[http] Difference between Pragma and Cache-Control headers?

I read about Pragma header on Wikipedia which says:

"The Pragma: no-cache header field is an HTTP/1.0 header intended for use in requests. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource, not for the server to tell the browser not to cache the resource. Some user agents do pay attention to this header in responses, but the HTTP/1.1 RFC specifically warns against relying on this behavior."

But I haven't understood what it does? What is the difference between the Cache-Control header whose value is no-cache and Pragma whose value is also no-cache?

If it's after 1999, and you're still using Expires or Pragma, you're doing it wrong.

I'm looking at you Stackoverflow:

200 OK
Pragma: no-cache
Content-Type: application/json
X-Frame-Options: SAMEORIGIN
X-Request-Guid: a3433194-4a03-4206-91ea-6a40f9bfd824
Strict-Transport-Security: max-age=15552000
Content-Length: 54
Accept-Ranges: bytes
Date: Tue, 03 Apr 2018 19:03:12 GMT
Via: 1.1 varnish
Connection: keep-alive
X-Served-By: cache-yyz8333-YYZ
X-Cache: MISS
X-Cache-Hits: 0
X-Timer: S1522782193.766958,VS0,VE30
Vary: Fastly-SSL
X-DNS-Prefetch-Control: off
Cache-Control: private

tl;dr: Pragma is a legacy of HTTP/1.0 and hasn't been needed since Internet Explorer 5, or Netscape 4.7. Unless you expect some of your users to be using IE5: it's safe to stop using it.

• Expires: [date] (deprecated - HTTP 1.0)
• Pragma: no-cache (deprecated - HTTP 1.0)
• Cache-Control: max-age=[seconds]
• Cache-Control: no-cache (must re-validate the cached copy every time)

And the conditional requests:

• Etag (entity tag) based conditional requests
  • Server: Etag: W/“1d2e7–1648e509289”
  • Client: If-None-Match: W/“1d2e7–1648e509289”
  • Server: 304 Not Modified
• Modified date based conditional requests
  • Server: last-modified: Thu, 09 May 2019 19:15:47 GMT
  • Client: If-Modified-Since: Fri, 13 Jul 2018 10:49:23 GMT
  • Server: 304 Not Modified

last-modified: Thu, 09 May 2019 19:15:47 GMT

There is no difference, except that Pragma is only defined as applicable to the requests by the client, whereas Cache-Control may be used by both the requests of the clients and the replies of the servers.

So, as far as standards go, they can only be compared from the perspective of the client making a requests and the server receiving a request from the client. The http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.32 defines the scenario as follows:

HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in HTTP.

  Note: because the meaning of "Pragma: no-cache as a response
  header field is not actually specified, it does not provide a
  reliable replacement for "Cache-Control: no-cache" in a response

The way I would read the above:

• if you're writing a client and need no-cache:

  • just use Pragma: no-cache in your requests, since you may not know if Cache-Control is supported by the server;
  • but in replies, to decide on whether to cache, check for Cache-Control

• if you're writing a server:

  • in parsing requests from the clients, check for Cache-Control; if not found, check for Pragma: no-cache, and execute the Cache-Control: no-cache logic;
  • in replies, provide Cache-Control.

Of course, reality might be different from what's written or implied in the RFC!