter>".
* This allows for the use of a wildcard route for 'broadcastRoutingPrefix'. See
* https://github.com/facebook/mcrouter/wiki/Routing-Prefix and
* https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup.
* - In order to reroute operations from "down" servers to spare ("gutter") servers, use
* "FailoverWithExptimeRoute" (failover_exptime=60) instead of "HashRoute|<datacenter>"
* in the "AllSyncRoute"/"AllAsyncRoute" child routes.
* The "gutter" pool is a set of memcached servers that only handle failover traffic.
* Such servers should be carefully spread over different rows and racks. See
* https://github.com/facebook/mcrouter/wiki/List-of-Route-Handles#failoverroute
* - B) Set up dynomite as the cache backend, using a memcached BagOStuff class for the 'cache'
* parameter. Note that with this setup, all key setting operations will be broadcasted,
* rather than just purges. Writes will be eventually consistent via the Dynamo replication
* model. See https://github.com/Netflix/dynomite.
*
* Broadcasted operations like delete() and touchCheckKey() are intended to run
* immediately in the local datacenter and asynchronously in remote datacenters.
*
* This means that callers in all datacenters may see older values for however many
* milliseconds that the purge took to reach that datacenter. As with any cache, this
* should not be relied on for cases where reads are used to determine writes to source
* (e.g. non-cache) data stores, except when reading immutable data.
*
* Internally, access to a given key actually involves the use of one or more "sister" keys.
* A sister key is constructed by prefixing the base key with "WANCache:" (used to distinguish
* WANObjectCache formatted keys) and suffixing a colon followed by a single-character sister
* key type. The sister key types include the following:
*
* - `v`: used to store "regular" values (metadata-wrapped) and temporary purge "tombstones".
* - `t`: used to store "last purge" timestamps for "check" keys.
* - `m`: used to store temporary mutex locks to avoid cache stampedes.
* - `i`: used to store temporary interim values (metadata-wrapped) for tombstoned keys.
* - `c`: used to store temporary "cool-off" indicators, which specify a period during which
* values cannot be stored, neither regularly nor using interim keys.
*
* @ingroup Cache
* @newable
* @since 1.26
*/
class WANObjectCache implements
ExpirationAwareness,
StorageAwareness,
IStoreKeyEncoder,
LoggerAwareInterface
{
/** @var BagOStuff The local datacenter cache */
protected $cache;
/** @var MapCacheLRU[] Map of group PHP instance caches */
protected $processCaches = [];
/** @var LoggerInterface */
protected $logger;
/** @var StatsdDataFactoryInterface */
protected $stats;
/** @var callable|null Function that takes a WAN cache callback and runs it later */
protected $asyncHandler;
/**
* Routing prefix for operations that should be broadcasted to all data centers.
*
* If null, the there is only one datacenter or a backend proxy broadcasts everything.
*
* @var string|null
*/
protected $broadcastRoute;
/** @var bool Whether to use "interim" caching while keys are tombstoned */
protected $useInterimHoldOffCaching = true;
/** @var float Unix timestamp of the oldest possible valid values */
protected $epoch;
/** @var string Stable secret used for hashing long strings into key components */
protected $secret;
/** @var int Scheme to use for key coalescing (Hash Tags or Hash Stops) */
protected $coalesceScheme;
/** @var array<int,array> List of (key, UNIX timestamp) tuples for get() cache misses */
private $missLog;
/** @var int Callback stack depth for getWithSetCallback() */
private $callbackDepth = 0;
/** @var mixed[] Temporary warm-up cache */
private $warmupCache = [];
/** @var int Key fetched */
private $warmupKeyMisses = 0;
/** @var float|null */
private $wallClockOverride;
/** Max expected seconds to pass between delete() and DB commit finishing */
private const MAX_COMMIT_DELAY = 3;
/** Max expected seconds of combined lag from replication and "view snapshots" */
private const MAX_READ_LAG = 7;
/** Seconds to tombstone keys on delete() and to treat keys as volatile after purges */
public const HOLDOFF_TTL = self::MAX_COMMIT_DELAY + self::MAX_READ_LAG + 1;
/** Consider regeneration if the key will expire within this many seconds */
private const LOW_TTL = 60;
/** Max TTL, in seconds, to store keys when a data source has high replication lag */
public const TTL_LAGGED = 30;
/** Expected time-till-refresh, in seconds, if the key is accessed once per second */
private const HOT_TTR = 900;
/** Minimum key age, in seconds, for expected time-till-refresh to be considered */
private const AGE_NEW = 60;
/** Idiom for getWithSetCallback() meaning "no cache stampede mutex" */
private const TSE_NONE = -1;
/** Idiom for set()/getWithSetCallback() meaning "no post-expiration persistence" */
public const STALE_TTL_NONE = 0;
/** Idiom for set()/getWithSetCallback() meaning "no post-expiration grace period" */
public const GRACE_TTL_NONE = 0;
/** Idiom for delete()/touchCheckKey() meaning "no hold-off period" */
public const HOLDOFF_TTL_NONE = 0;
/** @var float Idiom for getWithSetCallback() meaning "no minimum required as-of timestamp" */
public const MIN_TIMESTAMP_NONE = 0.0;
/** Default process cache name and max key count */
private const PC_PRIMARY = 'primary:1000';
/** Idiom for get()/getMulti() to return extra information by reference */
public const PASS_BY_REF = [];
/** Use twemproxy-style Hash Tag key scheme (e.g. "{...}") */
private const SCHEME_HASH_TAG = 1;
/** Use mcrouter-style Hash Stop key scheme (e.g. "...|#|") */
private const SCHEME_HASH_STOP = 2;
/** Seconds to keep dependency purge keys around */
private const CHECK_KEY_TTL = self::TTL_YEAR;
/** Seconds to keep interim value keys for tombstoned keys around */
private const INTERIM_KEY_TTL = 2;
/** Seconds to keep lock keys around */
private const LOCK_TTL = 10;
/** Seconds to ramp up the chance of regeneration due to expected time-till-refresh */
private const RAMPUP_TTL = 30;
/** @var float Tiny negative float to use when CTL comes up >= 0 due to clock skew */
private const TINY_NEGATIVE = -0.000001;
/** @var float Tiny positive float to use when using "minTime" to assert an inequality */
private const TINY_POSITIVE = 0.000001;
/** Min millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
private const RECENT_SET_LOW_MS = 50;
/** Max millisecond set() backoff during hold-off (far less than INTERIM_KEY_TTL) */
private const RECENT_SET_HIGH_MS = 100;
/** Consider value generation somewhat high if it takes this many seconds or more */
private const GENERATION_HIGH_SEC = 0.2;
/** Key to the tombstone entry timestamp */
private const PURGE_TIME = 0;
/** Key to the tombstone entry hold-off TTL */
private const PURGE_HOLDOFF = 1;
/** Cache format version number */
private const VERSION = 1;
/** Version number attribute for a key; keep value for b/c (< 1.36) */
public const KEY_VERSION = 'version';
/** Generation completion timestamp attribute for a key; keep value for b/c (< 1.36) */
public const KEY_AS_OF = 'asOf';
/** Logical TTL attribute for a key */
public const KEY_TTL = 'ttl';
/** Remaining TTL attribute for a key; keep value for b/c (< 1.36) */
public const KEY_CUR_TTL = 'curTTL';
/** Tomstone timestamp attribute for a key; keep value for b/c (< 1.36) */
public const KEY_TOMB_AS_OF = 'tombAsOf';
/** Highest "check" key timestamp for a key; keep value for b/c (< 1.36) */
public const KEY_CHECK_AS_OF = 'lastCKPurge';
/** Value for a key */
private const RES_VALUE = 0;
/** Version number attribute for a key */
private const RES_VERSION = 1;
/** Generation completion timestamp attribute for a key */
private const RES_AS_OF = 2;
/** Logical TTL attribute for a key */
private const RES_TTL = 3;
/** Tomstone timestamp attribute for a key */
private const RES_TOMB_AS_OF = 4;
/** Highest "check" key timestamp for a key */
private const RES_CHECK_AS_OF = 5;
/** Highest "touched" timestamp for a key */
private const RES_TOUCH_AS_OF = 6;
/** Remaining TTL attribute for a key */
private const RES_CUR_TTL = 7;
/** Key to WAN cache version number; stored in blobs */
private const FLD_FORMAT_VERSION = 0;
/** Key to the cached value; stored in blobs */
private const FLD_VALUE = 1;
/** Key to the original TTL; stored in blobs */
private const FLD_TTL = 2;
/** Key to the cache timestamp; stored in blobs */
private const FLD_TIME = 3;
/** Key to the flags bit field (reserved number) */
private const /** @noinspection PhpUnusedPrivateFieldInspection */ FLD_FLAGS = 4;
/** Key to collection cache version number; stored in blobs */
private const FLD_VALUE_VERSION = 5;
private const /** @noinspection PhpUnusedPrivateFieldInspection */ FLD_GENERATION_TIME = 6;
/** Single character component for value keys */
private const TYPE_VALUE = 'v';
/** Single character component for timestamp check keys */
private const TYPE_TIMESTAMP = 't';
/** Single character component for mutex lock keys */
private const TYPE_MUTEX = 'm';
/** Single character component for interium value keys */
private const TYPE_INTERIM = 'i';
/** Value prefix of purge values */
private const PURGE_VAL_PREFIX = 'PURGED';
/**
* @stable to call
* @param array $params
* - cache : BagOStuff object for a persistent cache
* - logger : LoggerInterface object
* - stats : StatsdDataFactoryInterface object
* - asyncHandler : A function that takes a callback and runs it later. If supplied,
* whenever a preemptive refresh would be triggered in getWithSetCallback(), the
* current cache value is still used instead. However, the async-handler function
* receives a WAN cache callback that, when run, will execute the value generation
* callback supplied by the getWithSetCallback() caller. The result will be saved
* as normal. The handler is expected to call the WAN cache callback at an opportune
* time (e.g. HTTP post-send), though generally within a few 100ms. [optional]
* - broadcastRoutingPrefix: a routing prefix used to broadcast certain operations to all
* datacenters; See also <https://github.com/facebook/mcrouter/wiki/Config-Files>.
* This prefix takes the form `/<datacenter>/<name of wan route>/`, where `datacenter`
* is usually a wildcard to select all matching routes (e.g. the WAN cluster in all DCs).
* See also <https://github.com/facebook/mcrouter/wiki/Multi-cluster-broadcast-setup>.
* This is required when using mcrouter as a multi-region backing store proxy. [optional]
* - epoch: lowest UNIX timestamp a value/tombstone must have to be valid. [optional]
* - secret: stable secret used for hashing long strings into key components. [optional]
* - coalesceScheme: which key scheme to use in order to encourage the backend to place any
* "helper" keys for a "value" key within the same cache server. This reduces network
* overhead and reduces the chance the a single downed cache server causes disruption.
* Use "hash_stop" with mcrouter and "hash_tag" with dynomite. [default: "hash_stop"]
*/
public function __construct( array $params ) {
$this->cache = $params['cache'];
$this->broadcastRoute = $params['broadcastRoutingPrefix'] ?? null;
$this->epoch = $params['epoch'] ?? 0;
$this->secret = $params['secret'] ?? (string)$this->epoch;
if ( ( $params['coalesceScheme'] ?? '' ) === 'hash_tag' ) {
// https://redis.io/topics/cluster-spec
// https://github.com/twitter/twemproxy/blob/v0.4.1/notes/recommendation.md#hash-tags
// https://github.com/Netflix/dynomite/blob/v0.7.0/notes/recommendation.md#hash-tags
$this->coalesceScheme = self::SCHEME_HASH_TAG;
} else {
// https://github.com/facebook/mcrouter/wiki/Key-syntax
$this->coalesceScheme = self::SCHEME_HASH_STOP;
}
$this->setLogger( $params['logger'] ?? new NullLogger() );
$this->stats = $params['stats'] ?? new NullStatsdDataFactory();
$this->asyncHandler = $params['asyncHandler'] ?? null;
$this->missLog = array_fill( 0, 10, [ '', 0.0 ] );
}
/**
* @param LoggerInterface $logger
*/
public function setLogger( LoggerInterface $logger ) {
$this->logger = $logger;
}
/**
* Get an instance that wraps EmptyBagOStuff
*
* @return WANObjectCache
*/
public static function newEmpty() {
return new static( [ 'cache' => new EmptyBagOStuff() ] );
}
/**
* Fetch the value of a key from cache
*
* If supplied, $curTTL is set to the remaining TTL (current time left):
* - a) INF; if $key exists, has no TTL, and is not purged by $checkKeys
* - b) float (>=0); if $key exists, has a TTL, and is not purged by $checkKeys
* - c) float (<0); if $key is tombstoned, stale, or existing but purged by $checkKeys
* - d) null; if $key does not exist and is not tombstoned
*
* If a key is tombstoned, $curTTL will reflect the time since delete().
*
* The timestamp of $key will be checked against the last-purge timestamp
* of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
* initialized to the current timestamp. If any of $checkKeys have a timestamp
* greater than that of $key, then $curTTL will reflect how long ago $key
* became invalid. Callers can use $curTTL to know when the value is stale.
* The $checkKeys parameter allow mass key purges by updating a single key:
* - a) Each "check" key represents "last purged" of some source data
* - b) Callers pass in relevant "check" keys as $checkKeys in get()
* - c) When the source data that "check" keys represent changes,
* the touchCheckKey() method is called on them
*
* Source data entities might exists in a DB that uses snapshot isolation
* (e.g. the default REPEATABLE-READ in innoDB). Even for mutable data, that
* isolation can largely be maintained by doing the following:
* - a) Calling delete() on entity change *and* creation, before DB commit
* - b) Keeping transaction duration shorter than the delete() hold-off TTL
* - c) Disabling interim key caching via useInterimHoldOffCaching() before get() calls
*
* However, pre-snapshot values might still be seen if an update was made
* in a remote datacenter but the purge from delete() didn't relay yet.
*
* Consider using getWithSetCallback(), which has cache slam avoidance and key
* versioning features, instead of bare get()/set() calls.
*
* Do not use this method on versioned keys accessed via getWithSetCallback().
*
* When using the $info parameter, it should be passed in as WANObjectCache::PASS_BY_REF.
* In that case, it becomes a key metadata map. Otherwise, for backwards compatibility,
* $info becomes the value generation timestamp (null if the key is nonexistant/tombstoned).
