Pull Request for Issue joomla-projects/soc21_website-cronjob#59

Pinging @ditsuke and @bembelimen since Scheduled Tasks is their baby and they're currently the only ones who can understand this PR.

Summary of Changes

Scheduled tasks can now return Status::WILL_RESUME to indicate they are not done and should be rescheduled immediately to resume execution.

This is useful for whenever you have long running tasks which would otherwise end up hitting a memory or timeout limit on your server. Some examples of such tasks:

• Sending a newsletter with thousands of subscribers
• Batch processing hundreds to thousands of images
• Synchronising folder contents across different servers or with external services (“poor man's rsync”)

Testing Instructions

The “Task - Demo Tasks!” plugin has been updated with a new sample task called “Resumable task”.

Create a new task using Resumable Task.

Set it to run once every 10 minutes.

Enable Individual Task Logs and set the Log Filename to resumable.log.php.

Trigger the task scheduler. On each execution, the task has a 60% chance of resuming and 40% chance of finishing.

Actual result BEFORE applying this Pull Request

N/A — Joomla did not support resumable tasks.

Expected result AFTER applying this Pull Request

The administrator/logs/resumable.log.php file will contain something like this:

2022-01-16	13:21:15	INFO	Running task#01 'Resumable Example'.
2022-01-16	13:21:15	INFO	Task> Starting new task 1
2022-01-16	13:21:15	INFO	Task> Starting 1s timeout
2022-01-16	13:21:16	INFO	Task> 1s timeout over!
2022-01-16	13:21:16	INFO	Task> Task 1 will resume
2022-01-16	13:21:16	INFO	Task#01 needs to perform more work.
2022-01-16	13:21:23	INFO	Running task#01 'Resumable Example'.
2022-01-16	13:21:23	INFO	Task> Resuming task 1
2022-01-16	13:21:23	INFO	Task> Starting 1s timeout
2022-01-16	13:21:24	INFO	Task> 1s timeout over!
2022-01-16	13:21:24	INFO	Task> Task 1 is now complete
2022-01-16	13:21:24	INFO	Successfully finished task#01 in 1.00 (net 1.00) seconds.

As you can see the task can detect whether it's being resumed.

Documentation Changes Required

N/A. There is no documentation on scheduled tasks beyond the demo tasks plugin. The new resumable demo task is self–documenting.

Postscriptum

I have still not managed to find time to integrate scheduled tasks in my own software. I did have, however, the time to study the code and understand better how Scheduled Tasks work. I can already see that they are very appealing. I am looking forward to implementing an integration with them in the next minor versions of my software tentatively scheduled for early Q2 2022. If lazy scheduling works in real world sites as reliably as it does on my test servers it will be a veritable game changer for Joomla!. That's the first time in years I am ecstatic about new code added to Joomla, let alone code added in a minor release. Well done!