Tracking mode lets Redis invalidate a client’s local cache, which is a genuinely good feature and is not free in PHP.
what it needs:
a connection that stays open to receive invalidation
messages — which php-fpm does not have between requests
what that leaves:
a long-running worker, where a process-local cache
survives long enough to be worth invalidating
where we used it: nowhere. the workers that would benefit
read each key once per job.The feature suits an application server with a persistent process and a working set that is read repeatedly, which is a shape PHP under FPM does not have. Writing this down mattered because it comes up every time the release notes are read — the answer is not “it does not work”, it is “the process model does not give it anywhere to live”.