Deadlock

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.