Eight years ago Yahoo reported on the browser cache experience of their users. The results showed more empty cache views than were expected:
40-60% of Yahoo!’s users have an empty cache experience and ~20% of all page views are done with an empty cache.
Earlier this year Facebook recreated that experiment: Web performance: Cache efficiency exercise.
Here are a few key data points:
- 24.8% of desktop requests and 26.9% of mobile were missing the cached image.
- On average, 44.6% of users are getting an empty cache.
- Mobile hits shows there is a 50% chance that a request will have a cache that is at most 12 hours old.
- There is a 42% chance that any request will have a cache that is, at most, 47 hours old on the desktop.
Overall our cache hit rate looks like it has improved since 2007. If we ignore Firefox v32 and newer (where we cannot log some cache hits), then the cache hit rate goes to 84.1%, up from about 80% in 2007. On the other hand, caches don’t stay populated for very long.
The bottom line: setting proper cache headers is good, but it doesn’t guarantee that browsers hitting your site will still have that content in their cache.