Does Varnish uses ETag?

[Geoff Simmons]

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On 7/7/11 11:14 PM, Kurt Kraut wrote:
> 
> Does Varnish used ETag to check cache freshness?

Mainline Varnish doesn't, but there's an experimental branch
'experimental-ims' on the source repository in which it's implemented.

> - If yes, how does it behave?

http://www.varnish-cache.org/trac/wiki/BackendConditionalRequests

As per the RFC, freshness checks against ETags are done with conditional
requests to the backend using If-None-Match.

> - If not, is it possible to achive through VCL?

I don't think so, because standard VCL doesn't have a way to access an
object in the cache that has an expired TTL, but otherwise matches the
request, and hence could be refreshed.

So you don't ordinarily know an ETag in VCL that could be used for the
refresh, and even if you did, and got a 304 response from the backend,
there's no way to tell Varnish to deliver the stale object after all.

If you get a chance to try the IMS branch, please let us know how it goes.


Best,
Geoff
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