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Locking Functions

TiDB supports most of the user-level locking functions available in MySQL 5.7.

Supported functions

GET_LOCK(lockName, timeout)Acquires an advisory lock. The lockName parameter must be NO longer than 64 characters. Waits maximum timeout seconds before timing out and returns a failure.
IS_FREE_LOCK(lockName)Checks if a lock is free.
IS_USED_LOCK(lockName)Checks if a lock is in use. If true, it returns the corresponding connection ID.
RELEASE_ALL_LOCKS()Releases all locks held by the current session.
RELEASE_LOCK(lockName)Releases a previously acquired lock. The lockName parameter must be NO longer than 64 characters.

MySQL compatibility

  • The minimum timeout permitted by TiDB is 1 second, and the maximum timeout is 1 hour (3600 seconds). This differs from MySQL, where both 0 second and unlimited timeouts (timeout=-1) are permitted. TiDB will automatically convert out-of-range values to the nearest permitted value and convert timeout=-1 to 3600 seconds.
  • TiDB does not automatically detect deadlocks caused by user-level locks. Deadlocked sessions will timeout after a maximum of 1 hour, but can also be manually resolved by using KILL on one of the affected sessions. You can also prevent deadlocks by always acquiring user-level locks in the same order.
  • Locks take effect on all TiDB servers in the cluster. This differs from MySQL Cluster and Group Replication where locks are local to a single server.
  • IS_USED_LOCK() returns 1 if it is called from another session and is unable to return the ID of the process that is holding the lock.
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