Difference between Pragma and Cache-Control headers?
HttpHttp HeadersRequestProtocolsHttpresponseHttp Problem Overview
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
?
Http Solutions
Solution 1 - Http
Pragma
is the HTTP/1.0 implementation and cache-control
is the HTTP/1.1 implementation of the same concept. They both are meant to prevent the client from caching the response. Older clients may not support HTTP/1.1 which is why that header is still in use.
Solution 2 - Http
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 ifCache-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 forPragma: no-cache
, and execute theCache-Control: no-cache
logic; -
in replies, provide
Cache-Control
.
Of course, reality might be different from what's written or implied in the RFC!
Solution 3 - Http
Stop using (HTTP 1.0) | Replaced with (HTTP 1.1 since 1999) |
---|---|
Expires: [date] | Cache-Control: max-age=[seconds] |
Pragma: no-cache | Cache-Control: 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
- Server:
- 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
- Server:
last-modified: Thu, 09 May 2019 19:15:47 GMT