README.md
# SecondLevelCache
[](http://badge.fury.io/rb/second_level_cache)
[](https://github.com/hooopo/second_level_cache/actions/workflows/build.yml)
[](https://codeclimate.com/github/hooopo/second_level_cache)
SecondLevelCache is a write-through and read-through caching library inspired by Cache Money and cache_fu, support ActiveRecord 4, ActiveRecord 5 and ActiveRecord 6.
Read-Through: Queries by ID, like `current_user.articles.find(params[:id])`, will first look in cache store and then look in the database for the results of that query. If there is a cache miss, it will populate the cache.
Write-Through: As objects are created, updated, and deleted, all of the caches are automatically kept up-to-date and coherent.
## Install
In your gem file:
ActiveRecord 7
```ruby
gem 'second_level_cache', '~> 2.7'
```
ActiveRecord 5.2 and 6.0:
```ruby
gem 'second_level_cache', '~> 2.6.3'
```
ActiveRecord 5.0.x, 5.1.x:
```ruby
gem 'second_level_cache', '~> 2.3.0'
```
For ActiveRecord 4:
```ruby
gem "second_level_cache", "~> 2.1.9"
```
For ActiveRecord 3:
```ruby
gem "second_level_cache", "~> 1.6"
```
## Usage
For example, cache User objects:
```ruby
class User < ActiveRecord::Base
second_level_cache expires_in: 1.week
end
```
Then it will fetch cached object in this situations:
```ruby
User.find(1)
user.articles.find(1)
User.where(status: 1).find(1)
User.where(id: 1).first # or .last
article.user
```
Cache key:
```ruby
user = User.find(1)
user.second_level_cache_key # We will get the key looks like "slc/user/1/0"
```
Expires cache:
```ruby
user = User.find(1)
user.expire_second_level_cache
```
or expires cache using class method:
```ruby
User.expire_second_level_cache(1)
```
Disable SecondLevelCache:
```ruby
User.without_second_level_cache do
user = User.find(1)
# ...
end
```
Only `SELECT *` query will be cached:
```ruby
# this query will NOT be cached
User.select("id, name").find(1)
```
## Notice
- SecondLevelCache cache by model name and id, so only find_one query will work.
- Only equal conditions query WILL get cache; and SQL string query like `User.where("name = 'Hooopo'").find(1)` WILL NOT work.
- SecondLevelCache sync cache after transaction commit:
```ruby
# user and account's write_second_level_cache operation will invoke after the logger.
ActiveRecord::Base.transaction do
user.save
account.save
Rails.logger.info "info"
end # <- Cache write
# if you want to do something after user and account's write_second_level_cache operation, do this way:
ActiveRecord::Base.transaction do
user.save
account.save
end # <- Cache write
Rails.logger.info "info"
```
- If you are using SecondLevelCache with database_cleaner, you should set cleaning strategy to `:truncation`:
```ruby
DatabaseCleaner.strategy = :truncation
```
## Configure
In production env, we recommend to use [Dalli](https://github.com/mperham/dalli) as Rails cache store.
```ruby
config.cache_store = [:dalli_store, APP_CONFIG["memcached_host"], { namespace: "ns", compress: true }]
```
## Tips:
- When you want to clear only second level cache apart from other cache for example fragment cache in cache store,
you can only change the `cache_key_prefix` (default: `slc`):
```ruby
SecondLevelCache.configure.cache_key_prefix = "slc1"
```
- SecondLevelCache was added model schema digest as cache version, this means when you add/remove/change columns, the caches of this Model will expires.
- When your want change the model cache version by manualy, just add the `version` option like this:
```ruby
class User < ActiveRecord::Base
second_level_cache version: 2, expires_in: 1.week
end
```
- It provides a great feature, not hits db when fetching record via unique key (not primary key).
```ruby
# this will fetch from cache
user = User.fetch_by_uniq_keys(nick_name: "hooopo")
post = Post.fetch_by_uniq_keys(user_id: 2, slug: "foo")
# this also fetch from cache
user = User.fetch_by_uniq_keys!(nick_name: "hooopo") # this will raise `ActiveRecord::RecordNotFound` Exception when nick name not exists.
```
- You can use Rails's [Eager Loading](http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations) feature as normal. Even better, second_level_cache will transform the `IN` query into a Rails.cache.multi_read operation. For example:
```ruby
Answer.includes(:question).limit(10).order("id DESC").each{|answer| answer.question.title}
Answer Load (0.2ms) SELECT `answers`.* FROM `answers` ORDER BY id DESC LIMIT 10 # Only one SQL query and one Rails.cache.read_multi fetching operation.
```
[Details for read_multi feature](http://hooopo.writings.io/articles/a9cae5e0).
## Original design by:
- [chloerei](https://github.com/chloerei)
- [hooopo](https://github.com/hooopo)
## Contributors
[Contributor List](https://github.com/hooopo/second_level_cache/graphs/contributors)
## License
MIT License