[Varnish] #1696: Varnish leaves connections in CLOSE_WAIT

[Varnish]

#1696: Varnish leaves connections in CLOSE_WAIT
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 Reporter:  rwimmer   |       Owner:
     Type:  defect    |      Status:  closed
 Priority:  high      |   Milestone:
Component:  build     |     Version:  3.0.5
 Severity:  critical  |  Resolution:  invalid
 Keywords:            |
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Changes (by daghf):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 Hi

 What happens here is sessions piling up in the waitinglists, due to
 backend request serialization caused by a vcl_fetch misconfiguration.

 On a cache miss, Varnish will kick off a backend request, and any
 subsequent request for that object while it is being fetched will be
 waitlisted, on the premise that the response  from the request that was
 first kicked off is indeed cacheable and can thus also be delivered to the
 waiting clients.

 To avoid waitlisting for non-cacheable resources, Varnish has the concept
 of hit_for_pass, which basically means that we cache the decision not to
 cache this object - and thus don't do any waitlisting/request coalescing.

 However, a hit_for_pass object also carries a TTL, and if that TTL was set
 to 0s, it is immediately timed out and we're back to queueing up requests
 (for a resources that is not cacheable), and thus they will be dispatched
 in a serialized manner, one at the time. In a situation where the incoming
 request rate is higher than the rate that they are dispatched to the
 backend, you will then see a pileup of sessions stuck in the waiting
 lists.

 The built-in VCL has safeguards to prevent this (if the ttl was set to 0s
 it will create a 120s hit_for_pass object), but when an explicit return is
 done that prevents the built-in VCL from executing.

 Summary: If you do an explicit return from vcl_fetch, make sure beresp.ttl
 is different from 0s.

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1696#comment:3>
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