Cachability Tests

Occasionally, we get asked questions like:

• ``how many requests for my pages are served as cache hits?''
• ``What factor could I multiply my access log counts by to include requests served as hits from caches?''

To start trying to answer this, we set up a simple experiment. We added six small images to our home page. Three of them are cachable, and three of them are uncachable. For the cachable ones, we do nothing special. They have a Last-Modified header, but no Expires header. For the uncachable objects, we add two special headers:

Expires: 0
Cache-control: no-cache

We are interested to see how the requests for the cachable images compare to the uncachable requests. Some preliminary results are shown below.

The plot below shows number of requests per day for the six objects.

This plot shows the ``cachability factor.'' We could multiply our cachable access counts by this amount to account for cache hits which our server never sees.