METHOD OF MAINTAINING CONSISTENCY OF CACHED DATA IN A DATABASE SYSTEM

A method of maintaining the consistency of cached data in a client-server database system. Three new locks--a cache lock, a pending lock and an out-of-date lock--are added to a two-lock concurrency control system. A new long-running envelope transaction holds a cache lock on each object cached by a given client. A working transaction of the client works only with the cached object until commit time. If a second client's working transaction acquires an "X" lock on the object the cache lock is changed to a pending lock; if the transaction thereafter commits the pending lock is changed to an out-of-date lock. If the first client's working transaction thereafter attempts to commit, it waits for a pending lock to change; it aborts if it encounters an out-of-date lock; and otherwise it commits.