# SecondLevelCache

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SecondLevelCache is a write-through and read-through caching library inspired by Cache Money and cache_fu, support only Rails3 and ActiveRecord.

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:

```ruby
gem "second_level_cache", "~> 1.5"
```

## Usage

For example, cache User objects:

```ruby
class User < ActiveRecord::Base
  acts_as_cached(:version => 1, :expires_in => 1.week)
end
```

Then it will fetch cached object in this situations:

```ruby
User.find(1)
User.find_by_id(1)
User.find_by_id!(1)
User.find_by_id_and_name(1, "Hooopo")
User.where(:status => 1).find_by_id(1)
user.articles.find_by_id(1)
user.articles.find(1)
User.where(:status => 1).find(1)
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"
```

## 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`:

```ruby
SecondLevelCache.configure.cache_key_prefix = "slc1"
```
* When schema of your model changed, just change the `version` of the speical model, avoding clear all the cache.

```ruby
class User < ActiveRecord::Base
  acts_as_cached(: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_key("hooopo", :nick_name)

# this also fetch from cache
user = User.fetch_by_uniq_key!("hooopo", :nick_name) # this will raise `ActiveRecord::RecordNotFound` Exception when nick name not exists.
```

## Contributors

* [chloerei](https://github.com/chloerei)
* [reyesyang](https://github.com/reyesyang)
* [hooopo](https://github.com/hooopo)
* [sishen](https://github.com/sishen)

## License

MIT License

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.