Server-side counting on a fully cached site

We took a client site to a full-page cache with a ninety-eight per cent hit rate last spring. Load times halved. Analytics fell off a cliff, and the cliff was our fault.

The tracking had been running in a WordPress hook. When the cache started answering almost every request without waking WordPress up, almost every request stopped being counted. The reports looked like a traffic collapse, which is a fun thing to explain on a Monday.

The fix was to move the counting to something that runs regardless. The web server sees every request whether the response came from cache or not, and it can hand off a minimal record without touching the application.

It is worth checking on any site with aggressive caching. Compare a day of analytics against a day of server logs. If the two differ by more than a few per cent, the cache is answering requests nothing is counting.