Locking mechanisms

Functions to coordinate long-running operations across requests.

In most environments, multiple Drupal page requests (a.k.a. threads or processes) will execute in parallel. This leads to potential conflicts or race conditions when two requests execute the same code at the same time. A common example of this is a rebuild like menu_router_rebuild() where we invoke many hook implementations to get and process data from all active modules, and then delete the current data in the database to insert the new afterwards.

This is a cooperative, advisory lock system. Any long-running operation that could potentially be attempted in parallel by multiple requests should try to acquire a lock before proceeding. By obtaining a lock, one request notifies any other requests that a specific operation is in progress which must not be executed in parallel.

To use this API, pick a unique name for the lock. A sensible choice is the name of the function performing the operation. A very simple example use of this API:

function mymodule_long_operation() {
  if (lock()->acquire('mymodule_long_operation')) {
    // Do the long operation here.
    // ...
    lock()->release('mymodule_long_operation');
  }
}

If a function acquires a lock it should always release it when the operation is complete by calling lock()->release(), as in the example.

A function that has acquired a lock may attempt to renew a lock (extend the duration of the lock) by calling lock()->acquire() again during the operation. Failure to renew a lock is indicative that another request has acquired the lock, and that the current operation may need to be aborted.

If a function fails to acquire a lock it may either immediately return, or it may call lock()->wait() if the rest of the current page request requires that the operation in question be complete. After lock()->wait() returns, the function may again attempt to acquire the lock, or may simply allow the page request to proceed on the assumption that a parallel request completed the operation.

lock()->acquire() and lock()->wait() will automatically break (delete) a lock whose duration has exceeded the timeout specified when it was acquired.

File

drupal/core/includes/bootstrap.inc, line 3107
Functions that need to be loaded on every Drupal request.

Functions

Namesort descending Location Description
lock Deprecated drupal/core/includes/bootstrap.inc Get locking layer instance.