lib/concurrent-ruby/concurrent/agent.rb
require 'concurrent/configuration'
require 'concurrent/atomic/atomic_reference'
require 'concurrent/atomic/count_down_latch'
require 'concurrent/atomic/thread_local_var'
require 'concurrent/collection/copy_on_write_observer_set'
require 'concurrent/concern/observable'
require 'concurrent/synchronization/lockable_object'
module Concurrent
# `Agent` is inspired by Clojure's [agent](http://clojure.org/agents)
# function. An agent is a shared, mutable variable providing independent,
# uncoordinated, *asynchronous* change of individual values. Best used when
# the value will undergo frequent, complex updates. Suitable when the result
# of an update does not need to be known immediately. `Agent` is (mostly)
# functionally equivalent to Clojure's agent, except where the runtime
# prevents parity.
#
# Agents are reactive, not autonomous - there is no imperative message loop
# and no blocking receive. The state of an Agent should be itself immutable
# and the `#value` of an Agent is always immediately available for reading by
# any thread without any messages, i.e. observation does not require
# cooperation or coordination.
#
# Agent action dispatches are made using the various `#send` methods. These
# methods always return immediately. At some point later, in another thread,
# the following will happen:
#
# 1. The given `action` will be applied to the state of the Agent and the
# `args`, if any were supplied.
# 2. The return value of `action` will be passed to the validator lambda,
# if one has been set on the Agent.
# 3. If the validator succeeds or if no validator was given, the return value
# of the given `action` will become the new `#value` of the Agent. See
# `#initialize` for details.
# 4. If any observers were added to the Agent, they will be notified. See
# `#add_observer` for details.
# 5. If during the `action` execution any other dispatches are made (directly
# or indirectly), they will be held until after the `#value` of the Agent
# has been changed.
#
# If any exceptions are thrown by an action function, no nested dispatches
# will occur, and the exception will be cached in the Agent itself. When an
# Agent has errors cached, any subsequent interactions will immediately throw
# an exception, until the agent's errors are cleared. Agent errors can be
# examined with `#error` and the agent restarted with `#restart`.
#
# The actions of all Agents get interleaved amongst threads in a thread pool.
# At any point in time, at most one action for each Agent is being executed.
# Actions dispatched to an agent from another single agent or thread will
# occur in the order they were sent, potentially interleaved with actions
# dispatched to the same agent from other sources. The `#send` method should
# be used for actions that are CPU limited, while the `#send_off` method is
# appropriate for actions that may block on IO.
#
# Unlike in Clojure, `Agent` cannot participate in `Concurrent::TVar` transactions.
#
# ## Example
#
# ```
# def next_fibonacci(set = nil)
# return [0, 1] if set.nil?
# set + [set[-2..-1].reduce{|sum,x| sum + x }]
# end
#
# # create an agent with an initial value
# agent = Concurrent::Agent.new(next_fibonacci)
#
# # send a few update requests
# 5.times do
# agent.send{|set| next_fibonacci(set) }
# end
#
# # wait for them to complete
# agent.await
#
# # get the current value
# agent.value #=> [0, 1, 1, 2, 3, 5, 8]
# ```
#
# ## Observation
#
# Agents support observers through the {Concurrent::Observable} mixin module.
# Notification of observers occurs every time an action dispatch returns and
# the new value is successfully validated. Observation will *not* occur if the
# action raises an exception, if validation fails, or when a {#restart} occurs.
#
# When notified the observer will receive three arguments: `time`, `old_value`,
# and `new_value`. The `time` argument is the time at which the value change
# occurred. The `old_value` is the value of the Agent when the action began
# processing. The `new_value` is the value to which the Agent was set when the
# action completed. Note that `old_value` and `new_value` may be the same.
# This is not an error. It simply means that the action returned the same
# value.
#
# ## Nested Actions
#
# It is possible for an Agent action to post further actions back to itself.
# The nested actions will be enqueued normally then processed *after* the
# outer action completes, in the order they were sent, possibly interleaved
# with action dispatches from other threads. Nested actions never deadlock
# with one another and a failure in a nested action will never affect the
# outer action.
