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From Sudheer Vinukonda <sudheervinuko...@yahoo.com>
Subject Re: collapsed forwarding still leaking to origin
Date Fri, 19 Jun 2020 18:25:05 GMT
 Interesting - that sounds like a bug in identifying concurrent requests for refresh of stale
objects. 
The cache miss scenario is more straightforward to detect and is well tested in production,
but, I do recall the stale object refresh doesn't quite work as expected. It was never a huge
problem for us because we were more concerned with the heavier media segment files (which
are typically cache miss) and manifest files are lower in number and tiny sized in comparison.
I'll take a look to see why the refresh doesn't collapse as well as miss.
To your question, the expectation is to follow configured behavior. If the config says return
a 502, requests should not be leaked. Can you confirm if you are seeing any leaks at all for
segment files (cache misses)?
This does also bring up a good point that the newly builtin feature within the ATS core does
not support a config option to entirely prevent a leak of requests. As it stands setting the
config to "5" would fall back to proxying (leaking) to Origin right now. We can enhance that
to support an option to short-circuit in the event the Origin is slower.
    On Friday, June 19, 2020, 03:05:02 AM PDT, ezko <erez.koler@harmonicinc.com> wrote:
 
 
 Thanks for the feedback.

We will definitely check out 8.1.x (any schedule for official release ?)

When the leakage happens all client requests are leaked (not sure if clients
are redirected to origin or ATS is forward proxying). I should mention that
it usually happens for HLS manifest refresh (cache stale) in testing
environment using benchmark tools like apache-bench (so there are thousands
of concurrent requests for the same object).

One question we do have regarding ColFw (either as plugin , or builtin) is ,
what is the expected behavior when the origin response is being delayed for
a few seconds  ? (for example due to a momentary network congestion)
Will ColFw "give up" waiting for the origin headers ? and what will happen
then ? will requests start leaking ?

BR,
Erez 





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