[PLUGIN RELEASE] ActsAsReadonlyable

Introduction

ActsAsReadonlyable adds support of multiple read-only slave databases to ActiveRecord models. When a model is marked with acts_as_readonlyable, some of AR finders are overridden to run against a slave DB. The supported finders are find, find_by_sql, count_by_sql, find_[all_]by_*, and reload.

Finders can be forced to fall back to a default DB by passing the :readonly flag set to false.

Disclaimer

As our blog post points out, we wrote this plugin in preparation to using slave DBs but we are not going to have those until May 2007. So even though the code is covered with tests (see svn://rubyforge.org/var/svn/acts-as-with-ro/trunk/test/unit/read_write_model_test.rb), it has not been used outside of those. We would have a discovery period in May when the code is likely to be improved so for now you can use is at your own risk. Meanwhile, we would be happy to fix any issue revealed. Drop us a line at rails-trunk [ at ] revolution DOT com.

Using this plugin should not be your first step in application optimization/scaling or even the second one. Before installing it make sure you understand the implication of leveraging multiple DBs (for example, the potential for cross DB joins).

Usage

Add acts_as_readonlyable to your models backed up by slave DBs. If you want to apply ActsAsReadonlyable to all models, add this or similar code at the end of config/environment.rb:

class << ActiveRecord::Base
  
  def read_only_inherited(child)
    child.acts_as_readonlyable :read_only
    ar_inherited(child)
  end
  
  alias_method :ar_inherited, :inherited
  alias_method :inherited, :read_only_inherited

end

Example

Sample DB Config

dbs:

  database: master_db
  host: master-host

  read_only:
    database: slave_db
    host: slave-host

Note: There is no need for more than one read-only database configuration in your database.yml since you can leverage traditional load balancing solutions. If you still want to use database.yml to spread the load, define multiple entries there and use acts_as_readonlyable [:first_read_only, :second_read_only].


Sample Model

class Fruit < ActiveRecord::Base
  acts_as_readonlyable :read_only
end

Usage

r = Fruit.find(:first) # executes against the read-only db
r.field = 'value'
r.save! # executes against the read/write db

r.reload # executes against the read-only db
r.reload(:readonly => false) # executes against the read/write db


Installation

As plugin:
script/plugin install svn://rubyforge.org/var/svn/acts-as-with-ro/trunk/vendor/plugins/acts_as_readonlyable


License

ActsAsReadonlyable released under the MIT license.


Support

The plugin RubyForge page is http://rubyforge.org/projects/acts-as-with-ro

ActsAsWithReadonly to support read-only DB slave databases

Update: Released as ActsAsReadonlyable

In his latest post, DHH comments on scaling issues Twitter's team had with their rails application. One of the frequent mentioned problems with ActiveRecord is a weak support of multiple read-only slave databases. We were told by our DBAs some time ago that we needed to think how our rails application would utilize slaves DBs after the official release of our portal . Back then our teammate, Jeffrey Damick, came up with an idea of having an 'acts_as' plugin to support that. We have not yet started using slave DBs but some code has been written. We are releasing it on Rubyforge as soon as the project is approved.

The basic idea behind the plugin is that when an ActiveRecord model is marked with acts_as_with_readonly, most of AR finders are overloaded to run against a slave DB. It allows to do all reads from a read-only farm while saving updates to the read-write DB.

An example of usage:

*** database.yml

production:

  database: master_db
  host: master-host

  read_only:
    database: slave_db
    host: slave-host

*** Sample Model

class ReadWriteModel < ActiveRecord::Base
 acts_as_with_readonly :read_only
end

*** Code

record = ReadWriteModel.find(:first) # against slave_db
record.field = 'new value'
record.save! # against master_db