Detect request coalescing?

Geoff Simmons geoff at
Fri Jul 31 06:24:14 UTC 2020

On 7/30/20 21:28, Batanun B wrote:
> Just a quick question that I wasn't able to find any information on
> using regular google searches. Is it possible to detect request
> coalescing somehow? Meaning that, when looking at the response headers,
> can I somehow see that the request had to wait for another request for
> the same resource to finish, before this request could return? Can I
> somehow detect that in VCL, and add a custom header with that information?

Client request logs will include Timestamp:WaitingList when that
happens. The timestamp isn't there if the request didn't have to wait,
and if it is, the timestamp tells you how long.

If your goal is to gather information about request coalescing, you cn
get it by scanning the log. If you need to know about it at runtime for
further request processing, then no I don't think there's anything for
that in VCL.

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