ETag vs Header Expires
I've looked around but haven't been able to figure out if I should use both an ETag and an Expires Header or one or the other.
What I'm trying to do is make sure that my flash files (and other images and what not only get updated when there is a change to those files.
I don't want to do anything special like changing the filename or putting some weird chars on the end of the url to make it not get cached.
Also, is there anything I need to do programatically on my end in my PHP scripts to support this or is it all Apache?
Solution 1:
They are slightly different - the ETag does not have any information that the client can use to determine whether or not to make a request for that file again in the future. If ETag is all it has, it will always have to make a request. However, when the server reads the ETag from the client request, the server can then determine whether to send the file (HTTP 200) or tell the client to just use their local copy (HTTP 304). An ETag is basically just a checksum for a file that semantically changes when the content of the file changes.
The Expires header is used by the client (and proxies/caches) to determine whether or not it even needs to make a request to the server at all. The closer you are to the Expires date, the more likely it is the client (or proxy) will make an HTTP request for that file from the server.
So really what you want to do is use BOTH headers - set the Expires header to a reasonable value based on how often the content changes. Then configure ETags to be sent so that when clients DO send a request to the server, it can more easily determine whether or not to send the file back.
One last note about ETag - if you are using a load-balanced server setup with multiple machines running Apache you will probably want to turn off ETag generation. This is because inodes are used as part of the ETag hash algorithm which will be different between the servers. You can configure Apache to not use inodes as part of the calculation but then you'd want to make sure the timestamps on the files are exactly the same, to ensure the same ETag gets generated for all servers.
Solution 2:
Etag and Last-modified headers are validators.
They help the browser and/or the cache (reverse proxy) to understand if a file/page, has changed, even if it preserves the same name.
Expires and Cache-control are giving refresh information.
This means that they inform, the browser and the reverse in-between proxies, up to what time or for how long, they may keep the page/file at their cache.
So the question usually is which one validator to use, etag or last-modified, and which refresh infomation header to use, expires or cache-control.
Solution 3:
Expires and Cache-Control are "strong caching headers"
Last-Modified and ETag are "weak caching headers"
First the browser check Expires/Cache-Control to determine whether or not to make a request to the server
If have 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.
Solution 4:
Another summary:
You need to use both. ETags are a "server side" information. Expires are a "Client side" caching.
Use ETags except if you have a load-balanced server. They are safe and will let clients know they should get new versions of your server files every time you change something on your side.
Expires must be used with caution, as if you set a expiration date far in the future but want to change one of the files immediatelly (a JS file for instance), some users may not get the modified version until a long time!
Solution 5:
One additional thing I would like to mention that some of the answers may have missed is the downside to having both ETags and Expires/Cache-control in your headers.
Depending on your needs it may just add extra bytes in your headers which may increase packets which means more TCP overhead. Again, you should see if the overhead of having both things in your headers is necessary or will it just add extra weight in your requests which reduces performance.
You can read more about it on this excellent blog post by Kyle Simpson: http://calendar.perfplanet.com/2010/bloated-request-response-headers/