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Simple, conservative slave reads for ActiveRecord

Slavery - Simple, conservative slave reads for ActiveRecord

Slavery is a simple, easy to use plugin for ActiveRecord that enables conservative slave reads, which means it doesn't automatically redirect all SELECTs to slaves. Instead, it lets you specify Slavery.on_slave to send a particular query to a slave.

Probably you just start off with one single database. As your app grows, you would move to master-slave replication for redundancy. At this point, all queries still go to the master and slaves are just backups. With that configuration, it's tempting to run some long-running queries on the slave. And that's exactly what Slavery does.

  • Conservative - Safe by default. Installing Slavery won't change your app's current behavior.
  • Future proof - No dirty hacks, simply works as a proxy for ActiveRecord::Base.connection.
  • Simple - Only 100+ LOC, you can read the entire source and completely stay in control.

Slavery works with ActiveRecord 3 or later.


Add this line to your application's Gemfile:

gem 'slavery'

And create slave configs for each environment.

  database: myapp_development

  database: myapp_development

By convention, config keys with [env]_slave are automatically used for slave reads.

Notice that we just copied the settings of development to development_slave. For development and test, it's actually recommended as probably you don't want to have replicating multiple databases on your machine. Two connections to the same identical database should be fine for testing purpose.

In case you prefer DRYer definition, YAML's aliasing and key merging might help.

common: &common
  adapter: mysql2
  username: root
  database: myapp_development

  <<: *common

  <<: *common

At this point, Slavery does nothing. Run tests and confirm that nothing is broken.


To start using Slavery, you need to add Slavery.on_slave in your code. Queries in the Slavery.on_slave block run on the slave.

Slavery.on_slave { User.count }     # => runs on slave

You can nest on_slave and on_master interchangeably. The following code works as expected.

Slavery.on_slave do
  Slavery.on_master do

Alternatively, you may call on_slave directly on the scope, so that the query will be read from slave when it's executed.

User.on_slave.where(active: true).count

Caveat: pluck is not supported by the scope syntax, you still need Slavery.on_slave in this case.

Read-only user

For an extra safeguard, it is recommended to use a read-only user for slave access.

  <<: *common
  username: readonly

With MySQL, GRANT SELECT creates a read-only user.

GRANT SELECT ON *.* TO 'readonly'@'localhost';

With this user, writes on slave should raises an exception.

Slavery.on_slave { User.create }    # => ActiveRecord::StatementInvalid: Mysql2::Error: INSERT command denied...

It is a good idea to confirm this behavior in your test code as well.

Database failure

When one of the master or the slave goes down, you would rewrite database.yml to make all queries go to the surviving database, until you restore or rebuild the failed one.

In such an event, you don't want to manually remove Slavery.on_slave from your code. Instead, just put the following line in config/initializers/slavery.rb.

Slavely.disabled = true

With this line, Slavery stops connection switching and all queries go to the new master.

Support for non-Rails apps

If you're using ActiveRecord in a non-Rails app (e.g. Sinatra), be sure to set Slavery.env in the boot sequence.

Slavery.env = 'development'

ActiveRecord::Base.send(:include, Slavery)

ActiveRecord::Base.configurations = {
  'development' =>        { adapter: 'mysql2', ... },
  'development_slave' =>  { adapter: 'mysql2', ... }

Custom slave key in database.yml

This is useful for deploying on EngineYard where the configuration key in database.yml is simple "slave". Put the following line in config/initializers/slavery.rb.

Slavery.spec_key = "slave" #instead of production_slave

Alternatively you can pass it a lambda for dynamically setting this.

Slavery.spec_key = lambda{ "#{Slavery.env}_slave" }
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