I am running PHP on Apache, and am confused about how to implement server-side caching, in order to make the site load faster.
What is the difference between the
ETag headers, and which one should be used in what situation?
You can use the
Expires header in conjunction but regardless of the other two. It’s universally supported by proxies and browser caches.
The difference between
Last-Modified stamps is more semantic. ETags are opaque to clients. It’s usually a checksum. Whereas a Last-Modified header can be interpreted by clients. It’s understood that the last modified timestamp works linearly.
If a browser requests a resource with
If-Unmodified-Since, then a wide range of timestamps in the past can match such a condition. If your pages change frequently then a Last-Modified timestamp might be advantageous.
The ETag approach, on the other hand, leads to clients that save one last fingerprint per resource. (I’m not sure if browser caches remember multiple ETags). On requests, only one or a few possible
If-None-Match tokens are listed. This might mean more misses. Also, you have to compare multiple checksums, whereas with a Last-Modified timestamp you could have an arithmetic comparison.
The real advantage of ETags is that you can reliably compare fingerprints. The Last-Modified timestamps are a bit more vague, as they don’t verify if the actual page content changed.
Cache-Control are “strong caching headers”
ETag are “weak caching headers”
First the browser checks
Expires/Cache-Control to determine whether or not to make a request to the servers.
If it has to make a request, it will send
Last-Modified/ETag in the HTTP request. If the
Etag value of the document matches that, the server will send a 304 code instead of 200, and no content. The browser will load the contents from its cache.
I recommend using one of the strong caching headers, along with one of the weak caching headers.