Cron API changes in WordPress 5.1

WordPress 5.1 sees a number of developer focused changes to the Cron API. These changes were discussed and made in #21072, #32656, #45797, and #45976.

Meaningful return values

Prior to WordPress 5.1, the functions used to modify scheduled tasks would return ambiguous values. These now return values indicating whether the function call was successful or otherwise.

wp_schedule_single_event(), wp_schedule_event(), wp_reschedule_event() and wp_unschedule_event() each return true or false to indicate whether the cron schedule was successfully modified.

wp_clear_scheduled_hook() and wp_unschedule_hook() each return false if the update was unsuccessful, 0 if there were no events to unschedule or an integer indicating the number of events that were successfully deleted.

Return values have also been added to the functions predominantly for use by WordPress Core only: spawn_cron(), wp_cron(), and _set_cron_array().

New Functions

Two new functions have been added to assist with returning data.

wp_get_ready_cron_jobs(): retrieve cron jobs ready to be run.

Returns the results of _get_cron_array() limited to events ready to be run, ie, with a timestamp in the past. The new function returns the data only, it does not cause them to be run; that remains the role of other functions.

This function does not accept any arguments.

For more information, check out the source code for this function.

wp_get_scheduled_event(): retrieve a scheduled event.

Returns the full event object for a scheduled event.

The new function accepts three arguments:

  • $hook (string) Required, the action hook of the event.
  • $args (array) Optional. An array containing each separate argument to pass to the hook’s callback function.
  • $timestamp (integer or null) Optional. Unix timestamp (UTC) of the event. If not specified, the next scheduled event is returned.

The return value will be either the event object or false if the specified event has not been registered.

For more information, check out the source code for this function.

New filters for modifying cron storage

WordPress Core stores all cron events as an array in a single option. As the number of events increases, this method of storage can become somewhat unwieldy and unperformant.

In WordPress 5.1 writing a custom storage system for cron events will become substantially easier with filters added to each of the functions used for scheduling, re-scheduling, cancelling and returning a list of events.

Each of these hooks follows the pattern of bypassing the Core function if the filter returns a value other than null.

Unless you are writing your own system for storing Cron jobs, these hooks are best left unused. If you are writing a custom storage solution, you will need to use all of them.

Each hook can be used to hijack the following:

  • pre_schedule_event for both single and recurring events,
  • pre_reschedule_event for rescheduling an event,
  • pre_unschedule_event for unscheduling an event
  • pre_clear_scheduled_hook for unscheduling all events attached to a hook/argument combination
  • pre_unschedule_hook for unscheduling all events related to a hook
  • pre_get_scheduled_event for returning an event including by specifying its timestamp,
  • pre_get_ready_cron_jobs for returning all cron events that are ready to be run

Warning: Adding code to these hooks must be done very early, before any other plugins attempt to register cron events or WordPress attempts to run any scheduled cron events on the init action.

Therefore, it is recommended you run add_filter() as your plugin is included , rather than waiting for the plugins_loaded or muplugins_loaded actions to fire. For example:

// File name: my-cron-plugin/my-cron-plugin.php

// Bootstrap immediately.
add_filter( 'pre_schedule_event', 'mcp_schedule_event', 10, 2 );

 * Use custom storage solution for scheduling Cron jobs.
function mcp_schedule_event( $hijack, $event ) {
    if ( $hijack !== null ) {
        // Already hijacked.
        return $hijack;

    // Use my plugin's custom storage solution.

    return true; // or false.

Note: WordPress 5.1 Beta 1 and 2 included the filters pre_next_scheduled and next_scheduled in the function wp_next_scheduled(). These were later removed as the logic for returning the next scheduled event was moved to wp_get_scheduled_event() to improve performance.

#5-1, #dev-notes