Sample database.yml for multiple slaves/masters with DataMapper

connection_pool_adapter.rb

module DataMapper
	module Adapters
		
		class ConnectionPoolAdapter < AbstractAdapter
			def initialize(name, options)
				super
				
				assert_kind_of 'options', @options[:pool],  Array
				
				raise ArgumentError, "The are no adapters in the adapter pool" if @options[:pool].empty?
				
				@pool = []
				@options[:pool].each do |adapter_opts|
					@pool.push(::DataMapper.setup(name,  adapter_opts))
				end
				
				@number_generator = Random.new
			end
			
			def create(resources)
				random_adapter.create(resources)
			end
			
			def read(query)
				random_adapter.read(query)
			end
			
			def update(attributes, collection)
				random_adapter.update(attributes, collection)
			end
			
			def delete(collection)
				random_adapter.delete(collection)
			end
			
			def method_missing(meth, *args, &block)
				random_adapter.send(meth, *args, &block)
			end
			
			private
			
				def random_adapter
					@pool[@number_generator.rand(0...@pool.length)]
				end
		end
		
	end
end

database.yml

defaults: &defaults
  adapter: mysql
  encoding: UTF-8
  username: user
  password: resu
  host: localhost

development:
  adapter: master_slave
  master:
    <<: *defaults
    database: ourproduct
  slave:
    <<: *defaults
    database: ourproduct

production:
  adapter: master_slave
  master:
    <<: *defaults
    database: ourproduct
    host: master1.db.ourcloud.com
  slave:
    adapter: connection_pool
    pool:
      - <<: *defaults
        database: ourproduct
        host: slave1.db.ourcloud.com
      - <<: *defaults
        database: ourproduct
        host: slave2.db.ourcloud.com

test:
  adapter: master_slave
  master:
    <<: *defaults
    database: ourproduct_test
  slave:
    <<: *defaults
    database: ourproduct_test

master_slave_adapter.rb

require 'forwardable'

module DataMapper
	module Adapters
		
		class MasterSlaveAdapter < AbstractAdapter
			extend Forwardable
			
			def_delegators :@slave_adapter,  :read
			def_delegators :@master_adapter, :create, :update, :delete
			
			def initialize(name, options)
				super
				
				assert_kind_of 'options', @options[:slave],  Hash
				assert_kind_of 'options', @options[:master], Hash
				
				# Giving the master/slave the same name allows them to act as one wrt. storage_names
				@slave_adapter  = ::DataMapper.setup(name,  @options[:slave])
				@master_adapter = ::DataMapper.setup(name, @options[:master])
			end
			
			# It's safest to send anything we don't know about to the master
			def method_missing(meth, *args, &block)
				@master_adapter.send(meth, *args, &block)
			end
		end
		
	end
end