A multiple computer environment is disclosed in which an application program executes simultaneously on a plurality of computers (M1, M2, . . . Mn) interconnected by a communications network (53) and in which the local memory of each computer is maintained substantially the same by updating in due course. A lock mechanism is provided to permit exclusive access to an asset, object, or structure (ie memory location) by acquisition and release of the lock. In particular, before a new lock can be acquired by any other computer on a memory location previously locked by one computer, any re-written content(s) for the previously locked memory location are transmitted to all the other computers and their corresponding memory locations (before the in due course updating). Thus when the new lock is acquired all the corresponding memory locations of all computers have been updated.