* Key metadata map fields include:
* - WANObjectCache::KEY_VERSION: value version number; null if key is nonexistant
* - WANObjectCache::KEY_AS_OF: value generation timestamp (UNIX); null if key is nonexistant
* - WANObjectCache::KEY_TTL: assigned TTL (seconds); null if key is nonexistant/tombstoned
* - WANObjectCache::KEY_CUR_TTL: remaining TTL (seconds); null if key is nonexistant
* - WANObjectCache::KEY_TOMB_AS_OF: tombstone timestamp (UNIX); null if key is not tombstoned
* - WANObjectCache::KEY_CHECK_AS_OF: highest "check" key timestamp (UNIX); null if none
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param float|null &$curTTL Seconds of TTL left [returned]
* @param string[] $checkKeys Map of (integer or cache key => "check" key(s));
* "check" keys must also be made with makeKey()/makeGlobalKey()
* @param array &$info Metadata map [returned]
* @return mixed Value of cache key; false on failure
*/
final public function get( $key, &$curTTL = null, array $checkKeys = [], &$info = [] ) {
// Note that an undeclared variable passed as $info starts as null (not the default).
// Also, if no $info parameter is provided, then it doesn't matter how it changes here.
$legacyInfo = ( $info !== self::PASS_BY_REF );
$res = $this->fetchKeys( [ $key ], $checkKeys )[$key];
$curTTL = $res[self::RES_CUR_TTL];
$info = $legacyInfo
? $res[self::RES_AS_OF]
: [
self::KEY_VERSION => $res[self::RES_VERSION],
self::KEY_AS_OF => $res[self::RES_AS_OF],
self::KEY_TTL => $res[self::RES_TTL],
self::KEY_CUR_TTL => $res[self::RES_CUR_TTL],
self::KEY_TOMB_AS_OF => $res[self::RES_TOMB_AS_OF],
self::KEY_CHECK_AS_OF => $res[self::RES_CHECK_AS_OF]
];
if ( $curTTL === null || $curTTL <= 0 ) {
// Log the timestamp in case a corresponding set() call does not provide "walltime"
unset( $this->missLog[array_key_first( $this->missLog )] );
$this->missLog[] = [ $key, $this->getCurrentTime() ];
}
return $res[self::RES_VALUE];
}
/**
* Fetch the value of several keys from cache
*
* $curTTLs becomes a map of only present/tombstoned $keys to their current time-to-live.
*
* $checkKeys holds the "check" keys used to validate values of applicable keys. The
* integer indexes hold "check" keys that apply to all of $keys while the string indexes
* hold "check" keys that only apply to the cache key with that name. The logic of "check"
* keys otherwise works the same as in WANObjectCache::get().
*
* When using the $info parameter, it should be passed in as WANObjectCache::PASS_BY_REF.
* In that case, it becomes a mapping of all the $keys to their metadata maps, each in the
* style of WANObjectCache::get(). Otherwise, for backwards compatibility, $info becomes a
* map of only present/tombstoned $keys to their value generation timestamps.
*
* @see WANObjectCache::get()
*
* @param string[] $keys List/map with makeKey()/makeGlobalKey() cache keys as values
* @param array<string,float> &$curTTLs Map of (key => seconds of TTL left) [returned]
* @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
* "check" keys must also be made with makeKey()/makeGlobalKey()
* @param array<string,array> &$info Map of (key => metadata map) [returned]
* @return array<string,mixed> Map of (key => value) for existing values in order of $keys
*/
final public function getMulti(
array $keys,
&$curTTLs = [],
array $checkKeys = [],
&$info = []
) {
// Note that an undeclared variable passed as $info starts as null (not the default).
// Also, if no $info parameter is provided, then it doesn't matter how it changes here.
$legacyInfo = ( $info !== self::PASS_BY_REF );
$curTTLs = [];
$info = [];
$valuesByKey = [];
$resByKey = $this->fetchKeys( $keys, $checkKeys );
foreach ( $resByKey as $key => $res ) {
if ( $res[self::RES_VALUE] !== false ) {
$valuesByKey[$key] = $res[self::RES_VALUE];
}
if ( $res[self::RES_CUR_TTL] !== null ) {
$curTTLs[$key] = $res[self::RES_CUR_TTL];
}
$info[$key] = $legacyInfo
? $res[self::RES_AS_OF]
: [
self::KEY_VERSION => $res[self::RES_VERSION],
self::KEY_AS_OF => $res[self::RES_AS_OF],
self::KEY_TTL => $res[self::RES_TTL],
self::KEY_CUR_TTL => $res[self::RES_CUR_TTL],
self::KEY_TOMB_AS_OF => $res[self::RES_TOMB_AS_OF],
self::KEY_CHECK_AS_OF => $res[self::RES_CHECK_AS_OF]
];
}
return $valuesByKey;
}
/**
* Fetch the value and key metadata of several keys from cache
*
* $checkKeys holds the "check" keys used to validate values of applicable keys.
* The integer indexes hold "check" keys that apply to all of $keys while the string
* indexes hold "check" keys that only apply to the cache key with that name.
*
* @param string[] $keys List/map with makeKey()/makeGlobalKey() cache keys as values
* @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
* "check" keys must also be made with makeKey()/makeGlobalKey()
* @param callable|null $touchedCb Callback yielding a UNIX timestamp from a value, or null
* @return array<string,array> Map of (key => WANObjectCache::RESULT_* map) in order of $keys
* @note Callable type hints are not used to avoid class-autoloading
*/
protected function fetchKeys( array $keys, array $checkKeys, $touchedCb = null ) {
$resByKey = [];
// List of all sister keys that need to be fetched from cache
$allSisterKeys = [];
// Order-corresponding value sister key list for the base key list ($keys)
$valueSisterKeys = [];
// List of "check" sister keys to compare all value sister keys against
$checkSisterKeysForAll = [];
// Map of (base key => additional "check" sister key(s) to compare against)
$checkSisterKeysByKey = [];
foreach ( $keys as $key ) {
$sisterKey = $this->makeSisterKey( $key, self::TYPE_VALUE );
$allSisterKeys[] = $sisterKey;
$valueSisterKeys[] = $sisterKey;
}
foreach ( $checkKeys as $i => $checkKeyOrKeyGroup ) {
// Note: avoid array_merge() inside loop in case there are many keys
if ( is_int( $i ) ) {
// Single "check" key that applies to all base keys
$sisterKey = $this->makeSisterKey( $checkKeyOrKeyGroup, self::TYPE_TIMESTAMP );
$allSisterKeys[] = $sisterKey;
$checkSisterKeysForAll[] = $sisterKey;
} else {
// List of "check" keys that apply to a specific base key
foreach ( (array)$checkKeyOrKeyGroup as $checkKey ) {
$sisterKey = $this->makeSisterKey( $checkKey, self::TYPE_TIMESTAMP );
$allSisterKeys[] = $sisterKey;
$checkSisterKeysByKey[$i][] = $sisterKey;
}
}
}
if ( $this->warmupCache ) {
// Get the wrapped values of the sister keys from the warmup cache
$wrappedBySisterKey = $this->warmupCache;
$sisterKeysMissing = array_diff( $allSisterKeys, array_keys( $wrappedBySisterKey ) );
if ( $sisterKeysMissing ) {
$this->warmupKeyMisses += count( $sisterKeysMissing );
$wrappedBySisterKey += $this->cache->getMulti( $sisterKeysMissing );
}
} else {
// Fetch the wrapped values of the sister keys from the backend
$wrappedBySisterKey = $this->cache->getMulti( $allSisterKeys );
}
// Pessimistically treat the "current time" as the time when any network I/O finished
$now = $this->getCurrentTime();
// List of "check" sister key purge timestamps to compare all value sister keys against
$ckPurgesForAll = $this->processCheckKeys(
$checkSisterKeysForAll,
$wrappedBySisterKey,
$now
);
// Map of (base key => extra "check" sister key purge timestamp(s) to compare against)
$ckPurgesByKey = [];
foreach ( $checkSisterKeysByKey as $keyWithCheckKeys => $checkKeysForKey ) {
$ckPurgesByKey[$keyWithCheckKeys] = $this->processCheckKeys(
$checkKeysForKey,
$wrappedBySisterKey,
$now
);
}
// Unwrap and validate any value found for each base key (under the value sister key)
reset( $keys );
foreach ( $valueSisterKeys as $valueSisterKey ) {
// Get the corresponding base key for this value sister key
$key = current( $keys );
next( $keys );
if ( array_key_exists( $valueSisterKey, $wrappedBySisterKey ) ) {
// Key exists as either a live value or tombstone value
$wrapped = $wrappedBySisterKey[$valueSisterKey];
} else {
// Key does not exist
$wrapped = false;
}
$res = $this->unwrap( $wrapped, $now );
$value = $res[self::RES_VALUE];
foreach ( array_merge( $ckPurgesForAll, $ckPurgesByKey[$key] ?? [] ) as $ckPurge ) {
$res[self::RES_CHECK_AS_OF] = max(
$ckPurge[self::PURGE_TIME],
$res[self::RES_CHECK_AS_OF]
);
// Timestamp marking the end of the hold-off period for this purge
$holdoffDeadline = $ckPurge[self::PURGE_TIME] + $ckPurge[self::PURGE_HOLDOFF];
// Check if the value was generated during the hold-off period
if ( $value !== false && $holdoffDeadline >= $res[self::RES_AS_OF] ) {
// How long ago this value was purged by *this* "check" key
$ago = min( $ckPurge[self::PURGE_TIME] - $now, self::TINY_NEGATIVE );
// How long ago this value was purged by *any* known "check" key
$res[self::RES_CUR_TTL] = min( $res[self::RES_CUR_TTL], $ago );
}
}
if ( $touchedCb !== null && $value !== false ) {
$touched = $touchedCb( $value );
if ( $touched !== null && $touched >= $res[self::RES_AS_OF] ) {
$res[self::RES_CUR_TTL] = min(
$res[self::RES_CUR_TTL],
$res[self::RES_AS_OF] - $touched,
self::TINY_NEGATIVE
);
}
} else {
$touched = null;
}
$res[self::RES_TOUCH_AS_OF] = max( $res[self::RES_TOUCH_AS_OF], $touched );
$resByKey[$key] = $res;
}
return $resByKey;
}
/**
* @param string[] $checkSisterKeys List of "check" sister keys
* @param mixed[] $wrappedBySisterKey Preloaded map of (sister key => wrapped value)
* @param float $now UNIX timestamp
* @return array[] List of purge value arrays
*/
private function processCheckKeys(
array $checkSisterKeys,
array $wrappedBySisterKey,
float $now
) {
$purges = [];
foreach ( $checkSisterKeys as $timeKey ) {
$purge = isset( $wrappedBySisterKey[$timeKey] )
? $this->parsePurgeValue( $wrappedBySisterKey[$timeKey] )
: null;
if ( $purge === null ) {
// No holdoff when lazy creating a check key, use cache right away (T344191)
$wrapped = $this->makeCheckPurgeValue( $now, self::HOLDOFF_TTL_NONE, $purge );
$this->cache->add(
$timeKey,
$wrapped,
self::CHECK_KEY_TTL,
$this->cache::WRITE_BACKGROUND
);
}
$purges[] = $purge;
}
return $purges;
}
/**
* Set the value of a key in cache
*
* Simply calling this method when source data changes is not valid because
* the changes do not replicate to the other WAN sites. In that case, delete()
* should be used instead. This method is intended for use on cache misses.
*
* If data was read using "view snapshots" (e.g. innodb REPEATABLE-READ),
* use 'since' to avoid the following race condition:
* - a) T1 starts
* - b) T2 updates a row, calls delete(), and commits
* - c) The HOLDOFF_TTL passes, expiring the delete() tombstone
* - d) T1 reads the row and calls set() due to a cache miss
* - e) Stale value is stuck in cache
*
* Setting 'lag' and 'since' help avoids keys getting stuck in stale states.
*
* Be aware that this does not update the process cache for getWithSetCallback()
* callers. Keys accessed via that method are not generally meant to also be set
* using this primitive method.
*
* Consider using getWithSetCallback(), which has cache slam avoidance and key
* versioning features, instead of bare get()/set() calls.
*
* Do not use this method on versioned keys accessed via getWithSetCallback().
*
* Example usage:
* @code
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* $setOpts = Database::getCacheSetOptions( $dbr );
* // Fetch the row from the DB
* $row = $dbr->selectRow( ... );
* $key = $cache->makeKey( 'building', $buildingId );
* $cache->set( $key, $row, $cache::TTL_DAY, $setOpts );
* @endcode
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param mixed $value Value to set for the cache key
* @param int $ttl Seconds to live. Special values are:
* - WANObjectCache::TTL_INDEFINITE: Cache forever (default)
* - WANObjectCache::TTL_UNCACHEABLE: Do not cache (if the key exists, it is not deleted)
* @param array $opts Options map:
* - lag: Highest seconds of replication lag potentially affecting reads used to generate
* the value. This should not be affected by the duration of transaction "view snapshots"
* (e.g. innodb REPEATABLE-READ) nor the time elapsed since the first read (though both
* increase staleness). For reads using view snapshots, only the replication lag during
* snapshot initialization matters. Use false if replication is stopped/broken on a
* replica server involved in the reads.
* Default: 0 seconds
* - since: UNIX timestamp indicative of the highest possible staleness caused by the
* duration of transaction "view snapshots" (e.g. innodb REPEATABLE-READ) and the time
* elapsed since the first read. This should not be affected by replication lag.
* Default: 0 seconds
* - pending: Whether this data is possibly from an uncommitted write transaction.
* Generally, other threads should not see values from the future and
* they certainly should not see ones that ended up getting rolled back.
* Default: false
* - lockTSE: If excessive replication/snapshot lag is detected, then store the value
* with this TTL and flag it as stale. This is only useful if the reads for this key
* use getWithSetCallback() with "lockTSE" set. Note that if "staleTTL" is set
* then it will still add on to this TTL in the excessive lag scenario.
* Default: WANObjectCache::TSE_NONE
* - staleTTL: Seconds to keep the key around if it is stale. The get()/getMulti()
* methods return such stale values with a $curTTL of 0, and getWithSetCallback()
* will call the generation callback in such cases, passing in the old value
* and its as-of time to the callback. This is useful if adaptiveTTL() is used
* on the old value's as-of time when it is verified as still being correct.
* Default: WANObjectCache::STALE_TTL_NONE
* - segmentable: Allow partitioning of the value if it is a large string.
* Default: false.
* - creating: Optimize for the case where the key does not already exist.
* Default: false
* - version: Integer version number signifying the format of the value.
