Record Cache

Cache Active Model Records in Rails 3

Record Cache transparantly stores Records in a Cache Store and retrieve those Records from the store when queried (by ID) using Active Model. Cache invalidation is performed automatically when Records are created, updated or destroyed. Currently only Active Record is supported, but more data stores may be added in the future.

Usage

Installation

Add the following line to your Gemfile:

gem 'record-cache'

Initializer

In /config/initializers/record_cache.rb:

# --- Version Store
# All Workers that use the Record Cache should point to the same Version Store
# E.g. a MemCached cluster or a Redis Store (defaults to Rails.cache)
RecordCache::Base.version_store = Rails.cache

# --- Record Stores
# Register Cache Stores for the Records themselves
# Note: A different Cache Store could be used per Model, but in most configurations the following 2 stores will suffice:

# The :local store is used to keep records in Worker memory
RecordCache::Base.register_store(:local, ActiveSupport::Cache.lookup_store(:memory_store))

# The :shared store is used to share Records between multiple Workers
RecordCache::Base.register_store(:shared, Rails.cache)

Models

Define the Caching Strategy in your models.

Typical Example: /app/models/person.rb:

class Person < ActiveRecord::Base
  cache_records :store => :shared, :key => "pers"
end

Example with Index Cache: /app/models/permission.rb:

class Permission < ActiveRecord::Base
  cache_records :store => :shared, :key => "perm", :index => [:person_id]

  belongs_to :person
end

Example with Request Cache: /app/models/account.rb:

class Account < ActiveRecord::Base
  cache_records :store => :local, :key => "acc", :request_cache => true
end

The following options are available:

• :store: The name of the Cache Store for the Records (default: Rails.cache)

  @see Initializer section above how to define named Cache Stores

• :key: Provide a short (unique) name to be used in the cache keys (default: <model>.name)

  Using shorter cache keys will improve performance as less data is sent to the Cache Stores

• :request_cache: Set to true to switch on Request Caching (default: false)

  In case the same Record is (always) queried multiple times during a single request from different locations, e.g. from a helper and from a model, the Record can be cached in the Request Scope by setting this option to +true+.
  Important: Add to application_controller.rb: prepend_before_filter { |c| RecordCache::Strategy::RequestCache.clear }
  Note: In most cases you should be able to use an instance variable in the controller (or helper) instead.

• :index: An array of :belongs_to attributes to cache :has_many relations (default: [])

  has_many relations will lead to queries like: SELECT * FROM permissions WHERE permission.person_id = 10
  As Record Cache only caches records by ID, this query would always hit the DB. If an index is set on person_id (like in the example above), Record Cache will keep track of the Permission IDs per Person ID.
  Using that information the query will be translated to: SELECT * FROM permissions WHERE permission.id IN (14,15,...) and the permissions can be retrieved from cache.
  Note: The administration overhead for the Permission IDs per Person ID leads to more calls to the Version Store and the Record Store. Whether or not it is profitable to add specific indexes for has_many relations will differ per use-case.

Tests

To switch off Record Cache during the tests, add the following line to /config/environments/test.rb:

RecordCache::Base.disable!

But it is also possible (and preferable during Integration Tests) to keep the Record Cache switched on. To make sure the cache is invalidated for all updated Records after each test/scenario, require the resettable_version_store and reset the Version Store after each test/scenario.

RSpec 2 example, in spec/spec_helper.rb:

require 'record_cache/test/resettable_version_store'

RSpec.configure do |config|
  config.after(:each) do
    RecordCache::Base.version_store.reset!
  end
end

Cucumber example, in features/support/env.rb:

require 'record_cache/test/resettable_version_store'

After do |scenario|
  RecordCache::Base.version_store.reset!
end

Restrictions

1. This gem is dependent on Rails 3

2. Only Active Record is supported as a data store.

3. Models that do not have an id attribute cannot be cached.

4. All servers that host Workers should be time-synchronized (otherwise the Version Store may return stale results).

Caveats

1. Record Cache sorting mimics the MySQL sort order being case-insensitive and using collation. If you need a different sort order, check out the code in <gem>/lib/record_cache/strategy/util.rb.

2. Using update_all to modify attributes used in the :index option will lead to stale results.

3. When using <model>.transaction do ... end, make sure wrap it in RecordCache::Base.without_record_cache do ... end. During the transaction the after_commit callbacks are delayed until the whole transaction completed successfully. As a result the records fetched from the Record Cache within that transaction will not contain the uncommitted changes yet.

4. (Uncommon) If you have a model (A) with a has_many :autosave => true relation to another model (B) that defines a :counter_cache back to model A, the <model B>_count attribute will contain stale results. To solve this, add an after_save hook to model A and update the <model B>_count attribute there in case the has_many relation was loaded.

