1. @cklosowski cklosows
Wrangling WP_Cron Scheduling Your Tasks the WordPress Way

2. Chris Klosowski
• Plugin Developer (WP-Push.com, PostPromoterPro.com,
GitHub, WordPress.org)
• Contributing Developer and Support Tech for Easy Digital
Downloads
• Hang out with my Wife and Kid
@cklosowski cklosows

3. WP_Cron
A tool to replace a tool

4. What is “Cron”
• A time-based job scheduler in Unix-like computer
operating systems.
• Used to schedule jobs (commands or shell scripts) to run
periodically at fixed times, dates, or intervals.
• Cleanup, populate data into cache, etc.

5. WP_Cron
The platform-agnostic task
scheduler

6. WP_Cron vs Cron
• Works on Windows based systems
• Works when you don’t have access to cron
• Easily allows access to the WordPress core
• “Reliably” available to Theme and Plugin developers

7. Basic Usage
<?php
wp_schedule_event( $timestamp, $recurrence, $hook, $args );
Simple Right?

8. Sort Of
Some Caveats

9. $timestamp
• The first time you want this item to run
• UNIX Timestamp format
• Always in UTC (not localized timezone)
• Use time(); as it uses GMT for right now
• No Default
get_option( 'gmt_offset' );

10. $recurrence
• The name of the schedule value
• hourly
• twicedaily
• daily
• Using cron_schedules filter, you can add your own recurrences.
• Not a timestamp, but a string that references a timestamp
• Number of seconds between occurrences

11. $hook
• Essentially does a do_action() when it runs. You would
then attach a function to this hook you’re creating.
• Possible issue on some systems where the hook must
not contain underscores or uppercase characters. (Have
not seen this myself)

12. $args
• Optional array of data to pass through the hook
• Great for when doing post or page specific crons
• Used later when doing lookups for scheduled crons.
• Must be an indexed array (no numeric keys)

13. // Be Kind, Rewind
register_deactivation_hook( __FILE__, 'ck_deactivation' );
function ck_deactivation() {
wp_clear_scheduled_hook( 'ck_hourly_event_hook' );
}
<?php
register_activation_hook( __FILE__, 'ck_activation' );
!
function ck_activation() {
wp_schedule_event( time(), 'hourly', 'ck_hourly_event_hook' );
}
!
add_action( 'ck_hourly_event_hook', 'ck_do_this_hourly' );
function ck_do_this_hourly() {
// do something every hour
}

14. <?php
// Situation of single event
$args = array( 'post_id' => 42, 'service' => 'twitter' );
wp_schedule_single_event( time(), 'ck_single_event', $args );
!
!
// Clear with same hook and EXACT same args
$args2 = array( 'post_id' => 42, 'service' => 'twitter' );
wp_clear_scheduled_hook( 'ck_single_event', $args2 );

15. Why Not WP_Cron?
Not all unicorns and rainbows

16. Because WP_Cron _______________
• Why WP Cron Sucks
http://www.lucasrolff.com/wordpress/why-wp-cron-sucks/
• Disabling WP-Cron in WordPress
http://www.limecanvas.com/disabling-wp-cron-in-wordpress/
• Understanding WP Cron, and why I don’t use it
http://chrislema.com/understanding-wp-cron/
…I could go on…

17. Low Traffic Sites
• WP_Cron is based off traffic
• No visits, no cron
• Unreliable for time specific
tasks

18. High Traffic Sites
• Possibility of many concurrent
cron events.
• Expensive cron events can
cause load issues.
• If needing EXACT times, still
not 100%

19. Race Conditions
• Identified by the same
scheduled task running twice,
at the same time.
• Due to PHP threads, they
aren’t aware of each other

20. Missed Tasks
• They will run.
• Scenario
• Tasks scheduled 8a and 9a.
• No cron run until 9:15a
• Both tasks will run at 9:15a
• Be aware of this when:
• Pushing to Web Services
• APIs

21. How to Wrangle WP_Cron
• 3rd party ping service (Pingdom)
• Setup an actual cron if available
http://domain.com/wp-cron.php?doing_wp_cron=1
• Limit threads with DB or file locks
• Know what crons are scheduled, and what they do
define('DISABLE_WP_CRON', true);

22. @cklosowski cklosows
Questions? Scheduling Your Tasks the WordPress Way