* Default: null
* - walltime: How long the value took to generate in seconds. Default: null
* @phpcs:ignore Generic.Files.LineLength
* @phan-param array{lag?:float|int,since?:float|int,pending?:bool,lockTSE?:int,staleTTL?:int,creating?:bool,version?:int,walltime?:int|float,segmentable?:bool} $opts
* @note Options added in 1.28: staleTTL
* @note Options added in 1.33: creating
* @note Options added in 1.34: version, walltime
* @note Options added in 1.40: segmentable
* @return bool Success
*/
final public function set( $key, $value, $ttl = self::TTL_INDEFINITE, array $opts = [] ) {
$keygroup = $this->determineKeyGroupForStats( $key );
$ok = $this->setMainValue(
$key,
$value,
$ttl,
$opts['version'] ?? null,
$opts['walltime'] ?? null,
$opts['lag'] ?? 0,
$opts['since'] ?? null,
$opts['pending'] ?? false,
$opts['lockTSE'] ?? self::TSE_NONE,
$opts['staleTTL'] ?? self::STALE_TTL_NONE,
$opts['segmentable'] ?? false,
$opts['creating'] ?? false
);
$this->stats->increment( "wanobjectcache.$keygroup.set." . ( $ok ? 'ok' : 'error' ) );
return $ok;
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param mixed $value
* @param int|float $ttl
* @param int|null $version
* @param float|null $walltime
* @param float|int|bool $dataReplicaLag
* @param float|int|null $dataReadSince
* @param bool $dataPendingCommit
* @param int $lockTSE
* @param int $staleTTL
* @param bool $segmentable
* @param bool $creating
* @return bool Success
*/
private function setMainValue(
$key,
$value,
$ttl,
?int $version,
?float $walltime,
$dataReplicaLag,
$dataReadSince,
bool $dataPendingCommit,
int $lockTSE,
int $staleTTL,
bool $segmentable,
bool $creating
) {
if ( $ttl < 0 ) {
// not cacheable
return true;
}
$now = $this->getCurrentTime();
$ttl = (int)$ttl;
$walltime ??= $this->timeSinceLoggedMiss( $key, $now );
$dataSnapshotLag = ( $dataReadSince !== null ) ? max( 0, $now - $dataReadSince ) : 0;
$dataCombinedLag = $dataReplicaLag + $dataSnapshotLag;
// Forbid caching data that only exists within an uncommitted transaction. Also, lower
// the TTL when the data has a "since" time so far in the past that a delete() tombstone,
// made after that time, could have already expired (the key is no longer write-holed).
// The mitigation TTL depends on whether this data lag is assumed to systemically effect
// regeneration attempts in the near future. The TTL also reflects regeneration wall time.
if ( $dataPendingCommit ) {
// Case A: data comes from an uncommitted write transaction
$mitigated = 'pending writes';
// Data might never be committed; rely on a less problematic regeneration attempt
$mitigationTTL = self::TTL_UNCACHEABLE;
} elseif ( $dataSnapshotLag > self::MAX_READ_LAG ) {
// Case B: high snapshot lag
$pregenSnapshotLag = ( $walltime !== null ) ? ( $dataSnapshotLag - $walltime ) : 0;
if ( ( $pregenSnapshotLag + self::GENERATION_HIGH_SEC ) > self::MAX_READ_LAG ) {
// Case B1: generation started when transaction duration was already long
$mitigated = 'snapshot lag (late generation)';
// Probably non-systemic; rely on a less problematic regeneration attempt
$mitigationTTL = self::TTL_UNCACHEABLE;
} else {
// Case B2: slow generation made transaction duration long
$mitigated = 'snapshot lag (high generation time)';
// Probably systemic; use a low TTL to avoid stampedes/uncacheability
$mitigationTTL = self::TTL_LAGGED;
}
} elseif ( $dataReplicaLag === false || $dataReplicaLag > self::MAX_READ_LAG ) {
// Case C: low/medium snapshot lag with high replication lag
$mitigated = 'replication lag';
// Probably systemic; use a low TTL to avoid stampedes/uncacheability
$mitigationTTL = self::TTL_LAGGED;
} elseif ( $dataCombinedLag > self::MAX_READ_LAG ) {
$pregenCombinedLag = ( $walltime !== null ) ? ( $dataCombinedLag - $walltime ) : 0;
// Case D: medium snapshot lag with medium replication lag
if ( ( $pregenCombinedLag + self::GENERATION_HIGH_SEC ) > self::MAX_READ_LAG ) {
// Case D1: generation started when read lag was too high
$mitigated = 'read lag (late generation)';
// Probably non-systemic; rely on a less problematic regeneration attempt
$mitigationTTL = self::TTL_UNCACHEABLE;
} else {
// Case D2: slow generation made read lag too high
$mitigated = 'read lag (high generation time)';
// Probably systemic; use a low TTL to avoid stampedes/uncacheability
$mitigationTTL = self::TTL_LAGGED;
}
} else {
// Case E: new value generated with recent data
$mitigated = null;
// Nothing to mitigate
$mitigationTTL = null;
}
if ( $mitigationTTL === self::TTL_UNCACHEABLE ) {
$this->logger->warning(
"Rejected set() for {cachekey} due to $mitigated.",
[
'cachekey' => $key,
'lag' => $dataReplicaLag,
'age' => $dataSnapshotLag,
'walltime' => $walltime
]
);
// no-op the write for being unsafe
return true;
}
// TTL to use in staleness checks (does not effect persistence layer TTL)
$logicalTTL = null;
if ( $mitigationTTL !== null ) {
// New value was generated from data that is old enough to be risky
if ( $lockTSE >= 0 ) {
// Persist the value as long as normal, but make it count as stale sooner
$logicalTTL = min( $ttl ?: INF, $mitigationTTL );
} else {
// Persist the value for a shorter duration
$ttl = min( $ttl ?: INF, $mitigationTTL );
}
$this->logger->warning(
"Lowered set() TTL for {cachekey} due to $mitigated.",
[
'cachekey' => $key,
'lag' => $dataReplicaLag,
'age' => $dataSnapshotLag,
'walltime' => $walltime
]
);
}
// Wrap that value with time/TTL/version metadata
$wrapped = $this->wrap( $value, $logicalTTL ?: $ttl, $version, $now );
$storeTTL = $ttl + $staleTTL;
$flags = $this->cache::WRITE_BACKGROUND;
if ( $segmentable ) {
$flags |= $this->cache::WRITE_ALLOW_SEGMENTS;
}
if ( $creating ) {
$ok = $this->cache->add(
$this->makeSisterKey( $key, self::TYPE_VALUE ),
$wrapped,
$storeTTL,
$flags
);
} else {
$ok = $this->cache->merge(
$this->makeSisterKey( $key, self::TYPE_VALUE ),
static function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
// A string value means that it is a tombstone; do nothing in that case
return ( is_string( $cWrapped ) ) ? false : $wrapped;
},
$storeTTL,
$this->cache::MAX_CONFLICTS_ONE,
$flags
);
}
return $ok;
}
/**
* Purge a key from all datacenters
*
* This should only be called when the underlying data (being cached)
* changes in a significant way. This deletes the key and starts a hold-off
* period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
* This is done to avoid the following race condition:
* - a) Some DB data changes and delete() is called on a corresponding key
* - b) A request refills the key with a stale value from a lagged DB
* - c) The stale value is stuck there until the key is expired/evicted
*
* This is implemented by storing a special "tombstone" value at the cache
* key that this class recognizes; get() calls will return false for the key
* and any set() calls will refuse to replace tombstone values at the key.
* For this to always avoid stale value writes, the following must hold:
* - a) Replication lag is bounded to being less than HOLDOFF_TTL; or
* - b) If lag is higher, the DB will have gone into read-only mode already
*
* Note that set() can also be lag-aware and lower the TTL if it's high.
*
* Be aware that this does not clear the process cache. Even if it did, callbacks
* used by getWithSetCallback() might still return stale data in the case of either
* uncommitted or not-yet-replicated changes (callback generally use replica DBs).
*
* When using potentially long-running ACID transactions, a good pattern is
* to use a pre-commit hook to issue the delete. This means that immediately
* after commit, callers will see the tombstone in cache upon purge relay.
* It also avoids the following race condition:
* - a) T1 begins, changes a row, and calls delete()
* - b) The HOLDOFF_TTL passes, expiring the delete() tombstone
* - c) T2 starts, reads the row and calls set() due to a cache miss
* - d) T1 finally commits
* - e) Stale value is stuck in cache
*
* Example usage:
* @code
* $dbw->startAtomic( __METHOD__ ); // start of request
* ... <execute some stuff> ...
* // Update the row in the DB
* $dbw->update( ... );
* $key = $cache->makeKey( 'homes', $homeId );
* // Purge the corresponding cache entry just before committing
* $dbw->onTransactionPreCommitOrIdle( function() use ( $cache, $key ) {
* $cache->delete( $key );
* } );
* ... <execute some stuff> ...
* $dbw->endAtomic( __METHOD__ ); // end of request
* @endcode
*
* The $ttl parameter can be used when purging values that have not actually changed
* recently. For example, user-requested purges or cache cleanup scripts might not need
* to invoke a hold-off period on cache backfills, so they can use HOLDOFF_TTL_NONE.
*
* Note that $ttl limits the effective range of 'lockTSE' for getWithSetCallback().
*
* If called twice on the same key, then the last hold-off TTL takes precedence. For
* idempotence, the $ttl should not vary for different delete() calls on the same key.
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param int $ttl Tombstone TTL; Default: WANObjectCache::HOLDOFF_TTL
* @return bool True if the item was purged or not found, false on failure
*/
final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
// Purge values must be stored under the value key so that WANObjectCache::set()
// can atomically merge values without accidentally undoing a recent purge and thus
// violating the holdoff TTL restriction.
$valueSisterKey = $this->makeSisterKey( $key, self::TYPE_VALUE );
if ( $ttl <= 0 ) {
// A client or cache cleanup script is requesting a cache purge, so there is no
// volatility period due to replica DB lag. Any recent change to an entity cached
// in this key should have triggered an appropriate purge event.
$ok = $this->relayNonVolatilePurge( $valueSisterKey );
} else {
// A cacheable entity recently changed, so there might be a volatility period due
// to replica DB lag. Clients usually expect their actions to be reflected in any
// of their subsequent web request. This is attainable if (a) purge relay lag is
// lower than the time it takes for subsequent request by the client to arrive,
// and, (b) DB replica queries have "read-your-writes" consistency due to DB lag
// mitigation systems.
$now = $this->getCurrentTime();
// Set the key to the purge value in all datacenters
$purge = $this->makeTombstonePurgeValue( $now );
$ok = $this->relayVolatilePurge( $valueSisterKey, $purge, $ttl );
}
$keygroup = $this->determineKeyGroupForStats( $key );
$this->stats->increment( "wanobjectcache.$keygroup.delete." . ( $ok ? 'ok' : 'error' ) );
return $ok;
}
/**
* Fetch the value of a timestamp "check" key
*
* The key will be *initialized* to the current time if not set,
* so only call this method if this behavior is actually desired
*
* The timestamp can be used to check whether a cached value is valid.
* Callers should not assume that this returns the same timestamp in
* all datacenters due to relay delays.
*
* The level of staleness can roughly be estimated from this key, but
* if the key was evicted from cache, such calculations may show the
* time since expiry as ~0 seconds.
*
* Note that "check" keys won't collide with other regular keys.
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @return float UNIX timestamp
*/
final public function getCheckKeyTime( $key ) {
return $this->getMultiCheckKeyTime( [ $key ] )[$key];
}
/**
* Fetch the values of each timestamp "check" key
*
* This works like getCheckKeyTime() except it takes a list of keys
* and returns a map of timestamps instead of just that of one key
*
* This might be useful if both:
* - a) a class of entities each depend on hundreds of other entities
* - b) these other entities are depended upon by millions of entities
*
* The later entities can each use a "check" key to purge their dependee entities.
* However, it is expensive for the former entities to verify against all of the relevant
* "check" keys during each getWithSetCallback() call. A less expensive approach is to do
* these verifications only after a "time-till-verify" (TTV) has passed. This is a middle
* ground between using blind TTLs and using constant verification. The adaptiveTTL() method
* can be used to dynamically adjust the TTV. Also, the initial TTV can make use of the
* last-modified times of the dependent entities (either from the DB or the "check" keys).
*
* Example usage:
* @code
* $value = $cache->getWithSetCallback(
* $cache->makeGlobalKey( 'wikibase-item', $id ),
* self::INITIAL_TTV, // initial time-till-verify
* function ( $oldValue, &$ttv, &$setOpts, $oldAsOf ) use ( $checkKeys, $cache ) {
* $now = microtime( true );
* // Use $oldValue if it passes max ultimate age and "check" key comparisons
* if ( $oldValue &&
* $oldAsOf > max( $cache->getMultiCheckKeyTime( $checkKeys ) ) &&
* ( $now - $oldValue['ctime'] ) <= self::MAX_CACHE_AGE
* ) {
* // Increase time-till-verify by 50% of last time to reduce overhead
* $ttv = $cache->adaptiveTTL( $oldAsOf, self::MAX_TTV, self::MIN_TTV, 1.5 );
* // Unlike $oldAsOf, "ctime" is the ultimate age of the cached data
* return $oldValue;
* }
*
* $mtimes = []; // dependency last-modified times; passed by reference
* $value = [ 'data' => $this->fetchEntityData( $mtimes ), 'ctime' => $now ];
* // Guess time-till-change among the dependencies, e.g. 1/(total change rate)
* $ttc = 1 / array_sum( array_map(
* function ( $mtime ) use ( $now ) {
* return 1 / ( $mtime ? ( $now - $mtime ) : 900 );
* },
* $mtimes
* ) );
* // The time-to-verify should not be overly pessimistic nor optimistic
* $ttv = min( max( $ttc, self::MIN_TTV ), self::MAX_TTV );
*
* return $value;
* },
* [ 'staleTTL' => $cache::TTL_DAY ] // keep around to verify and re-save
* );
* @endcode
*
* @see WANObjectCache::getCheckKeyTime()
* @see WANObjectCache::getWithSetCallback()
*
* @param string[] $keys Cache keys made with makeKey()/makeGlobalKey()
* @return float[] Map of (key => UNIX timestamp)
* @since 1.31
*/
final public function getMultiCheckKeyTime( array $keys ) {
$checkSisterKeysByKey = [];
foreach ( $keys as $key ) {
$checkSisterKeysByKey[$key] = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );
}
$wrappedBySisterKey = $this->cache->getMulti( $checkSisterKeysByKey );
$wrappedBySisterKey += array_fill_keys( $checkSisterKeysByKey, false );
$now = $this->getCurrentTime();
$times = [];
foreach ( $checkSisterKeysByKey as $key => $checkSisterKey ) {
$purge = $this->parsePurgeValue( $wrappedBySisterKey[$checkSisterKey] );
if ( $purge === null ) {
$wrapped = $this->makeCheckPurgeValue( $now, self::HOLDOFF_TTL_NONE, $purge );
$this->cache->add(
$checkSisterKey,
$wrapped,
self::CHECK_KEY_TTL,
$this->cache::WRITE_BACKGROUND
);
}
$times[$key] = $purge[self::PURGE_TIME];
}
return $times;
}
/**
* Increase the last-purge timestamp of a "check" key in all datacenters
*
* This method should only be called when some heavily referenced data changes in
* a significant way, such that it is impractical to call delete() on all the cache
* keys that should be purged. The get*() method calls used to fetch these keys must
* include the given "check" key in the relevant "check" keys argument/option.