#
# Nested actions can be called using the Agent reference from the enclosing
# scope or by passing the reference in as a "send" argument. Nested actions
# cannot be post using `self` from within the action block/proc/lambda; `self`
# in this context will not reference the Agent. The preferred method for
# dispatching nested actions is to pass the Agent as an argument. This allows
# Ruby to more effectively manage the closing scope.
#
# Prefer this:
#
# ```
# agent = Concurrent::Agent.new(0)
# agent.send(agent) do |value, this|
# this.send {|v| v + 42 }
# 3.14
# end
# agent.value #=> 45.14
# ```
#
# Over this:
#
# ```
# agent = Concurrent::Agent.new(0)
# agent.send do |value|
# agent.send {|v| v + 42 }
# 3.14
# end
# ```
#
# @!macro agent_await_warning
#
# **NOTE** Never, *under any circumstances*, call any of the "await" methods
# ({#await}, {#await_for}, {#await_for!}, and {#wait}) from within an action
# block/proc/lambda. The call will block the Agent and will always fail.
# Calling either {#await} or {#wait} (with a timeout of `nil`) will
# hopelessly deadlock the Agent with no possibility of recovery.
#
# @!macro thread_safe_variable_comparison
#
# @see http://clojure.org/Agents Clojure Agents
# @see http://clojure.org/state Values and Change - Clojure's approach to Identity and State
class Agent < Synchronization::LockableObject
include Concern::Observable
ERROR_MODES = [:continue, :fail].freeze
private_constant :ERROR_MODES
AWAIT_FLAG = ::Object.new
private_constant :AWAIT_FLAG
AWAIT_ACTION = ->(value, latch) { latch.count_down; AWAIT_FLAG }
private_constant :AWAIT_ACTION
DEFAULT_ERROR_HANDLER = ->(agent, error) { nil }
private_constant :DEFAULT_ERROR_HANDLER
DEFAULT_VALIDATOR = ->(value) { true }
private_constant :DEFAULT_VALIDATOR
Job = Struct.new(:action, :args, :executor, :caller)
private_constant :Job
# Raised during action processing or any other time in an Agent's lifecycle.
class Error < StandardError
def initialize(message = nil)
message ||= 'agent must be restarted before jobs can post'
super(message)
end
end
# Raised when a new value obtained during action processing or at `#restart`
# fails validation.
class ValidationError < Error
def initialize(message = nil)
message ||= 'invalid value'
super(message)
end
end
# The error mode this Agent is operating in. See {#initialize} for details.
attr_reader :error_mode
# Create a new `Agent` with the given initial value and options.
#
# The `:validator` option must be `nil` or a side-effect free proc/lambda
# which takes one argument. On any intended value change the validator, if
# provided, will be called. If the new value is invalid the validator should
# return `false` or raise an error.
#
# The `:error_handler` option must be `nil` or a proc/lambda which takes two
# arguments. When an action raises an error or validation fails, either by
# returning false or raising an error, the error handler will be called. The
# arguments to the error handler will be a reference to the agent itself and
# the error object which was raised.
#
# The `:error_mode` may be either `:continue` (the default if an error
# handler is given) or `:fail` (the default if error handler nil or not
# given).
#
# If an action being run by the agent throws an error or doesn't pass
# validation the error handler, if present, will be called. After the
# handler executes if the error mode is `:continue` the Agent will continue
# as if neither the action that caused the error nor the error itself ever
# happened.
#
# If the mode is `:fail` the Agent will become {#failed?} and will stop
# accepting new action dispatches. Any previously queued actions will be
# held until {#restart} is called. The {#value} method will still work,
# returning the value of the Agent before the error.
#
# @param [Object] initial the initial value
# @param [Hash] opts the configuration options
#
# @option opts [Symbol] :error_mode either `:continue` or `:fail`
# @option opts [nil, Proc] :error_handler the (optional) error handler
# @option opts [nil, Proc] :validator the (optional) validation procedure
def initialize(initial, opts = {})
super()
synchronize { ns_initialize(initial, opts) }
end
# The current value (state) of the Agent, irrespective of any pending or
# in-progress actions. The value is always available and is non-blocking.
#
# @return [Object] the current value
def value
@current.value # TODO (pitr 12-Sep-2015): broken unsafe read?
