Re: dont-revalidate Cache-Control header
Ben Maurer <ben.maurer@gmail.com> Thu, 16 July 2015 15:36 UTC
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Date: Thu, 16 Jul 2015 16:32:33 +0100
Message-ID: <CABgOVaLnpnmd7JvY6O=tXXboVuvCCn-p1KLzu8wKVkg-yon79w@mail.gmail.com>
From: Ben Maurer <ben.maurer@gmail.com>
To: Amos Jeffries <squid3@treenet.co.nz>
Cc: HTTP Working Group <ietf-http-wg@w3.org>
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Subject: Re: dont-revalidate Cache-Control header
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Thanks for the feedback! On Thu, Jul 16, 2015 at 1:35 PM, Amos Jeffries <squid3@treenet.co.nz> wrote: > > changes then giving each such URI an extremely distant expiration date. > > > > The static HTTP Cache-Control extension clarifies that a resource is > > guaranteed never to change and allows caches to optimize based on these > > semantics. For example, it allows user agents to avoid revalidating > static > > resources when a user presses the reload button. It also signals to > caches > > that the expiration date of the object may be set further in the future > > than the actual expected lifetime of the object. > > I dont think that last statement is correct. "may be set" implies that > heuristic lifetimes are applicable. But this control is explicitly > setting maximum lifetime when Expires/max-age/s-maxage are absent. That > is not a heuristic estimation but the absolute "infinity" value for the > cache. What I was trying to say here is that if I have a resource foo-v1.js with a lifetime of 1 year, I'm not actually expecting the resource to be useful for 1 year, merely that the value of the resource won't change. "static" might help signal this and change cache ranking algorithms. That said, this was a side note, and I don't know of anybody who uses expiration dates in this way, and could probably be deleted. > > > > 2. The static Cache-Control extension > > > > When present in an HTTP response, the static Cache-Control extension > > indicates that the semantic content of the response will never change in > > the future. A server MUST NOT either in the past or future serve > different > > semantic content for the same URI. If a server accidentally serves > > different content on the URI, it MUST alter all resources that reference > > that URI to reference a different URI. A server MAY either in the past or > > future serve an error response for the URI. The static cache-control > header > > MUST be used with either the "public" or "private" cache-control > directive. > > Why? content is always either public or private no matter what > Cache-Controls are used. Yeah, this may be better removed. > > > It MUST NOT be used in combination with "no-cache", "no-store", or > > "must-revalidate". > > Or proxy-revalidate, or stale-while-revalidate, ... and the as yet > undefined ones? > > Also, what if it does happen? Effectively any combination of cache > controls can be sent. > > IMHO its probably best to say that when this control is present in > responses any other controls causing revalidation MUST NOT be generated > by senders, and recipients must ignore such revalidation controls. With > the list of named controls just an example set. > Personally, I'd prefer to have static overridden by by other headers -- better to be more conservative and and make the browser refresh more. > > > > The server MUST send a max-age directive and SHOULD use > > a delta-age of at least 30 days. > > Why the MUST? "static" by itself could mean caching for maximum lifetime > permitted. (ie ~68 years). > > The SHOULD and delta-age seems arbitrary. I thought the intent of > "static" was to prevent heuristic cache expiry/revalidation limits being > applied anyways. > I guess it doesn't need to be a must, but it seems like a poor decision for one to have a short lifetime that would apply to UAs that did not implement this extension. Maybe it should be "SHOULD specify a long max-age (eg, 30 days or more) for the sake of caches that have not implemented this extension". > > > > > A cache MUST treat a response with the static Cache-Control extension as > > having the maximum allowable lifetime for that cache. > > There you go. :-) the max-age bit conflicts here. The idea is that static should take precedence over max-age. You need to support max-age for legacy caches. I could have been more clear here. > > The cache SHOULD NOT > > attempt to revalidate the response. > > s/SHOULD NOT/MUST NOT/ and this one line encompasses almost all the > requirements about revalidation controls. > I debated about making this MUST NOT. Using MUST NOT would conflict with any type of heuristics (in the security considerations section) s/by the refreshing/ by reloading/ > > > extension when a URI is directly navigated to by a user rather than > > referenced by another page. > > Lots of MUST criteria, then a giant loophole of MAY ignore it all is a > bit rough. All the non-browser agents including middleware/shared caches > either cannot identify a "directly navigated" URL (or consider > *everything* as directly navigated) anyways so the MAY is just setting > up a worse problem of conflicting cache behaviour between software. > > Probably best to leave the client sent Cache-Control:max-age=0 (aka > force-reload) control operational as a non-conditional fetch. This is > already implied by the text at the end of section 2. > You're right that I hadn't been thinking about this with the perspective of browser with an intermediate cache. I guess the behavior I'm suggesting here is that a browser should treat a refresh on the main resource the same as it does today (send a max-age=0 request), however it should *not* do this for sub-resources that have the static extension. Open to other suggestions about behavior here -- mainly I want to provide a safety net if a hack/mistake were to make www.foo.com/ return a corrupt document with a static caching flag.
- dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Amos Jeffries
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Zhong Yu
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Mark Nottingham
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Adam Rice
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Ilya Grigorik
- Re: dont-revalidate Cache-Control header Bryan McQuade
- Re: dont-revalidate Cache-Control header Amos Jeffries
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Mark Nottingham
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Ilya Grigorik
- Re: dont-revalidate Cache-Control header Roy T. Fielding
- Re: dont-revalidate Cache-Control header Ilya Grigorik
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Martin Thomson
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Amos Jeffries
- Re: dont-revalidate Cache-Control header Mark Nottingham
- Re: dont-revalidate Cache-Control header Mark Nottingham
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Mark Nottingham
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Amos Jeffries
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Martin Thomson
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Martin Thomson
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Martin Thomson
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Roy T. Fielding
- Re: dont-revalidate Cache-Control header Ben Maurer
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Amos Jeffries
- Re: dont-revalidate Cache-Control header Guille -bisho-
- Re: dont-revalidate Cache-Control header Amos Jeffries