Difference between no-cache and must-revalidate

http cache-control

From the RFC 2616

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1

no-cache If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server. This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests.

So it directs agents to revalidate all responses.

Compared this to

must-revalidate When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server

So it directs agents to revalidate stale responses.

Particularly with regard to no-cache, is this how user agents actually, empirically treat this directive?

What's the point of no-cache if there's must-revalidate and max-age?

See this comment:

http://palpapers.plynt.com/issues/2008Jul/cache-control-attributes/

no-cache Though this directive sounds like it is instructing the browser not to cache the page, there’s a subtle difference. The “no-cache” directive, according to the RFC, tells the browser that it should revalidate with the server before serving the page from the cache. Revalidation is a neat technique that lets the application conserve band-width. If the page the browser has cached has not changed, the server just signals that to the browser and the page is displayed from the cache. Hence, the browser (in theory, at least), stores the page in its cache, but displays it only after revalidating with the server. In practice, IE and Firefox have started treating the no-cache directive as if it instructs the browser not to even cache the page. We started observing this behavior about a year ago. We suspect that this change was prompted by the widespread (and incorrect) use of this directive to prevent caching.

Has anyone got anything more official on this?

Update

The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation could result in incorrect operation, such as a silently unexecuted financial transaction.

That's something I've never taken to heart until now. The RFC is saying not to use must-revalidate lightly. The thing is, with web services, you have to take a negative view and assume the worst for your unknown client apps. Any stale resource has the potential to cause a problem.

And something else I've just considered, without Last-Modified or ETags, the browser can only fetch the whole resource again. However with ETags, I've observed that Chrome at least seems to revalidate on every request. Which makes both these directives moot or at least poorly named since they can't properly revalidate unless the request also includes other headers that then cause 'always revalidate' anyway.

I just want to make that last point clearer. By just setting must-revalidate but not including either an ETag or Last-Modified, the agent can only get the content again since it has nothing to send to the server to compare.

However, my empirical testing has shown that when ETag or modified header data is included in responses, the agents always revalidate anyway, regardless of the presence of the must-revalidate header.

So the point of must-revalidate is to force a 'bypass cache' when it goes stale, which can only happen when you have set a lifetime/age, thus if must-revalidate is set on a response with no age or other headers, it effectively becomes equivalent to no-cache since the response will be considered immediately stale.

-- So I'm going to finally mark Gili's answer!

So in theory the difference is validate-always vs validate-if-stale, whereas in practice no-cache gets treated by certain browsers as the comment you quoted says as never-validate … so you should make your choice of which of those to use based on what caching behavior you actually want to achieve in practice …

Please read greenbytes.de/tech/webdav/… and see whether this clarifies things for you.

possible duplicate of What's the difference between Cache-Control: max-age=0 and no-cache?

check this decision tree for answer stackoverflow.com/a/49925190/3748498

Anish B.

I believe that must-revalidate means :

Once the cache expires, refuse to return stale responses to the user even if they say that stale responses are acceptable.

Whereas no-cache implies :

must-revalidate plus the fact the response becomes stale right away.

If a response is cacheable for 10 seconds, then must-revalidate kicks in after 10 seconds, whereas no-cache implies must-revalidate after 0 seconds.

At least, that's my interpretation.

That's how I'm seeing it now. The interesting part is my last para, without an ETag or Last-Modified, the agent has nothing to use to validate what it has in cache and must download the whole payload again. So when the RFC says "revalidate" that probably means, re-fetch.

Which also means max-age=0, must-revalidate and no-cache are identical

@Anshul, at first I thought you were right that 'max-age=0, must-revalidate and no-cache are identical', but see Jeffrey Fox's answer which seems to indicate that's not quite right.

@Anshul No, must-revalidate and no-cache have different meaning for fresh responses: If a cached response is fresh (i.e, the response hasn't expired), must-revalidate will make the proxy serve it right away without revalidating with the server, whereas with no-cache the proxy must revalidate the cached response regardless of freshness. Source: "HTTP - The Definitive Guide", pages 182-183.

@MatthiasBraun Ah, I can see the source of confusion. May be I should have written no-cache and max-age=0, must-revalidate are identical

Jeffrey Fox

max-age=0, must-revalidate and no-cache aren't exactly identical. With must-revalidate, if the server doesn't respond to a revalidation request, the browser/proxy is supposed to return a 504 error. With no-cache, it would just show the cached content, which would be probably preferred by the user (better to have something stale than nothing at all). This is why must-revalidate is intended for critical transactions only.

Not sure about your no-cache interpretation. From the RFC 7234 The "no-cache" response directive indicates that the response MUST NOT be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to prevent a cache from using it to satisfy a request without contacting it, even by caches that have been configured to send stale responses. This sounds similar to restrictions for must-revalidate

Does Jeffrey have evidence that implementations behave how he has described?

I think this answer is correct for proxy / lb servers. But indeed, browsers don't return a 504 in that case.

So must-validate means must-refresh

developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control directly states "no-cache and max-age=0, must-revalidate indicates same meaning."

rogerdpack

With Jeffrey Fox's interpretation about no-cache, i've tested under chrome 52.0.2743.116 m, the result shows that no-cache has the same behavior as must-revalidate, they all will NOT use local cache when server is unreachable, and, they all will use cache while tap browser's Back/Forward button when server is unreachable. As above, i think max-age=0, must-revalidate is identical to no-cache, at least in implementation.

Will Chrome use local cache when the server is available to revalidate? (i.e. "If-Modified-Since"). In both cases?

D.jennis

Agreed with part of @Jeffrey Fox's answer:

max-age=0, must-revalidate and no-cache aren't exactly identical.

Not agreed with this part:

With no-cache, it would just show the cached content, which would be probably preferred by the user (better to have something stale than nothing at all).

What should implementations do when cache-control: no-cache revalidation failed is just not specified in the RFC document. It's all up to implementations. They may throw a 504 error like cache-control: must-revalidate or just serve a stale copy from cache.

Jason C

For what it's worth, the MDN page on HTTP validation directly addresses this (emphasis mine):

It is often stated that the combination of max-age=0 and must-revalidate has the same meaning as no-cache. Cache-Control: max-age=0, must-revalidate max-age=0 means that the response is immediately stale, and must-revalidate means that it must not be reused without revalidation once it is stale — so in combination, the semantics seem to be the same as no-cache. However, that usage of max-age=0 is a remnant of the fact that many implementations prior to HTTP/1.1 were unable to handle the no-cache directive — and so to deal with that limitation, max-age=0 was used as a workaround. But now that HTTP/1.1-conformant servers are widely deployed, there's no reason to ever use that max-age=0-and-must-revalidate combination — you should instead just use no-cache.

For reference (for our own personal cache control, heh) that MDN page was last updated on June 1, 2022; and I pulled that quote on June 10, 2022 (archive June 8).

Rich

I think there is a difference between max-age=0, must-revalidate and no-cache:

In the must-revalidate case the client is allowed to send a If-Modified-Since request and serve the response from cache if 304 Not Modified is returned.

In the no-cache case, the client must not cache the response, so should not use If-Modified-Since.

But no-cache does not mean no-store - with no-cache, the resource can still be cached in the client; it just must be re-validated before being used?

You are conflating no-cache and no-store. no-cache means the resource MUST be revalidated. Revalidate includes the option to use conditional requests, such as If-None-Match and If-Modified-Since.

@JulesRandolph: you may be right. Do you have any tests / demos? All the conflicting evidence-free assertions on this q are frustrating. Even the accepted answer just says "At least, that's my interpretation". I might set up a test bed and post it here if I get some time.

Difference between no-cache and must-revalidate

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