Streaming and backend conditional requests

[Geoff Simmons]

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On 1/13/12 2:18 PM, Poul-Henning Kamp wrote:
> 
> By default all backend fetches which can find a candidate for it, 
> will try to IMS against the backend.  If you do not want that, you 
> remove the bereq.http.IMS header in vcl_pass{}/vcl_miss{}

What got the conversation going about this is that in the current
implementation, a busy object can never be the candidate for a
validation. So in 3.0 streaming, it can never be an object that is
still streaming. If that's the right solution now and in the future,
then everything's fine.

When new streaming arrives, if I've understood correctly, a second
request for a streaming object can be served from the same object in
the cache. If this happens after TTL has already elapsed, then a
validation against the backend could avoid another fetch for a large
object that hasn't changed.

If we want that, the innards of IMS implementation would have to
change, because now it can only work with an object stored in entirety
in the cache. If not, IMS would at least have to recognize that an
object is currently streaming (to be able to exclude it), since Martin
has advised that busy flag won't be set.

Thanks for the update.


Geoff
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