What is Cache-Control: private?

Question

When I visit chesseng.herokuapp.com I get a response header that looks like

Cache-Control:private
Connection:keep-alive
Content-Encoding:gzip
Content-Type:text/css
Date:Tue, 16 Oct 2012 06:37:53 GMT
Last-Modified:Tue, 16 Oct 2012 03:13:38 GMT
Status:200 OK
transfer-encoding:chunked
Vary:Accept-Encoding
X-Rack-Cache:miss

and then I refresh the page and get

Cache-Control:private
Connection:keep-alive
Date:Tue, 16 Oct 2012 06:20:49 GMT
Status:304 Not Modified
X-Rack-Cache:miss

so it seems like caching is working. If that works for caching then what is the point of Expires and Cache-Control:max-age. To add to confusion, when I test the page at https://developers.google.com/speed/pagespeed/insights/ it tells me to "Leverage browser caching".

Answer 1

To answer your question about why caching is working, even though the web-server didn't include the headers:

• Expires: [a date]
• Cache-Control: max-age=[seconds]

The server kindly asked any intermediate proxies to not cache the contents (i.e. the item should only be cached in a private cache, i.e. only on your own local machine):

• Cache-Control: private

But the server forgot to include any sort of caching hints:

• they forgot to include Expires, so the browser knows to use the cached copy until that date
• they forgot to include Max-Age, so the browser knows how long the cached item is good for
• they forgot to include E-Tag, so the browser can do a conditional request

But they did include a Last-Modified date in the response:

Last-Modified: Tue, 16 Oct 2012 03:13:38 GMT

Because the browser knows the date the file was modified, it can perform a conditional request. It will ask the server for the file, but instruct the server to only send the file if it has been modified since 2012/10/16 3:13:38:

GET / HTTP/1.1
If-Modified-Since: Tue, 16 Oct 2012 03:13:38 GMT

The server receives the request, realizes that the client has the most recent version already. Rather than sending the client 200 OK, followed by the contents of the page, instead it tells you that your cached version is good:

304 Not Modified

Your browser did have to suffer the delay of sending a request to the server, and wait for a response, but it did save having to re-download the static content.

Why Max-Age? Why Expires?

Because Last-Modified sucks.

Not everything on the server has a date associated with it. If I'm building a page on the fly, there is no date associated with it - it's now. But I'm perfectly willing to let the user cache the homepage for 15 seconds:

200 OK
Cache-Control: max-age=15

If the user hammers F5, they'll keep getting the cached version for 15 seconds. If it's a corporate proxy, then all 67198 users hitting the same page in the same 15-second window will all get the same contents - all served from close cache. Performance win for everyone.

The virtue of adding Cache-Control: max-age is that the browser doesn't even have to perform a conditional request.

• if you specified only Last-Modified, the browser has to perform a request If-Modified-Since, and watch for a 304 Not Modified response
• if you specified max-age, the browser won't even have to suffer the network round-trip; the content will come right out of the caches

The difference between "Cache-Control: max-age" and "Expires"

Expires is a legacy equivalent of the modern (c. 1998) Cache-Control: max-age header:

• Expires: you specify a date (yuck)
• max-age: you specify seconds (goodness)

• And if both are specified, then the browser uses max-age:

  200 OK
  Cache-Control: max-age=60
  Expires: 20180403T192837 
  

Any web-site written after 1998 should not use Expires anymore, and instead use max-age.

What is ETag?

ETag is similar to Last-Modified, except that it doesn't have to be a date - it just has to be a something.

If I'm pulling a list of products out of a database, the server can send the last rowversion as an ETag, rather than a date:

200 OK
ETag: "247986"

My ETag can be the SHA1 hash of a static resource (e.g. image, js, css, font), or of the cached rendered page (i.e. this is what the Mozilla MDN wiki does; they hash the final markup):

200 OK
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

And exactly like in the case of a conditional request based on Last-Modified:

GET / HTTP/1.1
If-Modified-Since: Tue, 16 Oct 2012 03:13:38 GMT

304 Not Modified

I can perform a conditional request based on the ETag:

GET / HTTP/1.1
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

304 Not Modified

An ETag is superior to Last-Modified because it works for things besides files, or things that have a notion of date. It just is

Answer 2

Cache-Control: private

Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache, such as a proxy server.

From RFC2616 section 14.9.1

Answer 3

RFC 2616, section 14.9.1:

Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache...A private (non-shared) cache MAY cache the response.

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Browsers could use this information. Of course, the current "user" may mean many things: OS user, a browser user (e.g. Chrome's profiles), etc. It's not specified.

For me, a more concrete example of Cache-Control: private is that proxy servers (which typically have many users) won't cache it. It is meant for the end user, and no one else.

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FYI, the RFC makes clear that this does not provide security. It is about showing the correct content, not securing content.

This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content.

Answer 4

The Expires entity-header field gives the date/time after which the response is considered stale.The Cache-control:maxage field gives the age value (in seconds) bigger than which response is consider stale.

Althought above header field give a mechanism to client to decide whether to send request to the server. In some condition, the client send a request to sever and the age value of response is bigger then the maxage value ,dose it means server needs to send the resource to client? Maybe the resource never changed.

In order to resolve this problem, HTTP1.1 gives last-modifided head. The server gives the last modified date of the response to client. When the client need this resource, it will send If-Modified-Since head field to server. If this date is before the modified date of the resouce, the server will sends the resource to client and gives 200 code.Otherwise,it will returns 304 code to client and this means client can use the resource it cached.