I’ve been working on a third party API plugin for WordPress that requires a script that auto updates from the API. It consists of pulling a big list of items from the API and checking them against the local site entries, often adding new items or updating existing entries.
The problem was with a number of memory allocation errors with PHP as the script went on, often about half way through the execution. Memory exhausted and Fatal Allowed memory issues popping up. A memory limit change would fix the problem to a point, it would still leave issues though – to work through the full list of items would need a very high PHP memory limit and also what happens if the list gets bigger in future? When 64Mb stops being enough do we simply up it to 128Mb what about when that isn’t enough do we carry on upping it? That still leaves a period where errors are being thrown and updates are missed until it is picked up and the memory limit is upped.

I made sure I called wordpress without themes to reduce a little bit of load and a quick check through to make sure unset() was being used as it should be – I’ll admit my code discipline has been lax in the past with regards to this but as ever we are always learning and making sure that we can improve our techniques. Unsetting things properly to free up the memory is one of those things I now make sure I’m doing rather than just overwriting old variables.
So the script was unsetting as it should and with diligent debugging with php memory usage outputs it became apparent that it was around WordPress functions that the memory usage was going up and not being unset, gradually leading to the errors. Plenty of looking around and eventually I stumbled upon wp_cache_flush and it worked a treat – every time I went around a processing loop or finished a process I called the cache flush and it removed all the stuff WordPress had been storing. I’m sure this wouldn’t be the best fix all for every situation but it’s certainly something to be well aware of.