a connection pool for eventmachine & mysql2 with batch-query support (allows async transactions)

mysql2_connection_pool.rb

require 'mysql2'
require 'mysql2/em'

class MysqlConnectionPool

  def initialize(conf)
    @pool_size = conf[:size] || 10
    @query_queue = EM::Queue.new
    @connections = {}
    start_queue conf
  end

  def queue_worker_loop
    proc{ |connection|

      worker_state = @connections[connection]

      if (worker_state.size > 0)
        # process existing queries in worker_state
        stmt = worker_state.pop
        sql = stmt[:sql].is_a?(Proc) ? stmt[:sql].call(connection) : stmt[:sql]

        connection.query(sql, stmt[:opts]).callback do |result|
          stmt[:callback].call result if stmt[:callback]
          queue_worker_loop.call connection
        end
      else
        # add new queries in worker_state
        @query_queue.pop do |elem|
          if elem[:sql].is_a?(Array)
            elem[:sql].each do |stmt|
              worker_state << :sql => stmt, :opts => elem[:opts], :callback => elem[:callback]
            end
          else
            worker_state << elem
          end
          queue_worker_loop.call connection
        end
      end
    }
  end

  def start_queue(conf)
    @pool_size.times do
      connection = Mysql2::EM::Client.new(conf)
      @connections[connection] = []
      queue_worker_loop.call connection
    end
  end

  def query(sql, opts={}, &block)
    @query_queue.push :sql => sql, :opts => opts, :callback => block
  end
end

untested code, pending confirmation by buddy

l. 32, worker_state << should be replaced by worker_state.push to get this working IIRC.

Just for information, this piece of code allows mysql transactions bits to be sent asynchronously (transaction bits need to be on the same connection).
This implementation does not cover error handling, which you do need for proper retrying as the documentation points out (http://dev.mysql.com/doc/refman/5.0/en/innodb-deadlocks.html - "Always be prepared to re-issue a transaction if it fails due to deadlock. Deadlocks are not dangerous. Just try again.").