http缓存应该如何工作？

Question

I know this is actually a old thing but I really got questions about this and how it was supposed to work when it started. So I got at least these four important cache headers I can send back to the client ( Last-Modified , Cache-Control , Expires and ETag )

Example situation for question 2.2:

So let's say I got some site with articles. There might be a new article every 15 minutes to 7 days. So I set Cache-Control and Expires to cache for 15 min so the client always got the newest version of it.

What I think about ETag and Last-Modified at the moment:

I just put some hash of the content in there and I can send the client 304 if If-None-Match == ETag .

I can send the client 304 if If-Modified-Since >= Last-Modified .

Questions

1. Do I need Cache-Control and Expires to support all browsers or not ?
2. It looks like Cache-Control and Expires only tells my browser how long the content should be cached on the computer right?
   • So I can only use ETag and Last-Modified to find out when I should send 304 right?
   • So I could just set Cache-Control and Expires to forever and just send the client the new version if ETag or Last-Modified changed that?
     • Because this works in my browser but will this work in all browsers?
3. Do I need ETag and Last-Modified to support all browsers or not ?
4. Pragma looks like another cache header similar to Cache-Control , which browsers are using Pragma and do I need it?

Answer 1

Do I need Cache-Control and Expires to support all browsers or not ?

Cache-Control is introduced in HTTP 1.1 and hence only supported on clients supporting HTTP 1.1. Expires is supported since HTTP 1.0. If both are specified, then Cache-Control takes precedence and Expires is ignored altogether.

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It looks like Cache-Control and Expires only tells my browser how long the content should be cached on the computer right?

Basically, yes. If the Cache-Control is absent (and thus Expires will be used) or set right with max-age .

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So I can only use ETag and Last-Modified to find out when I should send 304 right?

Here, your question starts to be confusing. You actually need to check If-None-Match and If-Modified-Since headers for that. Usually, browser sends only one of them on a conditional GET request and if it matches with the current value of ETag or Last-Modified in the server, then return 304 along with the ETag header. Note that if browser sends them both for some reason, then you can just ignore If-Modified-Since .

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So I could just set Cache-Control and Expires to forever and just send the client the new version if ETag or Last-Modified changed that?

No. If you set it to forever, the client will never send a (conditional) request until the enduser pressed (Ctrl)+F5. You need to set the Cache-Control: max-age or Expires to the desired cache time (to 15 minutes in the future maybe?).

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Because this works in my browser but will this work in all browsers?

Watch out how you test: pressing F5 will always send a conditional GET request. But a simple page-to-page navigation and form submits (like as in real world) won't send a conditional GET request! Pressing Ctrl+F5 will ignore the cache and send a brand new GET request.

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Do I need ETag and Last-Modified to support all browsers or not ?

All browsers support both. Browsers are required to send ETag back via If-None-Match if it has been received from the server and browser needs to perform a conditional GET request. The Last-Modified is for them optional and usually only sent if ETag is absent on the resource. See also RFC 2616 section 13.3.4 .

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Pragma looks like another cache header similar to Cache-Control, which browsers are using Pragma and do I need it?

You don't need it if you want to cache. You only need it if you want to not cache (in order to cover HTTP 1.0 clients/proxies, whose population is currently steadily decreasing, though).

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If you're familiar with Java, or can at least decipher it, then you may find this article helpful: FileServiet supporting resume and caching and GZIP .

See also:

• Caching tutorial for webmasters
• Caching in HTTP (RFC2616 section 13)