A computer system which is capable of running a plurality of concurrent processes, the system being operable to establish a first queue in which items related to data for sending over the network are enqueued, and to which access is governed by a lock; and when a first of said processes is denied access to the first queue by the lock, to enqueue the items in to a second queue to which access is not governed by the lock, and to arrange for the items in the second queue to be dequeued with items in the first queue.