*
* A "check" key essentially represents a last-modified time of an entity. When the
* key is touched, the timestamp will be updated to the current time. Keys fetched
* using get*() calls, that include the "check" key, will be seen as purged.
*
* The timestamp of the "check" key is treated as being HOLDOFF_TTL seconds in the
* future by get*() methods in order to avoid race conditions where keys are updated
* with stale values (e.g. from a lagged replica DB). A high TTL is set on the "check"
* key, making it possible to know the timestamp of the last change to the corresponding
* entities in most cases. This might use more cache space than resetCheckKey().
*
* When a few important keys get a large number of hits, a high cache time is usually
* desired as well as "lockTSE" logic. The resetCheckKey() method is less appropriate
* in such cases since the "time since expiry" cannot be inferred, causing any get()
* after the reset to treat the key as being "hot", resulting in more stale value usage.
*
* Note that "check" keys won't collide with other regular keys.
*
* @see WANObjectCache::get()
* @see WANObjectCache::getWithSetCallback()
* @see WANObjectCache::resetCheckKey()
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param int $holdoff HOLDOFF_TTL or HOLDOFF_TTL_NONE constant
* @return bool True if the item was purged or not found, false on failure
*/
final public function touchCheckKey( $key, $holdoff = self::HOLDOFF_TTL ) {
$checkSisterKey = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );
$now = $this->getCurrentTime();
$purge = $this->makeCheckPurgeValue( $now, $holdoff );
$ok = $this->relayVolatilePurge( $checkSisterKey, $purge, self::CHECK_KEY_TTL );
$keygroup = $this->determineKeyGroupForStats( $key );
$this->stats->increment( "wanobjectcache.$keygroup.ck_touch." . ( $ok ? 'ok' : 'error' ) );
return $ok;
}
/**
* Clear the last-purge timestamp of a "check" key in all datacenters
*
* Similar to touchCheckKey(), in that keys fetched using get*() calls, that include
* the given "check" key, will be seen as purged. However, there are some differences:
* - a) The "check" key will be deleted from all caches and lazily
* re-initialized when accessed (rather than set everywhere)
* - b) Thus, dependent keys will be known to be stale, but not
* for how long (they are treated as "just" purged), which
* effects any lockTSE logic in getWithSetCallback()
* - c) Since "check" keys are initialized only on the server the key hashes
* to, any temporary ejection of that server will cause the value to be
* seen as purged as a new server will initialize the "check" key.
*
* The advantage over touchCheckKey() is that the "check" keys, which have high TTLs,
* will only be created when a get*() method actually uses those keys. This is better
* when a large number of "check" keys must be changed in a short period of time.
*
* Note that "check" keys won't collide with other regular keys.
*
* @see WANObjectCache::get()
* @see WANObjectCache::getWithSetCallback()
* @see WANObjectCache::touchCheckKey()
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @return bool True if the item was purged or not found, false on failure
*/
final public function resetCheckKey( $key ) {
$checkSisterKey = $this->makeSisterKey( $key, self::TYPE_TIMESTAMP );
$ok = $this->relayNonVolatilePurge( $checkSisterKey );
$keygroup = $this->determineKeyGroupForStats( $key );
$this->stats->increment( "wanobjectcache.$keygroup.ck_reset." . ( $ok ? 'ok' : 'error' ) );
return $ok;
}
/**
* Method to fetch/regenerate a cache key
*
* On cache miss, the key will be set to the callback result via set()
* (unless the callback returns false) and that result will be returned.
* The arguments supplied to the callback are:
* - $oldValue: prior cache value or false if none was present
* - &$ttl: alterable reference to the TTL to be assigned to the new value
* - &$setOpts: alterable reference to the set() options to be used with the new value
* - $oldAsOf: generation UNIX timestamp of $oldValue or null if not present (since 1.28)
* - $params: custom field/value map as defined by $cbParams (since 1.35)
*
* It is strongly recommended to set the 'lag' and 'since' fields to avoid race conditions
* that can cause stale values to get stuck at keys. Usually, callbacks ignore the current
* value, but it can be used to maintain "most recent X" values that come from time or
* sequence based source data, provided that the "as of" id/time is tracked. Note that
* preemptive regeneration and $checkKeys can result in a non-false current value.
*
* Usage of $checkKeys is similar to get() and getMulti(). However, rather than the caller
* having to inspect a "current time left" variable (e.g. $curTTL, $curTTLs), a cache
* regeneration will automatically be triggered using the callback.
*
* The $ttl argument and "hotTTR" option (in $opts) use time-dependent randomization
* to avoid stampedes. Keys that are slow to regenerate and either heavily used
* or subject to explicit (unpredictable) purges, may need additional mechanisms.
* The simplest way to avoid stampedes for such keys is to use 'lockTSE' (in $opts).
* If explicit purges are needed, also:
* - a) Pass $key into $checkKeys
* - b) Use touchCheckKey( $key ) instead of delete( $key )
*
* This applies cache server I/O stampede protection against duplicate cache sets.
* This is important when the callback is slow and/or yields large values for a key.
*
* Example usage (typical key):
* @code
* $catInfo = $cache->getWithSetCallback(
* // Key to store the cached value under
* $cache->makeKey( 'cat-attributes', $catId ),
* // Time-to-live (in seconds)
* $cache::TTL_MINUTE,
* // Function that derives the new key value
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* return $dbr->selectRow( ... );
* }
* );
* @endcode
*
* Example usage (key that is expensive and hot):
* @code
* $catConfig = $cache->getWithSetCallback(
* // Key to store the cached value under
* $cache->makeKey( 'site-cat-config' ),
* // Time-to-live (in seconds)
* $cache::TTL_DAY,
* // Function that derives the new key value
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* return CatConfig::newFromRow( $dbr->selectRow( ... ) );
* },
* [
* // Calling touchCheckKey() on this key purges the cache
* 'checkKeys' => [ $cache->makeKey( 'site-cat-config' ) ],
* // Try to only let one datacenter thread manage cache updates at a time
* 'lockTSE' => 30,
* // Avoid querying cache servers multiple times in a web request
* 'pcTTL' => $cache::TTL_PROC_LONG
* ]
* );
* @endcode
*
* Example usage (key with dynamic dependencies):
* @code
* $catState = $cache->getWithSetCallback(
* // Key to store the cached value under
* $cache->makeKey( 'cat-state', $cat->getId() ),
* // Time-to-live (seconds)
* $cache::TTL_HOUR,
* // Function that derives the new key value
* function ( $oldValue, &$ttl, array &$setOpts ) {
* // Determine new value from the DB
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* return CatState::newFromResults( $dbr->select( ... ) );
* },
* [
* // The "check" keys that represent things the value depends on;
* // Calling touchCheckKey() on any of them purges the cache
* 'checkKeys' => [
* $cache->makeKey( 'sustenance-bowls', $cat->getRoomId() ),
* $cache->makeKey( 'people-present', $cat->getHouseId() ),
* $cache->makeKey( 'cat-laws', $cat->getCityId() ),
* ]
* ]
* );
* @endcode
*
* Example usage (key that is expensive with too many DB dependencies for "check" keys):
* @code
* $catToys = $cache->getWithSetCallback(
* // Key to store the cached value under
* $cache->makeKey( 'cat-toys', $catId ),
* // Time-to-live (seconds)
* $cache::TTL_HOUR,
* // Function that derives the new key value
* function ( $oldValue, &$ttl, array &$setOpts ) {
* // Determine new value from the DB
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* return CatToys::newFromResults( $dbr->select( ... ) );
* },
* [
* // Get the highest timestamp of any of the cat's toys
* 'touchedCallback' => function ( $value ) use ( $catId ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* $ts = $dbr->selectField( 'cat_toys', 'MAX(ct_touched)', ... );
*
* return wfTimestampOrNull( TS_UNIX, $ts );
* },
* // Avoid DB queries for repeated access
* 'pcTTL' => $cache::TTL_PROC_SHORT
* ]
* );
* @endcode
*
* Example usage (hot key holding most recent 100 events):
* @code
* $lastCatActions = $cache->getWithSetCallback(
* // Key to store the cached value under
* $cache->makeKey( 'cat-last-actions', 100 ),
* // Time-to-live (in seconds)
* 10,
* // Function that derives the new key value
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* // Start off with the last cached list
* $list = $oldValue ?: [];
* // Fetch the last 100 relevant rows in descending order;
* // only fetch rows newer than $list[0] to reduce scanning
* $rows = iterator_to_array( $dbr->select( ... ) );
* // Merge them and get the new "last 100" rows
* return array_slice( array_merge( $new, $list ), 0, 100 );
* },
* [
* // Try to only let one datacenter thread manage cache updates at a time
* 'lockTSE' => 30,
* // Use a magic value when no cache value is ready rather than stampeding
* 'busyValue' => 'computing'
* ]
* );
* @endcode
*
* Example usage (key holding an LRU subkey:value map; this can avoid flooding cache with
* keys for an unlimited set of (constraint,situation) pairs, thereby avoiding elevated
* cache evictions and wasted memory):
* @code
* $catSituationTolerabilityCache = $this->cache->getWithSetCallback(
* // Group by constraint ID/hash, cat family ID/hash, or something else useful
* $this->cache->makeKey( 'cat-situation-tolerability-checks', $groupKey ),
* WANObjectCache::TTL_DAY, // rarely used groups should fade away
* // The $scenarioKey format is $constraintId:<ID/hash of $situation>
* function ( $cacheMap ) use ( $scenarioKey, $constraintId, $situation ) {
* $lruCache = MapCacheLRU::newFromArray( $cacheMap ?: [], self::CACHE_SIZE );
* $result = $lruCache->get( $scenarioKey ); // triggers LRU bump if present
* if ( $result === null || $this->isScenarioResultExpired( $result ) ) {
* $result = $this->checkScenarioTolerability( $constraintId, $situation );
* $lruCache->set( $scenarioKey, $result, 3 / 8 );
* }
* // Save the new LRU cache map and reset the map's TTL
* return $lruCache->toArray();
* },
* [
* // Once map is > 1 sec old, consider refreshing
* 'ageNew' => 1,
* // Update within 5 seconds after "ageNew" given a 1hz cache check rate
* 'hotTTR' => 5,
* // Avoid querying cache servers multiple times in a request; this also means
* // that a request can only alter the value of any given constraint key once
* 'pcTTL' => WANObjectCache::TTL_PROC_LONG
* ]
* );
* $tolerability = isset( $catSituationTolerabilityCache[$scenarioKey] )
* ? $catSituationTolerabilityCache[$scenarioKey]
* : $this->checkScenarioTolerability( $constraintId, $situation );
* @endcode
*
* @see WANObjectCache::get()
* @see WANObjectCache::set()
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param int $ttl Nominal seconds-to-live for newly computed values. Special values are:
* - WANObjectCache::TTL_INDEFINITE: Cache forever (subject to LRU-style evictions)
* - WANObjectCache::TTL_UNCACHEABLE: Do not cache (if the key exists, it is not deleted)
* @param callable $callback Value generation function
* @param array $opts Options map:
* - checkKeys: List of "check" keys. The key at $key will be seen as stale when either
* touchCheckKey() or resetCheckKey() is called on any of the keys in this list. This
* is useful if thousands or millions of keys depend on the same entity. The entity can
* simply have its "check" key updated whenever the entity is modified.
* Default: [].
* - graceTTL: If the key is stale due to a purge (by "checkKeys" or "touchedCallback")
* less than this many seconds ago, consider reusing the stale value. The odds of a
* refresh become more likely over time, becoming certain once the grace period is
* reached. This can reduce traffic spikes when millions of keys are compared to the
* same "check" key and touchCheckKey() or resetCheckKey() is called on that "check" key.
* This option is not useful for avoiding traffic spikes in the case of the key simply
* expiring on account of its TTL (use "lowTTL" instead).
* Default: WANObjectCache::GRACE_TTL_NONE.
* - lockTSE: If the value is stale and the "time since expiry" (TSE) is less than the given
* number of seconds ago, then reuse the stale value if another such thread is already
* regenerating the value. The TSE of the key is influenced by purges (e.g. via delete(),
* "checkKeys", "touchedCallback"), and various other options (e.g. "staleTTL"). A low
* enough TSE is assumed to indicate a high enough key access rate to justify stampede
* avoidance. Note that no cache value exists after deletion, expiration, or eviction
* at the storage-layer; to prevent stampedes during these cases, use "busyValue".
* Default: WANObjectCache::TSE_NONE.
* - busyValue: Specify a placeholder value to use when no value exists and another thread
* is currently regenerating it. This assures that cache stampedes cannot happen if the
* value falls out of cache. This also mitigates stampedes when value regeneration
* becomes very slow (greater than $ttl/"lowTTL"). If this is a closure, then it will
* be invoked to get the placeholder when needed.
* Default: null.
* - pcTTL: Process cache the value in this PHP instance for this many seconds. This avoids
* network I/O when a key is read several times. This will not cache when the callback
* returns false, however. Note that any purges will not be seen while process cached;
* since the callback should use replica DBs and they may be lagged or have snapshot
* isolation anyway, this should not typically matter.
* Default: WANObjectCache::TTL_UNCACHEABLE.
* - pcGroup: Process cache group to use instead of the primary one. If set, this must be
* of the format ALPHANUMERIC_NAME:MAX_KEY_SIZE, e.g. "mydata:10". Use this for storing
* large values, small yet numerous values, or some values with a high cost of eviction.
* It is generally preferable to use a class constant when setting this value.
* This has no effect unless pcTTL is used.
* Default: WANObjectCache::PC_PRIMARY.
