Orders received by an electronic trading system are processed in batches based on the instrument to which an order relates. An incoming order is assigned to a queue of a queue set that makes up the batch according to a random process. Where orders are received from related trading parties they are assigned to the same queue set according to their time of receipt. The batch has a random duration within defined minimum and maximum durations and at the end of the batch, the orders held in the queues are transferred to a matching thread of the trading system sequentially with one order being removed from each queue and a number of passes of the queues completed until orders have been removed.