Best way to access read-only database in rails

Question

Our app is deployed on Heroku with Unicorn and master-slave database configuration with all app requests (read and write) going to master DB.

What we need is to redirect some of the read traffic to follower (slave) database.

For achieving this, we added read_only_database_connection.rb -

class ReadOnlyDatabaseConnection < ActiveRecord::Base
  self.abstract_class = true
end

and added a mixin switch_connection.rb

module SwitchConnection
  def readonly_db_connection    
    current_conf = ReadOnlyDatabaseConnection.connection_config
    begin
      ReadOnlyDatabaseConnection.establish_connection(READONLY_CONFIG).tap do
        puts "Primary DB -> Follower DB"
      end

      yield
    ensure
      ReadOnlyDatabaseConnection.establish_connection(current_conf).tap do
        puts "Follower DB -> Primary DB"
      end
    end
  end
end

And then in application_controller.rb , I did -

include SwitchConnection
around_filter :readonly_db_connection, only: [:index, :show]

Is this the right way to do this? Is there any better or safer approach to redirect all show and index traffic to readonly database?

Answer 1

I think objects that inherit from ActiveRecord::Base should be responsible for dealing with the database. You can have models that check the read-only slave database.

class ReadOnlyModel < ActiveRecord::Base
  establish_connection 'readonly_db'

  def readonly?
    true
  end
end

class Controller
  def index
    @models = ReadOnlyModel.all
  end
end

# in config/database.yml
readonly_database:
  adapter: #...

If you want to use the same table names (which I suspect you do), you can use the table_name method.

class Model < ActiveRecord::Base
end

class ReadOnlyModel < ActiveRecord::Base
  self.table_name :models
end

Here's the documentation for the readonly? method.