[http] 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?

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/

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.

In my view, With Expire Header, server can tell the client when my data would be stale, while with Etag, server would check the etag value for client' each request.

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/

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.

ETag is used to determine whether a resource should use the copy one. and Expires Header like Cache-Control is told the client that before the cache decades, client should fetch the local resource.

In modern sites, There are often offer a file named hash, like app.98a3cf23.js, so that it's a good practice to use Expires Header. Besides this, it also reduce the cost of network.

Hope it helps ;)

In my view, With Expire Header, server can tell the client when my data would be stale, while with Etag, server would check the etag value for client' each request.

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.

ETag is used to determine whether a resource should use the copy one. and Expires Header like Cache-Control is told the client that before the cache decades, client should fetch the local resource.

In modern sites, There are often offer a file named hash, like app.98a3cf23.js, so that it's a good practice to use Expires Header. Besides this, it also reduce the cost of network.

Hope it helps ;)

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!

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.