1. A computer system comprising one or more processors programmed to: (a) receive data describing a trading order for a security; (b) assign execution of at least part of said trading order to a first trading algorithm; (c) transmit a request to cancel execution of a part of said trading order assigned to but not yet executed by said first trading algorithm; and (d) prior to receiving a cancel confirmation for said part of said trading order assigned to but not yet executed by said first trading algorithm, commence execution by a second trading algorithm of at least a portion of said part of said trading order assigned to but not yet executed by said first trading algorithm.