This week in caching: a triage checklist for 'I deployed but users see old content'

This week in caching: a triage checklist for 'I deployed but users see old content'

Work the layers outward — the bug is almost never where you look first.

— Start at the browser — hard-reload and check if it's gone; if so, your HTML had a long max-age it shouldn't. Fix the document Cache-Control.
— Then the CDN — check the Age header; a high Age means the edge is still serving the old object and your deploy purge didn't fire.
— Then full-page cache — Varnish/nginx/plugin may not have purged; confirm the cache-status header reads MISS after deploy.
— Then OPcache — with validate_timestamps=0, old PHP runs until you reset OPcache; this is the sneakiest one.
— Last, the object cache — a stale serialized object in Redis can outlive the code that changed; flush the relevant keys, not everything.

Credit to countless post-mortems naming OPcache as the layer everyone forgets.

Bookmark: a one-page 'cache layers, outermost to innermost' diagram — tape it to your deploy doc.