Number of simultaneous clients

Pierre Baillet oct at
Fri Aug 6 10:11:42 EDT 2010


We've working on some improvement on our caching system and while
testing it, I encountered errors between Varnish and Unicorn. Namely,
Varnish claims that there is no backend connection and will not send
any data from Unicorn.

After some investigations (tcpdump), I found out that sometimes, I got
no reply from my backend server. See for example this complete
conversation between my Varnish server and my Unicorn server:

GET /items/422qq2nh931gk-aqXuxXfs/esi/card/sitemap/nil.xml/http HTTP/1.1
Accept: */*
Accept-Encoding: identity
X-Ftn-Is-Logged: no
X-Varnish: 1063393845

Yes, that's all. The last packet sent by the backend is an ACK to the
data packet sent by Varnish.

This happens when I really overload the Varnish with incoming
requests. I suspect it opens a lot of connection to our backend. But I
fail to see why Unicorn would not reply (even at a slow rate). Of
course, the issue can be reproduced more or less easily. Trying to
reproduce the issue by directly hitting the Unicorn has not succeeded.

Any thoughts ? Cheers,
Pierre Baillet <oct at>

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