* - version: Integer version number. This lets callers make breaking changes to the format
* of cached values without causing problems for sites that use non-instantaneous code
* deployments. Old and new code will recognize incompatible versions and purges from
* both old and new code will been seen by each other. When this method encounters an
* incompatibly versioned value at the provided key, a "variant key" will be used for
* reading from and saving to cache. The variant key is specific to the key and version
* number provided to this method. If the variant key value is older than that of the
* provided key, or the provided key is non-existant, then the variant key will be seen
* as non-existant. Therefore, delete() calls purge the provided key's variant keys.
* The "checkKeys" and "touchedCallback" options still apply to variant keys as usual.
* Avoid storing class objects, as this reduces compatibility (due to serialization).
* Default: null.
* - minAsOf: Reject values if they were generated before this UNIX timestamp.
* This is useful if the source of a key is suspected of having possibly changed
* recently, and the caller wants any such changes to be reflected.
* Default: WANObjectCache::MIN_TIMESTAMP_NONE.
* - hotTTR: Expected time-till-refresh (TTR) in seconds for keys that average ~1 hit per
* second (e.g. 1Hz). Keys with a hit rate higher than 1Hz will refresh sooner than this
* TTR and vise versa. Such refreshes won't happen until keys are "ageNew" seconds old.
* This uses randomization to avoid triggering cache stampedes. The TTR is useful at
* reducing the impact of missed cache purges, since the effect of a heavily referenced
* key being stale is worse than that of a rarely referenced key. Unlike simply lowering
* $ttl, seldomly used keys are largely unaffected by this option, which makes it
* possible to have a high hit rate for the "long-tail" of less-used keys.
* Default: WANObjectCache::HOT_TTR.
* - lowTTL: Consider pre-emptive updates when the current TTL (seconds) of the key is less
* than this. It becomes more likely over time, becoming certain once the key is expired.
* This helps avoid cache stampedes that might be triggered due to the key expiring.
* Default: WANObjectCache::LOW_TTL.
* - ageNew: Consider popularity refreshes only once a key reaches this age in seconds.
* Default: WANObjectCache::AGE_NEW.
* - staleTTL: Seconds to keep the key around if it is stale. This means that on cache
* miss the callback may get $oldValue/$oldAsOf values for keys that have already been
* expired for this specified time. This is useful if adaptiveTTL() is used on the old
* value's as-of time when it is verified as still being correct.
* Default: WANObjectCache::STALE_TTL_NONE
* - touchedCallback: A callback that takes the current value and returns a UNIX timestamp
* indicating the last time a dynamic dependency changed. Null can be returned if there
* are no relevant dependency changes to check. This can be used to check against things
* like last-modified times of files or DB timestamp fields. This should generally not be
* used for small and easily queried values in a DB if the callback itself ends up doing
* a similarly expensive DB query to check a timestamp. Usages of this option makes the
* most sense for values that are moderately to highly expensive to regenerate and easy
* to query for dependency timestamps. The use of "pcTTL" reduces timestamp queries.
* Default: null.
* @param array $cbParams Custom field/value map to pass to the callback (since 1.35)
* @phpcs:ignore Generic.Files.LineLength
* @phan-param array{checkKeys?:string[],graceTTL?:int,lockTSE?:int,busyValue?:mixed,pcTTL?:int,pcGroup?:string,version?:int,minAsOf?:float|int,hotTTR?:int,lowTTL?:int,ageNew?:int,staleTTL?:int,touchedCallback?:callable} $opts
* @return mixed Value found or written to the key
* @note Options added in 1.28: version, busyValue, hotTTR, ageNew, pcGroup, minAsOf
* @note Options added in 1.31: staleTTL, graceTTL
* @note Options added in 1.33: touchedCallback
* @note Callable type hints are not used to avoid class-autoloading
*/
final public function getWithSetCallback(
$key, $ttl, $callback, array $opts = [], array $cbParams = []
) {
$version = $opts['version'] ?? null;
$pcTTL = $opts['pcTTL'] ?? self::TTL_UNCACHEABLE;
$pCache = ( $pcTTL >= 0 )
? $this->getProcessCache( $opts['pcGroup'] ?? self::PC_PRIMARY )
: null;
// Use the process cache if requested as long as no outer cache callback is running.
// Nested callback process cache use is not lag-safe with regard to HOLDOFF_TTL since
// process cached values are more lagged than persistent ones as they are not purged.
if ( $pCache && $this->callbackDepth == 0 ) {
$cached = $pCache->get( $key, $pcTTL, false );
if ( $cached !== false ) {
$this->logger->debug( "getWithSetCallback($key): process cache hit" );
return $cached;
}
}
[ $value, $valueVersion, $curAsOf ] = $this->fetchOrRegenerate( $key, $ttl, $callback, $opts, $cbParams );
if ( $valueVersion !== $version ) {
// Current value has a different version; use the variant key for this version.
// Regenerate the variant value if it is not newer than the main value at $key
// so that purges to the main key propagate to the variant value.
$this->logger->debug( "getWithSetCallback($key): using variant key" );
[ $value ] = $this->fetchOrRegenerate(
$this->makeGlobalKey( 'WANCache-key-variant', md5( $key ), (string)$version ),
$ttl,
$callback,
[ 'version' => null, 'minAsOf' => $curAsOf ] + $opts,
$cbParams
);
}
// Update the process cache if enabled
if ( $pCache && $value !== false ) {
$pCache->set( $key, $value );
}
return $value;
}
/**
* Do the actual I/O for getWithSetCallback() when needed
*
* @see WANObjectCache::getWithSetCallback()
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param int $ttl
* @param callable $callback
* @param array $opts
* @param array $cbParams
* @return array Ordered list of the following:
* - Cached or regenerated value
* - Cached or regenerated value version number or null if not versioned
* - Timestamp of the current cached value at the key or null if there is no value
* @note Callable type hints are not used to avoid class-autoloading
*/
private function fetchOrRegenerate( $key, $ttl, $callback, array $opts, array $cbParams ) {
$checkKeys = $opts['checkKeys'] ?? [];
$graceTTL = $opts['graceTTL'] ?? self::GRACE_TTL_NONE;
$minAsOf = $opts['minAsOf'] ?? self::MIN_TIMESTAMP_NONE;
$hotTTR = $opts['hotTTR'] ?? self::HOT_TTR;
$lowTTL = $opts['lowTTL'] ?? min( self::LOW_TTL, $ttl );
$ageNew = $opts['ageNew'] ?? self::AGE_NEW;
$touchedCb = $opts['touchedCallback'] ?? null;
$startTime = $this->getCurrentTime();
$keygroup = $this->determineKeyGroupForStats( $key );
// Get the current key value and its metadata
$curState = $this->fetchKeys( [ $key ], $checkKeys, $touchedCb )[$key];
$curValue = $curState[self::RES_VALUE];
// Use the cached value if it exists and is not due for synchronous regeneration
if ( $this->isAcceptablyFreshValue( $curState, $graceTTL, $minAsOf ) ) {
if ( !$this->isLotteryRefreshDue( $curState, $lowTTL, $ageNew, $hotTTR, $startTime ) ) {
$this->stats->timing(
"wanobjectcache.$keygroup.hit.good",
1e3 * ( $this->getCurrentTime() - $startTime )
);
return [ $curValue, $curState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
} elseif ( $this->scheduleAsyncRefresh( $key, $ttl, $callback, $opts, $cbParams ) ) {
$this->logger->debug( "fetchOrRegenerate($key): hit with async refresh" );
$this->stats->timing(
"wanobjectcache.$keygroup.hit.refresh",
1e3 * ( $this->getCurrentTime() - $startTime )
);
return [ $curValue, $curState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
} else {
$this->logger->debug( "fetchOrRegenerate($key): hit with sync refresh" );
}
}
$isKeyTombstoned = ( $curState[self::RES_TOMB_AS_OF] !== null );
// Use the interim key as a temporary alternative if the key is tombstoned
if ( $isKeyTombstoned ) {
$volState = $this->getInterimValue( $key, $minAsOf, $startTime, $touchedCb );
$volValue = $volState[self::RES_VALUE];
} else {
$volState = $curState;
$volValue = $curValue;
}
// During the volatile "hold-off" period that follows a purge of the key, the value
// will be regenerated many times if frequently accessed. This is done to mitigate
// the effects of backend replication lag as soon as possible. However, throttle the
// overhead of locking and regeneration by reusing values recently written to cache
// tens of milliseconds ago. Verify the "as of" time against the last purge event.
$lastPurgeTime = max(
// RES_TOUCH_AS_OF depends on the value (possibly from the interim key)
$volState[self::RES_TOUCH_AS_OF],
$curState[self::RES_TOMB_AS_OF],
$curState[self::RES_CHECK_AS_OF]
);
$safeMinAsOf = max( $minAsOf, $lastPurgeTime + self::TINY_POSITIVE );
if ( $this->isExtremelyNewValue( $volState, $safeMinAsOf, $startTime ) ) {
$this->logger->debug( "fetchOrRegenerate($key): volatile hit" );
$this->stats->timing(
"wanobjectcache.$keygroup.hit.volatile",
1e3 * ( $this->getCurrentTime() - $startTime )
);
return [ $volValue, $volState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
}
$lockTSE = $opts['lockTSE'] ?? self::TSE_NONE;
$busyValue = $opts['busyValue'] ?? null;
$staleTTL = $opts['staleTTL'] ?? self::STALE_TTL_NONE;
$segmentable = $opts['segmentable'] ?? false;
$version = $opts['version'] ?? null;
// Determine whether one thread per datacenter should handle regeneration at a time
$useRegenerationLock =
// Note that since tombstones no-op set(), $lockTSE and $curTTL cannot be used to
// deduce the key hotness because |$curTTL| will always keep increasing until the
// tombstone expires or is overwritten by a new tombstone. Also, even if $lockTSE
// is not set, constant regeneration of a key for the tombstone lifetime might be
// very expensive. Assume tombstoned keys are possibly hot in order to reduce
// the risk of high regeneration load after the delete() method is called.
$isKeyTombstoned ||
// Assume a key is hot if requested soon ($lockTSE seconds) after purge.
// This avoids stampedes when timestamps from $checkKeys/$touchedCb bump.
(
$curState[self::RES_CUR_TTL] !== null &&
$curState[self::RES_CUR_TTL] <= 0 &&
abs( $curState[self::RES_CUR_TTL] ) <= $lockTSE
) ||
// Assume a key is hot if there is no value and a busy fallback is given.
// This avoids stampedes on eviction or preemptive regeneration taking too long.
( $busyValue !== null && $volValue === false );
// If a regeneration lock is required, threads that do not get the lock will try to use
// the stale value, the interim value, or the $busyValue placeholder, in that order. If
// none of those are set then all threads will bypass the lock and regenerate the value.
$hasLock = $useRegenerationLock && $this->claimStampedeLock( $key );
if ( $useRegenerationLock && !$hasLock ) {
// Determine if there is stale or volatile cached value that is still usable
// @phan-suppress-next-line PhanTypeMismatchArgumentNullable False positive
if ( $this->isValid( $volValue, $volState[self::RES_AS_OF], $minAsOf ) ) {
$this->logger->debug( "fetchOrRegenerate($key): returning stale value" );
$this->stats->timing(
"wanobjectcache.$keygroup.hit.stale",
1e3 * ( $this->getCurrentTime() - $startTime )
);
return [ $volValue, $volState[self::RES_VERSION], $curState[self::RES_AS_OF] ];
} elseif ( $busyValue !== null ) {
$miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
$this->logger->debug( "fetchOrRegenerate($key): busy $miss" );
$this->stats->timing(
"wanobjectcache.$keygroup.$miss.busy",
1e3 * ( $this->getCurrentTime() - $startTime )
);
$placeholderValue = $this->resolveBusyValue( $busyValue );
return [ $placeholderValue, $version, $curState[self::RES_AS_OF] ];
}
}
// Generate the new value given any prior value with a matching version
$setOpts = [];
$preCallbackTime = $this->getCurrentTime();
++$this->callbackDepth;
// https://github.com/phan/phan/issues/4419
$value = null;
try {
$value = $callback(
( $curState[self::RES_VERSION] === $version ) ? $curValue : false,
$ttl,
$setOpts,
( $curState[self::RES_VERSION] === $version ) ? $curState[self::RES_AS_OF] : null,
$cbParams
);
} finally {
--$this->callbackDepth;
}
$postCallbackTime = $this->getCurrentTime();
// How long it took to fetch, validate, and generate the value
$elapsed = max( $postCallbackTime - $startTime, 0.0 );
// How long it took to generate the value
$walltime = max( $postCallbackTime - $preCallbackTime, 0.0 );
$this->stats->timing( "wanobjectcache.$keygroup.regen_walltime", 1e3 * $walltime );
// Attempt to save the newly generated value if applicable
if (
// Callback yielded a cacheable value
( $value !== false && $ttl >= 0 ) &&
// Current thread was not raced out of a regeneration lock or key is tombstoned
( !$useRegenerationLock || $hasLock || $isKeyTombstoned )
) {
$this->stats->timing( "wanobjectcache.$keygroup.regen_set_delay", 1e3 * $elapsed );
// If the key is write-holed then use the (volatile) interim key as an alternative
if ( $isKeyTombstoned ) {
$this->setInterimValue(
$key,
$value,
$lockTSE,
$version,
$segmentable
);
} else {
$this->setMainValue(
$key,
$value,
$ttl,
$version,
$walltime,
// @phan-suppress-next-line PhanCoalescingAlwaysNull
$setOpts['lag'] ?? 0,
// @phan-suppress-next-line PhanCoalescingAlwaysNull
$setOpts['since'] ?? $preCallbackTime,
// @phan-suppress-next-line PhanCoalescingAlwaysNull
$setOpts['pending'] ?? false,
$lockTSE,
$staleTTL,
$segmentable,
( $curValue === false )
);
}
}
$this->yieldStampedeLock( $key, $hasLock );
$miss = is_infinite( $minAsOf ) ? 'renew' : 'miss';
$this->logger->debug( "fetchOrRegenerate($key): $miss, new value computed" );
$this->stats->timing(
"wanobjectcache.$keygroup.$miss.compute",
1e3 * ( $this->getCurrentTime() - $startTime )
);
return [ $value, $version, $curState[self::RES_AS_OF] ];
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @return bool Success
*/
private function claimStampedeLock( $key ) {
$checkSisterKey = $this->makeSisterKey( $key, self::TYPE_MUTEX );
// Note that locking is not bypassed due to I/O errors; this avoids stampedes
return $this->cache->add( $checkSisterKey, 1, self::LOCK_TTL );
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param bool $hasLock
*/
private function yieldStampedeLock( $key, $hasLock ) {
if ( $hasLock ) {
$checkSisterKey = $this->makeSisterKey( $key, self::TYPE_MUTEX );
$this->cache->delete( $checkSisterKey, $this->cache::WRITE_BACKGROUND );
}
}
/**
* Get sister keys that should be collocated with their corresponding base cache keys
*
* The key will bear the WANCache prefix and use the configured coalescing scheme
*
* @param string[] $baseKeys Cache keys made with makeKey()/makeGlobalKey()
* @param string $type Consistent hashing agnostic suffix character matching [a-zA-Z]
* @param string|null $route Routing prefix (optional)
* @return string[] Order-corresponding list of sister keys
*/
private function makeSisterKeys( array $baseKeys, string $type, string $route = null ) {
$sisterKeys = [];
foreach ( $baseKeys as $baseKey ) {
$sisterKeys[] = $this->makeSisterKey( $baseKey, $type, $route );
}
return $sisterKeys;
}
/**
* Get a sister key that should be collocated with a base cache key
*
* The keys will bear the WANCache prefix and use the configured coalescing scheme
*
* @param string $baseKey Cache key made with makeKey()/makeGlobalKey()
* @param string $typeChar Consistent hashing agnostic suffix character matching [a-zA-Z]
* @param string|null $route Routing prefix (optional)
* @return string Sister key
*/
private function makeSisterKey( string $baseKey, string $typeChar, string $route = null ) {
if ( $this->coalesceScheme === self::SCHEME_HASH_STOP ) {
// Key style: "WANCache:<base key>|#|<character>"
$sisterKey = 'WANCache:' . $baseKey . '|#|' . $typeChar;
} else {
// Key style: "WANCache:{<base key>}:<character>"
$sisterKey = 'WANCache:{' . $baseKey . '}:' . $typeChar;
}
if ( $route !== null ) {
$sisterKey = $this->prependRoute( $sisterKey, $route );
}
return $sisterKey;
}
/**
* Check if a key value is non-false, new enough, and has an "as of" time almost equal to now
*
* If the value was just written to cache, and it did not take an unusually long time to
* generate, then it is probably not worth regenerating yet. For example, replica databases
* might still return lagged pre-purge values anyway.
