code: function (callback) {
          var self = this;
  
          if (!Multiplexer.sources[self._connectionName]) {
              return callback(new AdapterError(AdapterError.UNKNOWN_CONNECTION));
          }
  
          Multiplexer.sources[self._connectionName].getConnection(function (error, connection) {
              if (error) { return callback(error); }
  
              // give a unique id to the connection and store it if not already
              self._threadId = util.uid();
              Multiplexer.connections[self._threadId] = connection;
  
              callback(null, self._threadId, connection);
          });
      }
docs: Retrieves a new connection for initialising queries from the pool specified as parameter.
  
  @param  {function} callback receives `error`, `threadId`, `connection` as parameter