Sweet Baby

By default, most scripts won’t return a validator (a Last-Modified or ETag response header) or freshness information (Expires or Cache-Control). While some scripts really are dynamic (meaning that they return a different response for every request), many (like search engines and database-driven sites) can benefit from being cache-friendly.

Generally speaking, if a script produces output that is reproducible with the same request at a later time (whether it be minutes or days later), it should be cacheable. If the content of the script changes only depending on what’s in the URL, it is cacheable; if the output depends on a cookie, authentication information or other external criteria, it probably isn’t.

The best way to make a script cache-friendly (as well as perform better) is to dump its content to a plain file whenever it changes. The Web server can then treat it like any other Web page, generating and using validators, which makes your life easier. Remember to only write files that have changed, so the Last-Modified times are preserved.

Another way to make a script cacheable in a limited fashion is to set an age-related header for as far in the future as practical. Although this can be done with Expires, it’s probably easiest to do so with Cache-Control: max-age, which will make the request fresh for an amount of time after the request.

If you can’t do that, you’ll need to make the script generate a validator, and then respond to If-Modified-Since and/or If-None-Match requests. This can be done by parsing the HTTP headers, and then responding with 304 Not Modified when appropriate. Unfortunately, this is not a trival task.

Some other tips;

Don’t use POST unless it’s appropriate. Responses to the POST method aren’t kept by most caches; if you send information in the path or query (via GET), caches can store that information for the future.

Don’t embed user-specific information in the URL unless the content generated is completely unique to that user.

Don’t count on all requests from a user coming from the same host, because caches often work together.

Generate Content-Length response headers. It’s easy to do, and it will allow the response of your script to be used in a persistent connection. This allows clients to request multiple representations on one TCP/IP connection, instead of setting up a connection for every request. It makes your site seem much faster.

See the Implementation Notes for more specific information.