*
* @param array $res Current value WANObjectCache::RES_* data map
* @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
* @param float $now Current UNIX timestamp
* @return bool Whether the age of a volatile value is negligible
*/
private function isExtremelyNewValue( $res, $minAsOf, $now ) {
if ( $res[self::RES_VALUE] === false || $res[self::RES_AS_OF] < $minAsOf ) {
return false;
}
$age = $now - $res[self::RES_AS_OF];
return ( $age < mt_rand( self::RECENT_SET_LOW_MS, self::RECENT_SET_HIGH_MS ) / 1e3 );
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
* @param float $now Fetch time to determine "age" metadata
* @param callable|null $touchedCb Function to find the max "dependency touched" UNIX timestamp
* @return array<int,mixed> Result map/n-tuple from unwrap()
* @phan-return array{0:mixed,1:mixed,2:?float,3:?int,4:?float,5:?float,6:?float,7:?float}
* @note Callable type hints are not used to avoid class-autoloading
*/
private function getInterimValue( $key, $minAsOf, $now, $touchedCb ) {
if ( $this->useInterimHoldOffCaching ) {
$interimSisterKey = $this->makeSisterKey( $key, self::TYPE_INTERIM );
$wrapped = $this->cache->get( $interimSisterKey );
$res = $this->unwrap( $wrapped, $now );
if ( $res[self::RES_VALUE] !== false && $res[self::RES_AS_OF] >= $minAsOf ) {
if ( $touchedCb !== null ) {
// Update "last purge time" since the $touchedCb timestamp depends on $value
// Get the new "touched timestamp", accounting for callback-checked dependencies
$res[self::RES_TOUCH_AS_OF] = max(
$touchedCb( $res[self::RES_VALUE] ),
$res[self::RES_TOUCH_AS_OF]
);
}
return $res;
}
}
return $this->unwrap( false, $now );
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param mixed $value
* @param int|float $ttl
* @param int|null $version Value version number
* @param bool $segmentable
* @return bool Success
*/
private function setInterimValue(
$key,
$value,
$ttl,
?int $version,
bool $segmentable
) {
$now = $this->getCurrentTime();
$ttl = max( self::INTERIM_KEY_TTL, (int)$ttl );
// Wrap that value with time/TTL/version metadata
$wrapped = $this->wrap( $value, $ttl, $version, $now );
$flags = $this->cache::WRITE_BACKGROUND;
if ( $segmentable ) {
$flags |= $this->cache::WRITE_ALLOW_SEGMENTS;
}
return $this->cache->set(
$this->makeSisterKey( $key, self::TYPE_INTERIM ),
$wrapped,
$ttl,
$flags
);
}
/**
* @param mixed $busyValue
* @return mixed
*/
private function resolveBusyValue( $busyValue ) {
return ( $busyValue instanceof Closure ) ? $busyValue() : $busyValue;
}
/**
* Method to fetch multiple cache keys at once with regeneration
*
* This works the same as getWithSetCallback() except:
* - a) The $keys argument must be the result of WANObjectCache::makeMultiKeys()
* - b) The $callback argument expects a function that returns an entity value, using
* boolean "false" if it does not exist. The callback takes the following arguments:
* - $id: ID of the entity to query
* - $oldValue: prior cache value or false if none was present
* - &$ttl: reference to the TTL to be assigned to the new value (alterable)
* - &$setOpts: reference to the new value set() options (alterable)
* - $oldAsOf: generation UNIX timestamp of $oldValue or null if not present
* - c) The return value is a map of (cache key => value) in the order of $keyedIds
*
* @see WANObjectCache::getWithSetCallback()
* @see WANObjectCache::getMultiWithUnionSetCallback()
*
* Example usage:
* @code
* $rows = $cache->getMultiWithSetCallback(
* // Map of cache keys to entity IDs
* $cache->makeMultiKeys(
* $this->fileVersionIds(),
* function ( $id, $cache ) {
* return $cache->makeKey( 'file-version', $id );
* }
* ),
* // Time-to-live (in seconds)
* $cache::TTL_DAY,
* // Function that derives the new key value
* function ( $id, $oldValue, &$ttl, array &$setOpts ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* // Load the row for this file
* $queryInfo = File::getQueryInfo();
* $row = $dbr->selectRow(
* $queryInfo['tables'],
* $queryInfo['fields'],
* [ 'id' => $id ],
* __METHOD__,
* [],
* $queryInfo['joins']
* );
*
* return $row ? (array)$row : false;
* },
* [
* // Process cache for 30 seconds
* 'pcTTL' => 30,
* // Use a dedicated 500 item cache (initialized on-the-fly)
* 'pcGroup' => 'file-versions:500'
* ]
* );
* $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
* @endcode
*
* @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
* @param int $ttl Seconds to live for key updates
* @param callable $callback Callback that yields entity generation callbacks
* @param array $opts Options map similar to that of getWithSetCallback()
* @return mixed[] Map of (cache key => value) in the same order as $keyedIds
* @since 1.28
*/
final public function getMultiWithSetCallback(
ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
) {
// Batch load required keys into the in-process warmup cache
$this->warmupCache = $this->fetchWrappedValuesForWarmupCache(
$this->getNonProcessCachedMultiKeys( $keyedIds, $opts ),
$opts['checkKeys'] ?? []
);
$this->warmupKeyMisses = 0;
// The required callback signature includes $id as the first argument for convenience
// to distinguish different items. To reuse the code in getWithSetCallback(), wrap the
// callback with a proxy callback that has the standard getWithSetCallback() signature.
// This is defined only once per batch to avoid closure creation overhead.
$proxyCb = static function ( $oldValue, &$ttl, &$setOpts, $oldAsOf, $params )
use ( $callback )
{
return $callback( $params['id'], $oldValue, $ttl, $setOpts, $oldAsOf );
};
// Get the order-preserved result map using the warm-up cache
$values = [];
foreach ( $keyedIds as $key => $id ) {
$values[$key] = $this->getWithSetCallback(
$key,
$ttl,
$proxyCb,
$opts,
[ 'id' => $id ]
);
}
$this->warmupCache = [];
return $values;
}
/**
* Method to fetch/regenerate multiple cache keys at once
*
* This works the same as getWithSetCallback() except:
* - a) The $keys argument expects the result of WANObjectCache::makeMultiKeys()
* - b) The $callback argument expects a function that returns a map of (ID => new value),
* using boolean "false" for entities that could not be found, for all entity IDs in
* $ids. The callback takes the following arguments:
* - $ids: list of entity IDs that require value generation
* - &$ttls: reference to the (entity ID => new TTL) map (alterable)
* - &$setOpts: reference to the new value set() options (alterable)
* - c) The return value is a map of (cache key => value) in the order of $keyedIds
* - d) The "lockTSE" and "busyValue" options are ignored
*
* @see WANObjectCache::getWithSetCallback()
* @see WANObjectCache::getMultiWithSetCallback()
*
* Example usage:
* @code
* $rows = $cache->getMultiWithUnionSetCallback(
* // Map of cache keys to entity IDs
* $cache->makeMultiKeys(
* $this->fileVersionIds(),
* function ( $id ) use ( $cache ) {
* return $cache->makeKey( 'file-version', $id );
* }
* ),
* // Time-to-live (in seconds)
* $cache::TTL_DAY,
* // Function that derives the new key value
* function ( array $ids, array &$ttls, array &$setOpts ) {
* $dbr = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
* // Account for any snapshot/replica DB lag
* $setOpts += Database::getCacheSetOptions( $dbr );
*
* // Load the rows for these files
* $rows = array_fill_keys( $ids, false );
* $queryInfo = File::getQueryInfo();
* $res = $dbr->select(
* $queryInfo['tables'],
* $queryInfo['fields'],
* [ 'id' => $ids ],
* __METHOD__,
* [],
* $queryInfo['joins']
* );
* foreach ( $res as $row ) {
* $rows[$row->id] = $row;
* $mtime = wfTimestamp( TS_UNIX, $row->timestamp );
* $ttls[$row->id] = $this->adaptiveTTL( $mtime, $ttls[$row->id] );
* }
*
* return $rows;
* },
* ]
* );
* $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
* @endcode
*
* @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
* @param int $ttl Seconds to live for key updates
* @param callable $callback Callback that yields entity generation callbacks
* @param array $opts Options map similar to that of getWithSetCallback()
* @return mixed[] Map of (cache key => value) in the same order as $keyedIds
* @since 1.30
*/
final public function getMultiWithUnionSetCallback(
ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
) {
$checkKeys = $opts['checkKeys'] ?? [];
$minAsOf = $opts['minAsOf'] ?? self::MIN_TIMESTAMP_NONE;
// unset incompatible keys
unset( $opts['lockTSE'] );
unset( $opts['busyValue'] );
// Batch load required keys into the in-process warmup cache
$keysByIdGet = $this->getNonProcessCachedMultiKeys( $keyedIds, $opts );
$this->warmupCache = $this->fetchWrappedValuesForWarmupCache( $keysByIdGet, $checkKeys );
$this->warmupKeyMisses = 0;
// IDs of entities known to be in need of generation
$idsRegen = [];
// Find out which keys are missing/deleted/stale
$resByKey = $this->fetchKeys( $keysByIdGet, $checkKeys );
foreach ( $keysByIdGet as $id => $key ) {
$res = $resByKey[$key];
if (
$res[self::RES_VALUE] === false ||
$res[self::RES_CUR_TTL] < 0 ||
$res[self::RES_AS_OF] < $minAsOf
) {
$idsRegen[] = $id;
}
}
// Run the callback to populate the generation value map for all required IDs
$newSetOpts = [];
$newTTLsById = array_fill_keys( $idsRegen, $ttl );
$newValsById = $idsRegen ? $callback( $idsRegen, $newTTLsById, $newSetOpts ) : [];
$method = __METHOD__;
// The required callback signature includes $id as the first argument for convenience
// to distinguish different items. To reuse the code in getWithSetCallback(), wrap the
// callback with a proxy callback that has the standard getWithSetCallback() signature.
// This is defined only once per batch to avoid closure creation overhead.
$proxyCb = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf, $params )
use ( $callback, $newValsById, $newTTLsById, $newSetOpts, $method )
{
$id = $params['id'];
if ( array_key_exists( $id, $newValsById ) ) {
// Value was already regenerated as expected, so use the value in $newValsById
$newValue = $newValsById[$id];
$ttl = $newTTLsById[$id];
$setOpts = $newSetOpts;
} else {
// Pre-emptive/popularity refresh and version mismatch cases are not detected
// above and thus $newValsById has no entry. Run $callback on this single entity.
$ttls = [ $id => $ttl ];
$result = $callback( [ $id ], $ttls, $setOpts );
if ( !isset( $result[$id] ) ) {
// T303092
$this->logger->warning(
$method . ' failed due to {id} not set in result {result}', [
'id' => $id,
'result' => json_encode( $result )
] );
}
$newValue = $result[$id];
$ttl = $ttls[$id];
}
return $newValue;
};
// Get the order-preserved result map using the warm-up cache
$values = [];
foreach ( $keyedIds as $key => $id ) {
$values[$key] = $this->getWithSetCallback(
$key,
$ttl,
$proxyCb,
$opts,
[ 'id' => $id ]
);
}
$this->warmupCache = [];
return $values;
}
/**
* @see BagOStuff::makeGlobalKey()
* @since 1.27
* @param string $keygroup Key group component, should be under 48 characters.
* @param string|int ...$components Additional, ordered, key components for entity IDs
* @return string Colon-separated, keyspace-prepended, ordered list of encoded components
*/
public function makeGlobalKey( $keygroup, ...$components ) {
// @phan-suppress-next-line PhanParamTooFewUnpack Should infer non-emptiness
return $this->cache->makeGlobalKey( ...func_get_args() );
}
/**
* @see BagOStuff::makeKey()
* @since 1.27
* @param string $keygroup Key group component, should be under 48 characters.