end
alias_method :deref, :value
# When {#failed?} and {#error_mode} is `:fail`, returns the error object
# which caused the failure, else `nil`. When {#error_mode} is `:continue`
# will *always* return `nil`.
#
# @return [nil, Error] the error which caused the failure when {#failed?}
def error
@error.value
end
alias_method :reason, :error
# @!macro agent_send
#
# Dispatches an action to the Agent and returns immediately. Subsequently,
# in a thread from a thread pool, the {#value} will be set to the return
# value of the action. Action dispatches are only allowed when the Agent
# is not {#failed?}.
#
# The action must be a block/proc/lambda which takes 1 or more arguments.
# The first argument is the current {#value} of the Agent. Any arguments
# passed to the send method via the `args` parameter will be passed to the
# action as the remaining arguments. The action must return the new value
# of the Agent.
#
# * {#send} and {#send!} should be used for actions that are CPU limited
# * {#send_off}, {#send_off!}, and {#<<} are appropriate for actions that
# may block on IO
# * {#send_via} and {#send_via!} are used when a specific executor is to
# be used for the action
#
# @param [Array<Object>] args zero or more arguments to be passed to
# the action
# @param [Proc] action the action dispatch to be enqueued
#
# @yield [agent, value, *args] process the old value and return the new
# @yieldparam [Object] value the current {#value} of the Agent
# @yieldparam [Array<Object>] args zero or more arguments to pass to the
# action
# @yieldreturn [Object] the new value of the Agent
#
# @!macro send_return
# @return [Boolean] true if the action is successfully enqueued, false if
# the Agent is {#failed?}
def send(*args, &action)
enqueue_action_job(action, args, Concurrent.global_fast_executor)
end
# @!macro agent_send
#
# @!macro send_bang_return_and_raise
# @return [Boolean] true if the action is successfully enqueued
# @raise [Concurrent::Agent::Error] if the Agent is {#failed?}
def send!(*args, &action)
raise Error.new unless send(*args, &action)
true
end
# @!macro agent_send
# @!macro send_return
def send_off(*args, &action)
enqueue_action_job(action, args, Concurrent.global_io_executor)
end
alias_method :post, :send_off
# @!macro agent_send
# @!macro send_bang_return_and_raise
def send_off!(*args, &action)
raise Error.new unless send_off(*args, &action)
true
end
# @!macro agent_send
# @!macro send_return
# @param [Concurrent::ExecutorService] executor the executor on which the
# action is to be dispatched
def send_via(executor, *args, &action)
enqueue_action_job(action, args, executor)
end
# @!macro agent_send
# @!macro send_bang_return_and_raise
# @param [Concurrent::ExecutorService] executor the executor on which the
# action is to be dispatched
def send_via!(executor, *args, &action)
raise Error.new unless send_via(executor, *args, &action)
true
end
# Dispatches an action to the Agent and returns immediately. Subsequently,
# in a thread from a thread pool, the {#value} will be set to the return
# value of the action. Appropriate for actions that may block on IO.
#
# @param [Proc] action the action dispatch to be enqueued
# @return [Concurrent::Agent] self
# @see #send_off
def <<(action)
send_off(&action)
self
end
# Blocks the current thread (indefinitely!) until all actions dispatched
# thus far, from this thread or nested by the Agent, have occurred. Will
# block when {#failed?}. Will never return if a failed Agent is {#restart}
# with `:clear_actions` true.
#
# Returns a reference to `self` to support method chaining:
#
# ```
# current_value = agent.await.value
# ```
#
# @return [Boolean] self
#
# @!macro agent_await_warning
def await
wait(nil)
self
end
# Blocks the current thread until all actions dispatched thus far, from this
# thread or nested by the Agent, have occurred, or the timeout (in seconds)
# has elapsed.
#
# @param [Float] timeout the maximum number of seconds to wait
# @return [Boolean] true if all actions complete before timeout else false
#
# @!macro agent_await_warning
def await_for(timeout)
wait(timeout.to_f)
end
# Blocks the current thread until all actions dispatched thus far, from this
# thread or nested by the Agent, have occurred, or the timeout (in seconds)
# has elapsed.
#
# @param [Float] timeout the maximum number of seconds to wait
# @return [Boolean] true if all actions complete before timeout
#
# @raise [Concurrent::TimeoutError] when timout is reached
#
# @!macro agent_await_warning
def await_for!(timeout)
raise Concurrent::TimeoutError unless wait(timeout.to_f)
true
end
# Blocks the current thread until all actions dispatched thus far, from this
# thread or nested by the Agent, have occurred, or the timeout (in seconds)
# has elapsed. Will block indefinitely when timeout is nil or not given.
#
# Provided mainly for consistency with other classes in this library. Prefer
# the various `await` methods instead.
#
# @param [Float] timeout the maximum number of seconds to wait
# @return [Boolean] true if all actions complete before timeout else false
#
# @!macro agent_await_warning
def wait(timeout = nil)
latch = Concurrent::CountDownLatch.new(1)
enqueue_await_job(latch)
latch.wait(timeout)
end
# Is the Agent in a failed state?
#
# @see #restart
def failed?
!@error.value.nil?
end
alias_method :stopped?, :failed?
