Network Working Group M. Nottingham
Internet-Draft Yahoo! Inc.
Intended status: Informational May 9, 2008
Expires: November 10, 2008
The stale-while-revalidate HTTP Cache-Control Extension
draft-nottingham-http-stale-while-revalidate-01
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Abstract
The stale-while-revalidate HTTP response Cache-Control extension
allows servers to instruct caches to serve stale responses while
validating them, to avoid latency in some situations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. The stale-while-revalidate Cache-Control Extension . . . . . . 3
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
7. Normative References . . . . . . . . . . . . . . . . . . . . . 4
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . . . 6
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1. Introduction
The potential for latency (due to the network as well as server
processing) introduced by cache validation in HTTP [RFC2616] is often
undesirable; while subsequent requests can be served from the cache
quickly, the request that triggers validation sees degraded service.
In some situations, it may be useful to avoid this latency, at the
cost of serving slightly stale responses. the stale-while-revalidate
HTTP response Cache-Control extension allows caches to do this.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This specification uses the augmented Backus-Naur Form of RFC2616
[RFC2616], and includes the delta-seconds rule from that
specification.
3. The stale-while-revalidate Cache-Control Extension
When present in an HTTP response, the stale-while-revalidate Cache-
Control extension indicates that caches MAY serve the response it
appears in after it becomes stale, up to the indicated number of
seconds.
stale-while-revalidate = "stale-while-revalidate" "=" delta-seconds
If a cached response is served stale due to the presence of this
extension, the cache SHOULD attempt to revalidate it while still
serving stale responses (i.e., without blocking).
Note that 'stale' implies that the response will have a non-zero Age
header and a warning header, as per HTTP's requirements.
If delta-seconds passes without the cached entity being revalidated,
it MUST NOT continue to be served stale, absent other information.
4. Example
A response containing:
Cache-Control: max-age=600, stale-while-revalidate=30
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indicates that it is fresh for 600 seconds, and it may continue to be
served stale for up to 30 seconds while an asynchronous validation is
attempted. If validation is inconclusive, or if there is not traffic
that triggers it, after 30 seconds the stale-while-revalidate
function will cease to operate, and the cached response will be
"truly" stale (i.e., the next request will block and be handled
normally).
Generally, servers will want to set the combination of max-age and
stale-while-revalidate to the longest total potential freshness
lifetime that they can tolerate. For example, with both set to 600,
the server must be able to tolerate the response being served from
cache for up to 20 minutes.
Since asynchronous validation will only happen if a request occurs
after the response has become stale, but before the end of the stale-
while-revalidate window, the size of that window and the likelihood
of a request during it determines how likely it is that all requests
will be served without delay. if the window is too small, or traffic
too sparse, some requests will fall outside of it, and block until
the server can validate the cached response.
5. Security Considerations
This document provides origin servers with a mechanism for dictating
that stale content should be served from caches under certain
circumstances, with the expectation that the cached response will be
revalidated in the background. It is suggested that such validation
be predicated upon an incoming request, to avoid the possibility of
an amplification attack (as can be seen in some other pre-fetching
and automatic refresh mechanisms). Cache implementers should keep
this in mind when deciding the circumstances under which they will
generate a request that is not directly initiated by a user or
client.
6. IANA Considerations
This document has no actions for IANA.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
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Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
Appendix A. Acknowledgements
Thanks to Ben Drees, John Nienart, Henrik Nordstrom, Evan Torrie, and
Chris Westin for their suggestions. The author takes all
responsibility for errors and omissions.
Author's Address
Mark Nottingham
Yahoo! Inc.
Email: mnot@yahoo-inc.com
URI: http://www.mnot.net/
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