* @param string|int ...$components Additional, ordered, key components for entity IDs
* @return string Colon-separated, keyspace-prepended, ordered list of encoded components
*/
public function makeKey( $keygroup, ...$components ) {
// @phan-suppress-next-line PhanParamTooFewUnpack Should infer non-emptiness
return $this->cache->makeKey( ...func_get_args() );
}
/**
* Hash a possibly long string into a suitable component for makeKey()/makeGlobalKey()
*
* @param string $component A raw component used in building a cache key
* @return string 64 character HMAC using a stable secret for public collision resistance
* @since 1.34
*/
public function hash256( $component ) {
return hash_hmac( 'sha256', $component, $this->secret );
}
/**
* Get an iterator of (cache key => entity ID) for a list of entity IDs
*
* The $callback argument expects a function that returns the key for an entity ID via
* makeKey()/makeGlobalKey(). There should be no network nor filesystem I/O used in the
* callback. The entity ID/key mapping must be 1:1 or an exception will be thrown. Use
* the hash256() method for any hashing. The callback takes the following arguments:
* - $id: An entity ID
* - $cache: This WANObjectCache instance
*
* Example usage for the default keyspace:
* @code
* $keyedIds = $cache->makeMultiKeys(
* $modules,
* function ( $module, $cache ) {
* return $cache->makeKey( 'example-module', $module );
* }
* );
* @endcode
*
* Example usage for mixed default and global keyspace:
* @code
* $keyedIds = $cache->makeMultiKeys(
* $filters,
* function ( $filter, $cache ) {
* return self::isCentral( $filter )
* ? $cache->makeGlobalKey( 'example-filter', $filter )
* : $cache->makeKey( 'example-filter', $filter )
* }
* );
* @endcode
*
* Example usage with hashing:
* @code
* $keyedIds = $cache->makeMultiKeys(
* $urls,
* function ( $url, $cache ) {
* return $cache->makeKey( 'example-url', $cache->hash256( $url ) );
* }
* );
* @endcode
*
* @see WANObjectCache::makeKey()
* @see WANObjectCache::makeGlobalKey()
* @see WANObjectCache::hash256()
*
* @param string[]|int[] $ids List of entity IDs
* @param callable $keyCallback Function returning makeKey()/makeGlobalKey() on the input ID
* @return ArrayIterator Iterator of (cache key => ID); order of $ids is preserved
* @since 1.28
*/
final public function makeMultiKeys( array $ids, $keyCallback ) {
$idByKey = [];
foreach ( $ids as $id ) {
// Discourage triggering of automatic makeKey() hashing in some backends
if ( strlen( $id ) > 64 ) {
$this->logger->warning( __METHOD__ . ": long ID '$id'; use hash256()" );
}
$key = $keyCallback( $id, $this );
// Edge case: ignore key collisions due to duplicate $ids like "42" and 42
if ( !isset( $idByKey[$key] ) ) {
$idByKey[$key] = $id;
} elseif ( (string)$id !== (string)$idByKey[$key] ) {
throw new UnexpectedValueException(
"Cache key collision; IDs ('$id','{$idByKey[$key]}') map to '$key'"
);
}
}
return new ArrayIterator( $idByKey );
}
/**
* Get an (ID => value) map from (i) a non-unique list of entity IDs, and (ii) the list
* of corresponding entity values by first appearance of each ID in the entity ID list
*
* For use with getMultiWithSetCallback() and getMultiWithUnionSetCallback().
*
* *Only* use this method if the entity ID/key mapping is trivially 1:1 without exception.
* Key generation method must utilize the *full* entity ID in the key (not a hash of it).
*
* Example usage:
* @code
* $poems = $cache->getMultiWithSetCallback(
* $cache->makeMultiKeys(
* $uuids,
* function ( $uuid ) use ( $cache ) {
* return $cache->makeKey( 'poem', $uuid );
* }
* ),
* $cache::TTL_DAY,
* function ( $uuid ) use ( $url ) {
* return $this->http->run( [ 'method' => 'GET', 'url' => "$url/$uuid" ] );
* }
* );
* $poemsByUUID = $cache->multiRemap( $uuids, $poems );
* @endcode
*
* @see WANObjectCache::makeMultiKeys()
* @see WANObjectCache::getMultiWithSetCallback()
* @see WANObjectCache::getMultiWithUnionSetCallback()
*
* @param string[]|int[] $ids Entity ID list makeMultiKeys()
* @param mixed[] $res Result of getMultiWithSetCallback()/getMultiWithUnionSetCallback()
* @return mixed[] Map of (ID => value); order of $ids is preserved
* @since 1.34
*/
final public function multiRemap( array $ids, array $res ) {
if ( count( $ids ) !== count( $res ) ) {
// If makeMultiKeys() is called on a list of non-unique IDs, then the resulting
// ArrayIterator will have less entries due to "first appearance" de-duplication
$ids = array_keys( array_fill_keys( $ids, true ) );
if ( count( $ids ) !== count( $res ) ) {
throw new UnexpectedValueException( "Multi-key result does not match ID list" );
}
}
return array_combine( $ids, $res );
}
/**
* Get a "watch point" token that can be used to get the "last error" to occur after now
*
* @return int A token that the current error event
* @since 1.38
*/
public function watchErrors() {
return $this->cache->watchErrors();
}
/**
* Get the "last error" registry
*
* The method should be invoked by a caller as part of the following pattern:
* - The caller invokes watchErrors() to get a "since token"
* - The caller invokes a sequence of cache operation methods
* - The caller invokes getLastError() with the "since token"
*
* External callers can also invoke this method as part of the following pattern:
* - The caller invokes clearLastError()
* - The caller invokes a sequence of cache operation methods
* - The caller invokes getLastError()
*
* @param int $watchPoint Only consider errors from after this "watch point" [optional]
* @return int BagOStuff:ERR_* constant for the "last error" registry
* @note Parameters added in 1.38: $watchPoint
*/
final public function getLastError( $watchPoint = 0 ) {
$code = $this->cache->getLastError( $watchPoint );
switch ( $code ) {
case self::ERR_NONE:
return self::ERR_NONE;
case self::ERR_NO_RESPONSE:
return self::ERR_NO_RESPONSE;
case self::ERR_UNREACHABLE:
return self::ERR_UNREACHABLE;
default:
return self::ERR_UNEXPECTED;
}
}
/**
* Clear the "last error" registry
* @deprecated Since 1.38
*/
final public function clearLastError() {
$this->cache->clearLastError();
}
/**
* Clear the in-process caches; useful for testing
*
* @since 1.27
*/
public function clearProcessCache() {
$this->processCaches = [];
}
/**
* Enable or disable the use of brief caching for tombstoned keys
*
* When a key is purged via delete(), there normally is a period where caching
* is hold-off limited to an extremely short time. This method will disable that
* caching, forcing the callback to run for any of:
* - WANObjectCache::getWithSetCallback()
* - WANObjectCache::getMultiWithSetCallback()
* - WANObjectCache::getMultiWithUnionSetCallback()
*
* This is useful when both:
* - a) the database used by the callback is known to be up-to-date enough
* for some particular purpose (e.g. replica DB has applied transaction X)
* - b) the caller needs to exploit that fact, and therefore needs to avoid the
* use of inherently volatile and possibly stale interim keys
*
* @see WANObjectCache::delete()
* @param bool $enabled Whether to enable interim caching
* @since 1.31
*/
final public function useInterimHoldOffCaching( $enabled ) {
$this->useInterimHoldOffCaching = $enabled;
}
/**
* @param int $flag ATTR_* class constant
* @return int QOS_* class constant
* @since 1.28
*/
public function getQoS( $flag ) {
return $this->cache->getQoS( $flag );
}
/**
* Get a TTL that is higher for objects that have not changed recently
*
* This is useful for keys that get explicit purges and DB or purge relay
* lag is a potential concern (especially how it interacts with CDN cache)
*
* Example usage:
* @code
* // Last-modified time of page
* $mtime = wfTimestamp( TS_UNIX, $page->getTimestamp() );
* // Get adjusted TTL. If $mtime is 3600 seconds ago and $minTTL/$factor left at
* // defaults, then $ttl is 3600 * .2 = 720. If $minTTL was greater than 720, then
* // $ttl would be $minTTL. If $maxTTL was smaller than 720, $ttl would be $maxTTL.
* $ttl = $cache->adaptiveTTL( $mtime, $cache::TTL_DAY );
* @endcode
*
* Another use case is when there are no applicable "last modified" fields in the DB,
* and there are too many dependencies for explicit purges to be viable, and the rate of
* change to relevant content is unstable, and it is highly valued to have the cached value
* be as up-to-date as possible.
*
* Example usage:
* @code
* $query = "<some complex query>";
* $idListFromComplexQuery = $cache->getWithSetCallback(
* $cache->makeKey( 'complex-graph-query', $hashOfQuery ),
* GraphQueryClass::STARTING_TTL,
* function ( $oldValue, &$ttl, array &$setOpts, $oldAsOf ) use ( $query, $cache ) {
* $gdb = $this->getReplicaGraphDbConnection();
* // Account for any snapshot/replica DB lag
* $setOpts += GraphDatabase::getCacheSetOptions( $gdb );
*
* $newList = iterator_to_array( $gdb->query( $query ) );
* sort( $newList, SORT_NUMERIC ); // normalize
*
* $minTTL = GraphQueryClass::MIN_TTL;
* $maxTTL = GraphQueryClass::MAX_TTL;
* if ( $oldValue !== false ) {
* // Note that $oldAsOf is the last time this callback ran
* $ttl = ( $newList === $oldValue )
* // No change: cache for 150% of the age of $oldValue
* ? $cache->adaptiveTTL( $oldAsOf, $maxTTL, $minTTL, 1.5 )
* // Changed: cache for 50% of the age of $oldValue
* : $cache->adaptiveTTL( $oldAsOf, $maxTTL, $minTTL, .5 );
* }
*
* return $newList;
* },
* [
* // Keep stale values around for doing comparisons for TTL calculations.
* // High values improve long-tail keys hit-rates, though might waste space.
* 'staleTTL' => GraphQueryClass::GRACE_TTL
* ]
* );
* @endcode
*
* @param int|float|string|null $mtime UNIX timestamp; null if none
* @param int $maxTTL Maximum TTL (seconds)
* @param int $minTTL Minimum TTL (seconds); Default: 30
* @param float $factor Value in the range (0,1); Default: .2
* @return int Adaptive TTL
* @since 1.28
*/
public function adaptiveTTL( $mtime, $maxTTL, $minTTL = 30, $factor = 0.2 ) {
// handle fractional seconds and string integers
$mtime = (int)$mtime;
if ( $mtime <= 0 ) {
// no last-modified time provided
return $minTTL;
}
$age = (int)$this->getCurrentTime() - $mtime;
return (int)min( $maxTTL, max( $minTTL, $factor * $age ) );
}
/**
* @internal For use by unit tests only
* @return int
* @since 1.30
*/
final public function getWarmupKeyMisses() {
// Number of misses in $this->warmupCache during the last call to certain methods
return $this->warmupKeyMisses;
}
/**
* Set a sister key to a purge value in all datacenters
*
* This method should not wait for the operation to complete on remote datacenters
*
* Since older purge values can sometimes arrive after newer ones, use a relative expiry
* so that even if the older value replaces the newer value, the TTL will greater than the
* remaining TTL on the older value (assuming that all purges for a key use the same TTL).
*
* @param string $sisterKey A value key or "check" key
* @param string $purgeValue Result of makeTombstonePurgeValue()/makeCheckKeyPurgeValue()
* @param int $ttl Seconds to keep the purge value around
* @return bool Success
*/
protected function relayVolatilePurge( string $sisterKey, string $purgeValue, int $ttl ) {
if ( $this->broadcastRoute !== null ) {
$routeKey = $this->prependRoute( $sisterKey, $this->broadcastRoute );
} else {
$routeKey = $sisterKey;
}
return $this->cache->set(
$routeKey,
$purgeValue,
$ttl,
$this->cache::WRITE_BACKGROUND
);
}
/**
* Remove a sister key from all datacenters
*
* This method should not wait for the operation to complete on remote datacenters
*
* @param string $sisterKey A value key or "check" key
* @return bool Success
*/
protected function relayNonVolatilePurge( string $sisterKey ) {
if ( $this->broadcastRoute !== null ) {
$routeKey = $this->prependRoute( $sisterKey, $this->broadcastRoute );
} else {
$routeKey = $sisterKey;
}
return $this->cache->delete( $routeKey, $this->cache::WRITE_BACKGROUND );
}
/**
* @param string $sisterKey
* @param string $route Key routing prefix
* @return string
*/
protected function prependRoute( string $sisterKey, string $route ) {
if ( $sisterKey[0] === '/' ) {
throw new RuntimeException( "Sister key '$sisterKey' already contains a route." );
}
return $route . $sisterKey;
}
/**
* Schedule a deferred cache regeneration if possible
*
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param int $ttl Seconds to live
* @param callable $callback
* @param array $opts
* @param array $cbParams
* @return bool Success
* @note Callable type hints are not used to avoid class-autoloading
*/
private function scheduleAsyncRefresh( $key, $ttl, $callback, array $opts, array $cbParams ) {
if ( !$this->asyncHandler ) {
return false;
}
// Update the cache value later, such during post-send of an HTTP request. This forces
// cache regeneration by setting "minAsOf" to infinity, meaning that no existing value
// is considered valid. Furthermore, note that preemptive regeneration is not applicable
// to invalid values, so there is no risk of infinite preemptive regeneration loops.
$func = $this->asyncHandler;
$func( function () use ( $key, $ttl, $callback, $opts, $cbParams ) {
$opts['minAsOf'] = INF;
try {
$this->fetchOrRegenerate( $key, $ttl, $callback, $opts, $cbParams );
} catch ( Exception $e ) {
// Log some context for easier debugging
$this->logger->error( 'Async refresh failed for {key}', [
'key' => $key,
'ttl' => $ttl,
'exception' => $e
] );
throw $e;
}
} );
return true;
}
/**
* Check if a key value is non-false, new enough, and either fresh or "gracefully" stale
*
* @param array $res Current value WANObjectCache::RES_* data map
* @param int $graceTTL Consider using stale values if $curTTL is greater than this
* @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
* @return bool
*/
private function isAcceptablyFreshValue( $res, $graceTTL, $minAsOf ) {
if ( !$this->isValid( $res[self::RES_VALUE], $res[self::RES_AS_OF], $minAsOf ) ) {
// Value does not exists or is too old
return false;
}
$curTTL = $res[self::RES_CUR_TTL];
if ( $curTTL > 0 ) {
// Value is definitely still fresh
return true;
}
// Remaining seconds during which this stale value can be used
$curGraceTTL = $graceTTL + $curTTL;
return ( $curGraceTTL > 0 )
// Chance of using the value decreases as $curTTL goes from 0 to -$graceTTL
? !$this->worthRefreshExpiring( $curGraceTTL, $graceTTL, $graceTTL )
// Value is too stale to fall in the grace period
: false;
}
/**
* Check if a key is due for randomized regeneration due to near-expiration/popularity
*
* @param array $res Current value WANObjectCache::RES_* data map
* @param float $lowTTL Consider a refresh when $curTTL is less than this; the "low" threshold
* @param int $ageNew Age of key when this might recommend refreshing (seconds)
* @param int $hotTTR Age of key when it should be refreshed if popular (seconds)
* @param float $now The current UNIX timestamp
* @return bool
*/
protected function isLotteryRefreshDue( $res, $lowTTL, $ageNew, $hotTTR, $now ) {
$curTTL = $res[self::RES_CUR_TTL];
$logicalTTL = $res[self::RES_TTL];
$asOf = $res[self::RES_AS_OF];
return (
$this->worthRefreshExpiring( $curTTL, $logicalTTL, $lowTTL ) ||
$this->worthRefreshPopular( $asOf, $ageNew, $hotTTR, $now )
);
}
/**
* Check if a key is due for randomized regeneration due to its popularity
*
* This is used so that popular keys can preemptively refresh themselves for higher
* consistency (especially in the case of purge loss/delay). Unpopular keys can remain
* in cache with their high nominal TTL. This means popular keys keep good consistency,
* whether the data changes frequently or not, and long-tail keys get to stay in cache
* and get hits too. Similar to worthRefreshExpiring(), randomization is used.