# When an Agent is {#failed?}, changes the Agent {#value} to `new_value`
# then un-fails the Agent so that action dispatches are allowed again. If
# the `:clear_actions` option is give and true, any actions queued on the
# Agent that were being held while it was failed will be discarded,
# otherwise those held actions will proceed. The `new_value` must pass the
# validator if any, or `restart` will raise an exception and the Agent will
# remain failed with its old {#value} and {#error}. Observers, if any, will
# not be notified of the new state.
#
# @param [Object] new_value the new value for the Agent once restarted
# @param [Hash] opts the configuration options
# @option opts [Symbol] :clear_actions true if all enqueued but unprocessed
# actions should be discarded on restart, else false (default: false)
# @return [Boolean] true
#
# @raise [Concurrent:AgentError] when not failed
def restart(new_value, opts = {})
clear_actions = opts.fetch(:clear_actions, false)
synchronize do
raise Error.new('agent is not failed') unless failed?
raise ValidationError unless ns_validate(new_value)
@current.value = new_value
@error.value = nil
@queue.clear if clear_actions
ns_post_next_job unless @queue.empty?
end
true
end
class << self
# Blocks the current thread (indefinitely!) until all actions dispatched
# thus far to all the given Agents, from this thread or nested by the
# given Agents, have occurred. Will block when any of the agents are
# failed. Will never return if a failed Agent is restart with
# `:clear_actions` true.
#
# @param [Array<Concurrent::Agent>] agents the Agents on which to wait
# @return [Boolean] true
#
# @!macro agent_await_warning
def await(*agents)
agents.each { |agent| agent.await }
true
end
# Blocks the current thread until all actions dispatched thus far to all
# the given Agents, from this thread or nested by the given Agents, have
# occurred, or the timeout (in seconds) has elapsed.
#
# @param [Float] timeout the maximum number of seconds to wait
# @param [Array<Concurrent::Agent>] agents the Agents on which to wait
# @return [Boolean] true if all actions complete before timeout else false
#
# @!macro agent_await_warning
def await_for(timeout, *agents)
end_at = Concurrent.monotonic_time + timeout.to_f
ok = agents.length.times do |i|
break false if (delay = end_at - Concurrent.monotonic_time) < 0
break false unless agents[i].await_for(delay)
end
!!ok
end
# Blocks the current thread until all actions dispatched thus far to all
# the given Agents, from this thread or nested by the given Agents, have
# occurred, or the timeout (in seconds) has elapsed.
#
# @param [Float] timeout the maximum number of seconds to wait
# @param [Array<Concurrent::Agent>] agents the Agents on which to wait
# @return [Boolean] true if all actions complete before timeout
#
# @raise [Concurrent::TimeoutError] when timout is reached
# @!macro agent_await_warning
def await_for!(timeout, *agents)
raise Concurrent::TimeoutError unless await_for(timeout, *agents)
true
end
end
private
def ns_initialize(initial, opts)
@error_mode = opts[:error_mode]
@error_handler = opts[:error_handler]
if @error_mode && !ERROR_MODES.include?(@error_mode)
raise ArgumentError.new('unrecognized error mode')
elsif @error_mode.nil?
@error_mode = @error_handler ? :continue : :fail
end
@error_handler ||= DEFAULT_ERROR_HANDLER
@validator = opts.fetch(:validator, DEFAULT_VALIDATOR)
@current = Concurrent::AtomicReference.new(initial)
@error = Concurrent::AtomicReference.new(nil)
@caller = Concurrent::ThreadLocalVar.new(nil)
@queue = []
self.observers = Collection::CopyOnNotifyObserverSet.new
end
def enqueue_action_job(action, args, executor)
raise ArgumentError.new('no action given') unless action
job = Job.new(action, args, executor, @caller.value || Thread.current.object_id)
synchronize { ns_enqueue_job(job) }
end
def enqueue_await_job(latch)
synchronize do
if (index = ns_find_last_job_for_thread)
job = Job.new(AWAIT_ACTION, [latch], Concurrent.global_immediate_executor,
Thread.current.object_id)
ns_enqueue_job(job, index+1)
else
latch.count_down
true
end
end
end
def ns_enqueue_job(job, index = nil)
# a non-nil index means this is an await job
return false if index.nil? && failed?
index ||= @queue.length
@queue.insert(index, job)
# if this is the only job, post to executor
ns_post_next_job if @queue.length == 1
true
end
def ns_post_next_job
@queue.first.executor.post { execute_next_job }
end
def execute_next_job
job = synchronize { @queue.first }
old_value = @current.value
@caller.value = job.caller # for nested actions
new_value = job.action.call(old_value, *job.args)
@caller.value = nil
return if new_value == AWAIT_FLAG
if ns_validate(new_value)
@current.value = new_value
observers.notify_observers(Time.now, old_value, new_value)
else
handle_error(ValidationError.new)
end
rescue => error
handle_error(error)
ensure
synchronize do
@queue.shift
unless failed? || @queue.empty?
ns_post_next_job
end
end
end
def ns_validate(value)
@validator.call(value)
rescue
false
end
def handle_error(error)
# stop new jobs from posting
@error.value = error if @error_mode == :fail
@error_handler.call(self, error)
rescue
# do nothing
end
def ns_find_last_job_for_thread
@queue.rindex { |job| job.caller == Thread.current.object_id }
end
end
end