ActiveRecord Shards is an extension for ActiveRecord that provides support for sharded database and slaves. Basically it is just a nice way to switch between database connection. We've made the implementation very small, and have tried not to reinvent any wheels already present in ActiveRecord.
ActiveRecord Shards has only been used and tested on Rails 2.3.x, and has in some form or another been used on in production on a large rails app for more than a year!
$ gem install active_record_shards
and make sure to require 'active_record_shards' in some way.
Add the slave and shard configuration to config/database.yml:
production: adapter: mysql encoding: utf8 database: my_app_main pool: 5 host: db1 username: root password: slave: host: db1_slave shards: 1: host: db_shard1 database: my_app_shard slave: host: db_shard1_slave 2: host: db_shard2 database: my_app_shard slave: host: db_shard2_slave
basically connections inherit configuration from the parent the configuration file.
Normally you have some models that live on a shared database, and you might need to query this data in order to know what shard to switch to. All the model that live on the shared database must be marked as not_sharded:
class Account < ActiveRecord::Base not_sharded has_many :projects end class Project < ActiveRecord::Base belongs_to :account end
So in this setup the accounts live on the shared database, but the projects are sharded. If accounts had a shard_id column, you could lookup the account in a rack middleware and switch to the right shard:
class AccountMiddleware def initialize(app) @app = app end def call(env) account = lookup_account(env) if account ActiveRecord::Base.on_shard(account.shard_id) do @app.call(env) end else @app.call(env) end end def lookup_account(env) ... end end
Any where in your app, you can switch to the slave databases, by wrapping you code an on_slave block:
ActiveRecord::Base.on_slave do Account.find_by_big_expensive_query end
This will perform the query on the slave, and mark the returned instances as read only. There is also a shortcut for this:
Copyright (c) 2011 Zendesk. See LICENSE for details.
Mick Staugaard, Eric Chapweske