backend_abstraction.rb

#
# Inside Config file
#
backend_map = {'mongodb' => MyProject::Backends::MongoDB,
               'mysql'   => MyProject::Backends::MySQL}

conf = load_some_yaml_file_with_a_backend_URI_parameter()
uri = conf[:backend] 
backend_name = uri.split(':').first

config['backend_class'] = backend_map[backend_name]
config['uri'] = uri

config['db'] = 
  if backend_name == 'mysql' 
    EventMachine::Synchrony::ConnectionPool.new(size: 10) do
      Mysql2::EM::Client.new(...)
    end
  elsif backend_name == 'mongodb'
    EventMachine::Synchrony::ConnectionPool.new(size: 10) do
      # setup mongo connection    
    end
  end

# -----------------------------

#
# Inside Goliath server file
#
# after modify my backend classes to accept a 'conn' instance variable and use that inside of its methods instead
# of create a connection at each call

DB = backend_class.connect(uri: uri, conn: db) # and use it inside of routes