### Berkeley DB Reference Guide: Locking Subsystem |

# Configuring locking

The DB_ENV->set_lk_detect function specifies that the deadlock detector should be run whenever a lock blocks. This option provides for rapid detection of deadlocks at the expense of potentially frequent invocations of the deadlock detector. On a fast processor with a highly contentious application where response time is critical, this is a good choice. An argument to the DB_ENV->set_lk_detect function indicates which transaction to abort when a deadlock is detected. It can take on any one of the following values:

DB_LOCK_YOUNGEST
  Abort the most recently started transaction.
DB_LOCK_MAXLOCKS
  Abort the transaction holding the greatest number of locks.
DB_LOCK_MINLOCKS
  Abort the transaction holding the fewest locks.
DB_LOCK_MINWRITE
  Abort the transaction holding the fewest write locks.
DB_LOCK_OLDEST
  Abort the longest-lived transaction.
DB_LOCK_RANDOM
  Abort whatever transaction the deadlock detector happens to find first.
DB_LOCK_DEFAULT
  Use the default policy.

In general, DB_LOCK_DEFAULT is probably the correct first choice, and other options should only be selected based on evidence that they improve transaction throughput. If an application has long-running transactions, DB_LOCK_YOUNGEST will guarantee that transactions eventually complete, but it may do so at the expense of a large number of aborts.

The alternative to using the DB_ENV->set_lk_detect interface is to explicitly perform deadlock detection using the Berkeley DB lock_detect interface.

The DB_ENV->set_lk_conflicts function allows you to specify your own locking conflicts matrix. This is an advanced configuration option, and is almost never necessary.

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