GET_LOCK

Purpose

GET_LOCK() tries to obtain a lock with a name given by the string str. When a session successfully obtains a lock with a specific name, any other sessions attempting to obtain a lock with the same name will be blocked (that is, forced to wait) until the session holding the lock releases it.

You can use RELEASE_LOCK() to explicitly release a lock obtained with GET_LOCK(). In addition, when a session terminates, whether normally or abnormally, all locks held by the session are released implicitly.

Syntax

GET_LOCK('str', timeout)

Description

Parameters

• str: the name of the lock. The value is a string.

• timeout: the timeout period for waiting a lock to be released, in seconds. The valid values are as follows:

  • 0 or a positive number.
  • A negative number, indicating that the session will wait indefinitely until the lock is obtained.

Return value

• 1: The lock was obtained successfully. The session that called the GET_LOCK() function is then holding the lock. Any other attempts to obtain a lock with the same name will not succeed unless the lock is released.
• 0: The attempt to obtain the lock failed due to a timeout. This usually means that the lock is held by another session and not released within the timeout period.
• NULL: An error occurred while attempting to obtain the lock, which may be because the memory is insufficient or the thread for obtaining the lock is forcibly terminated.

Examples

1. In session 1, run the following command to obtain the lock named my_lock in a timeout of 10s:

   obclient [(none)]> SELECT GET_LOCK('my_lock', 10);
   

   The return result is as follows:

   +-------------------------+
   | GET_LOCK('my_lock', 10) |
   +-------------------------+
   |                       1 |
   +-------------------------+
   1 row in set
   

   1 is returned, indicating that the session has obtained the my_lock lock.

2. In session 2, run the following command to obtain the lock named my_lock in a timeout of 10s:

   obclient [(none)]> SELECT GET_LOCK('my_lock', 10);
   

   The return result is as follows:

   +-------------------------+
   | GET_LOCK('my_lock', 10) |
   +-------------------------+
   |                       0 |
   +-------------------------+
   1 row in set
   

   0 is returned, indicating that the attempt to obtain the my_lock lock failed due to a timeout.

References

RELEASE_LOCK