master_slave_db.rb

module ModelExt
  module MasterSlaveDb
 
    # This module allows you to send db queries directly to the slave database.
    # Please do not use this on the user frontend at the moment - administration interface only.
    #
    #   1) The slave is weaker (CPU, RAM) than the master.
    #   2) The slave will be down around lunch time for backup reasons.
    #
    # Usage:
    #   include ModelExt::MasterSlaveDb
    #
    #   Model.with_slave_db! do
    #     Model.slow_find
    #   end
    #
    def self.included(base)
      base.extend(ClassMethods)
    end
 
    module ClassMethods
      def with_slave_db!
        connect_with!(:slave)
        yield
      ensure
        connect_with!(:master)
      end
 
      private
        def connect_with!(type)
          if Rails.env.development?
            called_by = caller(2).first rescue "unknown"
 
            ::Rails.logger.debug("DB SWITCH -------------------------------------------")
            ::Rails.logger.debug("BY #{ called_by.gsub(Rails.root, '') }")
            ::Rails.logger.debug("TO #{ type }")
            ::Rails.logger.debug("-----------------------------------------------------")
          end
 
          clear_all_connections!      # Returns connections back to the pool
 
          case type
          when :master  then  establish_connection(configurations[Rails.env])
          when :slave   then  establish_connection(YamlLoader.file('database_slave').marshal_dump)
          end
        end
    end
  end
end