Standby is a simple, easy to use gem for ActiveRecord that enables conservative reading from standby databases, which means it won't automatically redirect all SELECTs to standbys.

Instead, you can do Standby.on_standby { User.count } to send a particular query to a standby.

Background: Probably your app started off with one single database. As it grows, you would upgrade to a primary-standby (or master-slave) replication for redundancy. At this point, all queries still go to the primary and standbys are just backups. With that configuration, it's tempting to run some long-running queries on one of the standbys. And that's exactly what Standby does.

• Conservative - Safe by default. Installing Standby won't change your app's current behavior.
• Future proof - No dirty hacks. Simply works as a proxy for ActiveRecord::Base.connection.
• Simple code - Intentionally small. You can read the entire source and completely stay in control.

Standby works with ActiveRecord 3 or later.

Add this line to your application's Gemfile:

gem 'standby'

And create standby configs for each environment.

development:
  database: myapp_development

development_standby:
  database: myapp_development

By convention, config keys with [env]_standby are automatically used for standby reads.

Notice that we just copied the settings of development to development_standby. 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

development:
  <<: *common

development_standby:
  <<: *common

Optionally, you can use a database url for your connections:

development: postgres://root:@localhost:5432/myapp_development
development_standby: postgres://root:@localhost:5432/myapp_development_standby

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

To start using Standby, you need to add Standby.on_standby in your code. Queries in the Standby.on_standby block run on the standby.

Standby.on_standby { User.count }   # => runs on standby
Standby.on_standby(:two) { User.count }  # => runs on another standby configured as `development_standby_two`

You can nest on_standby and on_primary interchangeably. The following code works as expected.

Standby.on_standby do
  ...
  Standby.on_primary do
    ...
  end
  ...
end

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

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

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

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

development_standby:
  <<: *common
  username: readonly

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

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

With this user, writes on a standby should raise an exception.

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

With Postgres you can set the entire database to be readonly:

ALTER DATABASE myapp_development_standby SET default_transaction_read_only = true;

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

You can quickly disable standby reads by dropping the following line in config/initializers/standby.rb.

Standby.disabled = true

With this line, Standby stops connection switching and all queries go to the primary.

This may be useful when one of the primary or the standby goes down. You would rewrite database.yml to make all queries go to the surviving database, until you restore or rebuild the failed one.

When use_transactional_fixtures is set to true, it's NOT recommended to write to the database besides fixtures, since the standby connection is not aware of changes performed in the primary connection due to transaction isolation.

In that case, you are suggested to disable Standby in the test environment by putting the following in test/test_helper.rb (or spec/spec_helper.rb for RSpec users):

Standby.disabled = true

The gem name has been changed from slavery to standby.

Update your Gemfile

gem 'standby'

Then

• Replace Slavery with Standby, on_slave with on_standby, and on_master with on_primary
• Update keys in database.yml (e.g. development_slave to development_standby)

Please note that Standby.spec_key= method has been removed from version 3.

If you're using ActiveRecord in a non-Rails app (e.g. Sinatra), be sure to set RACK_ENV environment variable in the boot sequence, then:

require 'standby'

ActiveRecord::Base.configurations = {
  'development' =>          { adapter: 'mysql2', ... },
  'development_standby' =>  { adapter: 'mysql2', ... }
}
ActiveRecord::Base.establish_connection(:development)

• v4.0.0: Rename gem from Slavery to Standby
• v3.0.0: Support for multiple standby targets (@punchh)
• v2.1.0: Debug log support / Database URL support / Rails 3.2 & 4.0 compatibility (Thanks to @citrus)
• v2.0.0: Rails 5 support