5. When using Dalli as a MemCache client, multi_read actions may be 50x slower than normal reads, @see petergoldstein/dalli#106 If the same applies to your environment, add the following at the top of /config/initializers/record_cache.rb: RecordCache::MultiRead.disable(ActiveSupport::Cache::DalliStore)

6. The combination of Mongrel (Rack) and the Dalli :threadsafe => false option will lead to the following errors in your log file: undefined method constantize’ for 0:Fixnum`. This is because Mongrel creates multiple threads. To overcome this, set thread_save to true, or consider using a different webserver like Unicorn.

Explain

Retrieval

Each query is parsed and sent to Record Cache before it is executed to check if the query is cacheable. A query is cacheable if:

• it contains at least one where(:id => ...) or where(<indexed attribute> => ...) clause, and

• it contains zero or more where(<attribute> => <single value>) clauses on attributes in the same model, and

• it has no limit(...) defined, or is limited to 1 record and has exactly one id in the where(:id => ...) clause, and

• it has no order(...) clause, or it is sorted on single attributes using ASC and DESC only

• it has no joins, calculations, group by, etc. clauses

When the query is accepted by Record Cache, all requested records will be retrieved and cached as follows:

ID queries:

1. The Version Store is called to retrieve the current version for each ID using a multi_read (keys rc/<model-name>/<id>).

2. A new version will be generated (using the current timestamp) for each ID unknown to the Version Store.

3. The Record Store is called to retrieve the latest data for each ID using a multi_read (keys rc/<model-name>/<id>v<current-version>).

4. The data of the missing records is retrieved directly from the Data Store (single query) and are subsequently cached in the Record Store.

5. The data of all records is deserialized to Active Model records.

6. The other (simple) where(<attribute> => <single value>) clauses are applied, if applicable.

7. The (simple) order(...) clause is applied, if applicable.

Index queries:

1. The Version Store is called to retrieve the current version for the group (key rc/<model-name>/<index>/<id>).

2. A new version will be generated (using the current timestamp) in case the current version is unknown to the Version Store.

3. The Record Store is called to retrieve the latest set of IDs in this group (key rc/<model-name>/<index>/<id>v<current-version>).

4. In case the IDs are missing, the IDs (only) will be retrieved from the Data Store (single query) and subsequently cached in the Record Store.

5. The IDs are passed as an ID query to the id-based-cache (see above).

Invalidation

The after_commit, :on => :create/:update/:destroy hooks are used to inform the Record Cache of changes to the cached records.

ID cache:

• :create: add a new version to the Version Store and cache the record in the Records Store

• :update: similar to :create

• :destroy: remove the record from the Version Store

Index cache:

• :create: increment Version Store for each index that contains the indexed attribute value of this record. In case the IDs in this group are cached and fresh, add the ID of the new record to the group and store the updated list of IDs in the Records Store.

• :update: For each index that is included in the changed attribute, apply the :destoy logic to the old value and the :create logic to the new value.

• :destroy: increment Version Store for each index that contains the indexed attribute value of this record. In case the IDs in this group are current cached and fresh, remove the ID of the record from the group and store the updated list of IDs in the Records Store.

The update_all method of Active Record Relation is also overridden to make sure that mass-updates are processed correctly, e.g. used by the :counter_cache. As the details of the change are not known, all records that match the IDs mentioned in the update_all statement are invalidated by removing them from the Version Store.

Finally for has_many relations, the after_commit hooks are not triggered on add and remove. Whether this is a bug or feature I do not know, but for Active Record the Has Many Association is patched to invalidate the Index Cache of the referenced (reflection) Record in case it has an :index on the reverse belongs_to relation.

Development

$ bundle

# run the specs
$ bundle exec rake spec

# run a single spec
$ bundle exec rspec ./spec/lib/strategy/base_spec.rb:61

# make sure Rails 3.0 support is also fine:
$ vi record-cache.gemspec
  replace [">= 3.0"] with [">= 3.0"], "< 3.1"
$ bundle update
$ bundle exec rake spec

Deploying the gem:

# Don't forget to update the version in lib/record_cache/version.rb
$ git tag -a v0.1.1 -m 'version 0.1.1'
$ git push origin master --tags
$ gem update --system
$ gem build record-cache.gemspec
$ gem push record-cache-0.1.1.gem

Debugging the gem:

Switch on DEBUG logging (config.log_level = :debug in development.rb) to get more information on cache hits and misses.

Release Notes

Version 1.0

First version, with the following Strategies:

1. Request Cache
2. ID Cache
3. Index Cache

Version 1.1

Added support for Rails 3.1

Version 1.2

Refactoring: Moved Serialization, Sorting and Filtering to separate Util class.

Now it is possible to re-use MySQL style sorting (with collation) in your own app, e.g. by calling RecordCache::Strategy::Util.sort!(Apple.all, :name).

Version 1.3

Fixed Bugs:

1. "\u0000" is also used by Arel as a parameter query binding marker.
2. orslumen#2: bypassing record_cache when selecting rows with lock

Added:

1. Release Notes ;)
2. orslumen#3: Ruby 1.9 fixes, has_one support, Remove Freeze for Dalli encoding (Bryan Mundie)
3. :unique_index option
4. :cache_all option

And updated the gemspec file.

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Copyright (c) 2011 Orslumen, released under the MIT license