*
* @param float $asOf UNIX timestamp of the value
* @param int $ageNew Age of key when this might recommend refreshing (seconds)
* @param int $timeTillRefresh Age of key when it should be refreshed if popular (seconds)
* @param float $now The current UNIX timestamp
* @return bool
*/
protected function worthRefreshPopular( $asOf, $ageNew, $timeTillRefresh, $now ) {
if ( $ageNew < 0 || $timeTillRefresh <= 0 ) {
return false;
}
$age = $now - $asOf;
$timeOld = $age - $ageNew;
if ( $timeOld <= 0 ) {
return false;
}
$popularHitsPerSec = 1;
// Lifecycle is: new, ramp-up refresh chance, full refresh chance.
// Note that the "expected # of refreshes" for the ramp-up time range is half
// of what it would be if P(refresh) was at its full value during that time range.
$refreshWindowSec = max( $timeTillRefresh - $ageNew - self::RAMPUP_TTL / 2, 1 );
// P(refresh) * (# hits in $refreshWindowSec) = (expected # of refreshes)
// P(refresh) * ($refreshWindowSec * $popularHitsPerSec) = 1 (by definition)
// P(refresh) = 1/($refreshWindowSec * $popularHitsPerSec)
$chance = 1 / ( $popularHitsPerSec * $refreshWindowSec );
// Ramp up $chance from 0 to its nominal value over RAMPUP_TTL seconds to avoid stampedes
$chance *= ( $timeOld <= self::RAMPUP_TTL ) ? $timeOld / self::RAMPUP_TTL : 1;
return ( mt_rand( 1, 1_000_000_000 ) <= 1_000_000_000 * $chance );
}
/**
* Check if a key is nearing expiration and thus due for randomized regeneration
*
* If $curTTL is greater than the "low" threshold (e.g. not nearing expiration) then this
* returns false. If $curTTL <= 0 (e.g. value already expired), then this returns false.
* Otherwise, the chance of this returning true increases steadily from 0% to 100% as
* $curTTL moves from the "low" threshold down to 0 seconds.
*
* The logical TTL will be used as the "low" threshold if it is less than $lowTTL.
*
* This method uses deadline-aware randomization in order to handle wide variations
* of cache access traffic without the need for configuration or expensive state.
*
* @param float $curTTL Approximate TTL left on the key
* @param float $logicalTTL Full logical TTL assigned to the key; 0 for "infinite"
* @param float $lowTTL Consider a refresh when $curTTL is less than this; the "low" threshold
* @return bool
*/
protected function worthRefreshExpiring( $curTTL, $logicalTTL, $lowTTL ) {
if ( $lowTTL <= 0 ) {
return false;
}
// T264787: avoid having keys start off with a high chance of being refreshed;
// the point where refreshing becomes possible cannot precede the key lifetime.
$effectiveLowTTL = min( $lowTTL, $logicalTTL ?: INF );
// How long the value was in the "low TTL" phase
$timeOld = $effectiveLowTTL - $curTTL;
if ( $timeOld <= 0 || $timeOld >= $effectiveLowTTL ) {
return false;
}
// Ratio of the low TTL phase that has elapsed (r)
$ttrRatio = $timeOld / $effectiveLowTTL;
// Use p(r) as the monotonically increasing "chance of refresh" function,
// having p(0)=0 and p(1)=1. The value expires at the nominal expiry.
$chance = $ttrRatio ** 4;
return ( mt_rand( 1, 1_000_000_000 ) <= 1_000_000_000 * $chance );
}
/**
* Check that a wrapper value exists and has an acceptable age
*
* @param array|false $value Value wrapper or false
* @param float $asOf Value generation "as of" timestamp
* @param float $minAsOf Minimum acceptable value "as of" UNIX timestamp
* @return bool
*/
protected function isValid( $value, $asOf, $minAsOf ) {
return ( $value !== false && $asOf >= $minAsOf );
}
/**
* @param mixed $value
* @param int $ttl Seconds to live or zero for "indefinite"
* @param int|null $version Value version number or null if not versioned
* @param float $now Unix Current timestamp just before calling set()
* @return array
*/
private function wrap( $value, $ttl, $version, $now ) {
// Returns keys in ascending integer order for PHP7 array packing:
// https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implementation.html
$wrapped = [
self::FLD_FORMAT_VERSION => self::VERSION,
self::FLD_VALUE => $value,
self::FLD_TTL => $ttl,
self::FLD_TIME => $now
];
if ( $version !== null ) {
$wrapped[self::FLD_VALUE_VERSION] = $version;
}
return $wrapped;
}
/**
* @param array|string|false $wrapped The entry at a cache key (false if key is nonexistant)
* @param float $now Unix Current timestamp (preferably pre-query)
* @return array<int,mixed> Result map/n-tuple that includes the following:
* - WANObjectCache::RES_VALUE: value or false if absent/tombstoned/malformed
* - WANObjectCache::KEY_VERSION: value version number; null if there is no value
* - WANObjectCache::KEY_AS_OF: value generation timestamp (UNIX); null if there is no value
* - WANObjectCache::KEY_TTL: assigned logical TTL (seconds); null if there is no value
* - WANObjectCache::KEY_TOMB_AS_OF: tombstone timestamp (UNIX); null if not tombstoned
* - WANObjectCache::RES_CHECK_AS_OF: null placeholder for highest "check" key timestamp
* - WANObjectCache::RES_TOUCH_AS_OF: null placeholder for highest "touched" timestamp
* - WANObjectCache::KEY_CUR_TTL: remaining logical TTL (seconds) (negative if tombstoned)
* @phan-return array{0:mixed,1:mixed,2:?float,3:?int,4:?float,5:?float,6:?float,7:?float}
*/
private function unwrap( $wrapped, $now ) {
// https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implementation.html
$res = [
// Attributes that only depend on the fetched key value
self::RES_VALUE => false,
self::RES_VERSION => null,
self::RES_AS_OF => null,
self::RES_TTL => null,
self::RES_TOMB_AS_OF => null,
// Attributes that depend on caller-specific "check" keys or "touched callbacks"
self::RES_CHECK_AS_OF => null,
self::RES_TOUCH_AS_OF => null,
self::RES_CUR_TTL => null
];
if ( is_array( $wrapped ) ) {
// Entry expected to be a cached value; validate it
if (
( $wrapped[self::FLD_FORMAT_VERSION] ?? null ) === self::VERSION &&
$wrapped[self::FLD_TIME] >= $this->epoch
) {
if ( $wrapped[self::FLD_TTL] > 0 ) {
// Get the approximate time left on the key
$age = $now - $wrapped[self::FLD_TIME];
$curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
} else {
// Key had no TTL, so the time left is unbounded
$curTTL = INF;
}
$res[self::RES_VALUE] = $wrapped[self::FLD_VALUE];
$res[self::RES_VERSION] = $wrapped[self::FLD_VALUE_VERSION] ?? null;
$res[self::RES_AS_OF] = $wrapped[self::FLD_TIME];
$res[self::RES_CUR_TTL] = $curTTL;
$res[self::RES_TTL] = $wrapped[self::FLD_TTL];
}
} else {
// Entry expected to be a tombstone; parse it
$purge = $this->parsePurgeValue( $wrapped );
if ( $purge !== null ) {
// Tombstoned keys should always have a negative "current TTL"
$curTTL = min( $purge[self::PURGE_TIME] - $now, self::TINY_NEGATIVE );
$res[self::RES_CUR_TTL] = $curTTL;
$res[self::RES_TOMB_AS_OF] = $purge[self::PURGE_TIME];
}
}
return $res;
}
/**
* @param string $key Cache key in the format `<keyspace>:<keygroup>[:<other components>]...`
* as formatted by WANObjectCache::makeKey() or ::makeKeyGlobal.
* @return string The key group of this cache key
*/
private function determineKeyGroupForStats( $key ) {
$parts = explode( ':', $key, 3 );
// Fallback in case the key was not made by makeKey.
// Replace dots because they are special in StatsD (T232907)
return strtr( $parts[1] ?? $parts[0], '.', '_' );
}
/**
* Extract purge metadata from cached value if it is a valid purge value
*
* Valid purge values come from makeTombstonePurgeValue()/makeCheckKeyPurgeValue()
*
* @param mixed $value Cached value
* @return array|null Tuple of (UNIX timestamp, hold-off seconds); null if value is invalid
*/
private function parsePurgeValue( $value ) {
if ( !is_string( $value ) ) {
return null;
}
$segments = explode( ':', $value, 3 );
$prefix = $segments[0];
if ( $prefix !== self::PURGE_VAL_PREFIX ) {
// Not a purge value
return null;
}
$timestamp = (float)$segments[1];
// makeTombstonePurgeValue() doesn't store hold-off TTLs
$holdoff = isset( $segments[2] ) ? (int)$segments[2] : self::HOLDOFF_TTL;
if ( $timestamp < $this->epoch ) {
// Purge value is too old
return null;
}
return [ self::PURGE_TIME => $timestamp, self::PURGE_HOLDOFF => $holdoff ];
}
/**
* @param float $timestamp UNIX timestamp
* @return string Wrapped purge value; format is "PURGED:<timestamp>"
*/
private function makeTombstonePurgeValue( float $timestamp ) {
return self::PURGE_VAL_PREFIX . ':' . (int)$timestamp;
}
/**
* @param float $timestamp UNIX timestamp
* @param int $holdoff In seconds
* @param array|null &$purge Unwrapped purge value array [returned]
* @return string Wrapped purge value; format is "PURGED:<timestamp>:<holdoff>"
*/
private function makeCheckPurgeValue( float $timestamp, int $holdoff, array &$purge = null ) {
$normalizedTime = (int)$timestamp;
// Purge array that matches what parsePurgeValue() would have returned
$purge = [ self::PURGE_TIME => (float)$normalizedTime, self::PURGE_HOLDOFF => $holdoff ];
return self::PURGE_VAL_PREFIX . ":$normalizedTime:$holdoff";
}
/**
* @param string $group
* @return MapCacheLRU
*/
private function getProcessCache( $group ) {
if ( !isset( $this->processCaches[$group] ) ) {
[ , $size ] = explode( ':', $group );
$this->processCaches[$group] = new MapCacheLRU( (int)$size );
if ( $this->wallClockOverride !== null ) {
$this->processCaches[$group]->setMockTime( $this->wallClockOverride );
}
}
return $this->processCaches[$group];
}
/**
* @param ArrayIterator $keys
* @param array $opts
* @return string[] Map of (ID => cache key)
*/
private function getNonProcessCachedMultiKeys( ArrayIterator $keys, array $opts ) {
$pcTTL = $opts['pcTTL'] ?? self::TTL_UNCACHEABLE;
$keysMissing = [];
if ( $pcTTL > 0 && $this->callbackDepth == 0 ) {
$pCache = $this->getProcessCache( $opts['pcGroup'] ?? self::PC_PRIMARY );
foreach ( $keys as $key => $id ) {
if ( !$pCache->has( $key, $pcTTL ) ) {
$keysMissing[$id] = $key;
}
}
}
return $keysMissing;
}
/**
* @param string[] $keys Cache keys made with makeKey()/makeGlobalKey()
* @param string[]|string[][] $checkKeys Map of (integer or cache key => "check" key(s));
* "check" keys must also be made with makeKey()/makeGlobalKey()
* @return array<string,mixed> Map of (sister key => value, or, false if not found)
*/
private function fetchWrappedValuesForWarmupCache( array $keys, array $checkKeys ) {
if ( !$keys ) {
return [];
}
// Get all the value keys to fetch...
$sisterKeys = $this->makeSisterKeys( $keys, self::TYPE_VALUE );
// Get all the "check" keys to fetch...
foreach ( $checkKeys as $i => $checkKeyOrKeyGroup ) {
// Note: avoid array_merge() inside loop in case there are many keys
if ( is_int( $i ) ) {
// Single "check" key that applies to all value keys
$sisterKeys[] = $this->makeSisterKey( $checkKeyOrKeyGroup, self::TYPE_TIMESTAMP );
} else {
// List of "check" keys that apply to a specific value key
foreach ( (array)$checkKeyOrKeyGroup as $checkKey ) {
$sisterKeys[] = $this->makeSisterKey( $checkKey, self::TYPE_TIMESTAMP );
}
}
}
$wrappedBySisterKey = $this->cache->getMulti( $sisterKeys );
$wrappedBySisterKey += array_fill_keys( $sisterKeys, false );
return $wrappedBySisterKey;
}
/**
* @param string $key Cache key made with makeKey()/makeGlobalKey()
* @param float $now Current UNIX timestamp
* @return float|null Seconds since the last logged get() miss for this key, or, null
*/
private function timeSinceLoggedMiss( $key, $now ) {
for ( end( $this->missLog ); $miss = current( $this->missLog ); prev( $this->missLog ) ) {
if ( $miss[0] === $key ) {
return ( $now - $miss[1] );
}
}
return null;
}
/**
* @return float UNIX timestamp
* @codeCoverageIgnore
*/
protected function getCurrentTime() {
return $this->wallClockOverride ?: microtime( true );
}
/**
* @param float|null &$time Mock UNIX timestamp for testing
* @codeCoverageIgnore
*/
public function setMockTime( &$time ) {
$this->wallClockOverride =& $time;
$this->cache->setMockTime( $time );
foreach ( $this->processCaches as $pCache ) {
$pCache->setMockTime( $time );
}
}
}