# Deadlocks

In a database, a deadlock is a situation in which two or more transactions are waiting for one another to give up locks.

For example, Transaction A might hold a lock on some rows in the Accounts table and needs to update some rows in the Orders table to finish. Transaction B holds locks on those very rows in the Orders table but needs to update the rows in the Accounts table held by Transaction A. Transaction A cannot complete its transaction because of the lock on Orders. Transaction B cannot complete its transaction because of the lock on Accounts. All activity comes to a halt and remains at a standstill forever unless the DBMS detects the deadlock and aborts one of the transactions.

Figure 1. A deadlock.

- Avoiding deadlocks
  Using both row-level locking and the TRANSACTION_READ_COMMITTED isolation level makes it likely that you will avoid deadlocks (both settings are Derby defaults). However, deadlocks are still possible.
- Deadlock detection
  When a transaction waits more than a specific amount of time to obtain a lock (called the deadlock timeout), Derby can detect whether the transaction is involved in a deadlock.
- Lock wait timeouts
  Even if a transaction is not involved in a deadlock, it might have to wait a considerable amount of time to obtain a lock because of a long-running transaction or transactions holding locks on the tables it needs.
- Configuring deadlock detection and lock wait timeouts
  You configure the amount of time a transaction waits before Derby does any deadlock checking with the derby.locks.deadlockTimeout property.
- Debugging Deadlocks
  If deadlocks occur frequently in your multi-user system with a particular application, you might need to do some debugging.
- Programming applications to handle deadlocks
  When you configure your system for deadlock and lockwait timeouts and an application could be chosen as a victim when the transaction times out, you should program your application to handle them.

Parent topic: Locking, concurrency, and isolation
Related concepts
Isolation levels and concurrency
Configuring isolation levels
Lock granularity
Types and scope of locks in Derby systems