5.6 Deadlocks

Deadlocks

Locking can be used to solve the problems of concurrency. However, locking can also introduce the problem of deadlock as shown in the example below.

Deadlock is a situation in which two or more transactions are in a simultaneous wait state, each of them waiting for one of the others to release a lock before it can proceed.

If a deadlock occurs, the system may detect it and break it. Detecting involves detecting a cycle in the “Wait-For Graph” (a graph which shows 'who is waiting for whom'). Breaking a deadlock implies choosing one of the deadlocked transactions as the victim and rolling it back, thereby releasing all its locks. This may allow some other transaction(s) to proceed.

Deadlock prevention can be done by not allowing any cyclic-waits.