We have been using nginx in combination with 3 mongrel instances on our servers. Recently we began testing the use of haproxy to act as a middle man to more efficiently handle our queues. When nginx was talking directly with our mongrels we had no issues, but now with haproxy we do seem to be experiencing some 502 errors. We monitor our mongrels using monit and when a mongrel instance begins to take up too large of a memory footprint we have monit restart the mongrel instance. It appears that the 502 errors are thrown on the last request that the mongrel handles prior to restarting (this is how it appears, but we can't be certain this is correct). The log from haproxy seems to imply that it was waiting for "complete, valid response HEADERS from the server" when it throws the 502 error. Is it possible that the mongrel headers are sent back differently on the last request prior to a restart? We do know that the last request is being processed because a user was trying to create a record and received a 502, but found that it did successfully create the record, so the problem seems to be solely in the response headers after the request has been processed.
If you are interested you can see the breakdown of the haproxy error here: http://pastie.org/private/nfvbtq236x8jp9ynj4dg I don't know if this is an issue with haproxy or with mongrel so I have also asked the folks at haproxy to look at the issue. Any insight you can offer from the mongrel point of view would be greatly appreciated. Thanks! Ben Fyvie
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