Do cached assets count?

We test all creatives with a completely cleared cache. Testing without cache is the only fair measurement as the results otherwise would be highly unpredictable because more or less all browsers viewing your site will have a different cache at the time of their visit.

One also needs to understand that a server can never dictate to a browser whether something should be cached or not. Caching is a voluntary client-side optimisation that the browser implements. In our opinion it should not be used as a reason to further increase the size of already incredibly bloated ads.

The server can make recommendations to the browser caching engine by specifying how long an asset can be kept in the cache. This is done by the Cache-Control header sent as part of the response. By looking at the response headers, the caching recommendation from the server can be determined.

If a server sends a max-age of say one week for a resource, most browsers will keep it in cache for that long - without reconfirming the resource with the server. This does save some bandwidth and connection overhead but it does absolutely nothing for the much bigger problem of JS bloat and execution performance.

Our recommendation: We do not recommend exempting javascript load sizes on the account they might be cached. If a publisher was to exempt all cached libraries from Sizmek, DCM and other vendors it would easily add up to a 100Mb+ list — which would effectively nullify creative weight limits altogether.

Please note: Weight limits are not only about download and bandwidth, they are equally (and sometime more so) about javascript execution and performance, which caching does not help with.

Whether or not a resource can be cached is always up to the visitor's browser, and it makes its decision based on several factors such as available disk space, optimisations and headers sent by the server which tell the browser when to update the resource.

Happy validating!