The DatabaseSelector Middleware provides a framework for automatically swapping from the primary to the replica database connection. Rails provides a basic framework to determine when to swap and allows for applications to write custom strategy classes to override the default behavior.

The resolver class defines when the application should switch (i.e. read from the primary if a write occurred less than 2 seconds ago) and a resolver context class that sets a value that helps the resolver class decide when to switch.

Rails default middleware uses the request's session to set a timestamp that informs the application when to read from a primary or read from a replica.

To use the DatabaseSelector in your application with default settings, run the provided generator.

bin/rails g active_record:multi_db

This will create a file named config/initializers/multi_db.rb with the following contents:

Rails.application.configure do
  config.active_record.database_selector = { delay: 2.seconds }
  config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
  config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
end

Alternatively you can set the options in your environment config or any other config file loaded on boot.

The default behavior can be changed by setting the config options to a custom class:

config.active_record.database_selector = { delay: 2.seconds }
config.active_record.database_resolver = MyResolver
config.active_record.database_resolver_context = MyResolver::MySession

Note: If you are using `rails new my_app –minimal` you will need to call `require “active_support/core_ext/integer/time”` to load the libaries for Time.

Methods

Attributes

[R] context_klass
[R] options
[R] resolver_klass

Class Public methods

new(app, resolver_klass = nil, context_klass = nil, options = {})

# File activerecord/lib/active_record/middleware/database_selector.rb, line 50
def initialize(app, resolver_klass = nil, context_klass = nil, options = {})
  @app = app
  @resolver_klass = resolver_klass || Resolver
  @context_klass = context_klass || Resolver::Session
  @options = options
end

Instance Public methods

call(env)

Middleware that determines which database connection to use in a multiple database application.

# File activerecord/lib/active_record/middleware/database_selector.rb, line 61
def call(env)
  request = ActionDispatch::Request.new(env)

  select_database(request) do
    @app.call(env)
  end
end