Deadlock
![Both processes need resources to continue execution. P1 requires additional resource R1 and is in possession of resource R2, P2 requires additional resource R2 and is in possession of R1; neither process can continue.](/Images/godic/202501/08/Process_deadlock.svg4741.png")
![Four processes (blue lines) compete for one resource (grey circle), following a right-before-left policy. A deadlock occurs when all processes lock the resource simultaneously (black lines). The deadlock can be resolved by breaking the symmetry.](/Images/godic/202501/08/Deadlock_at_a_four-way-stop4742.gif")
![Two processes competing for two resources in opposite order.
(A) A single process goes through.
(B) The later process has to wait.
(C) A deadlock occurs when the first process locks the first resource at the same time as the second process locks the second resource.
(D) The deadlock can be resolved by cancelling and restarting the first process.](/Images/godic/202501/08/Two_processes,_two_resources4742.gif")
![(A) Two processes competing for one resource, following a first-come, first-served policy.
(B) A deadlock occurs when both processes lock the resource simultaneously.
(C) The deadlock can be resolved by breaking the symmetry of the locks.
(D) The deadlock can be prevented by breaking the symmetry of the locking mechanism.](/Images/godic/202501/08/Avoiding_deadlock4742.gif")
In concurrent programming, a deadlock is a situation in which two or more competing actions are each waiting for the other to finish, and thus neither ever does.
In a transactional database, a deadlock happens when two processes each within its own transaction updates two rows of information but in the opposite order. For example, process A updates row 1 then row 2 in the exact timeframe that process B updates row 2 then row 1. Process A can't finish updating row 2 until process B is finished, but process B cannot finish updating row 1 until process A is finished. No matter how much time is allowed to pass, this situation will never resolve itself and because of this, database management systems will typically kill the transaction of the process that has done the least amount of work.