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From alin vasile <>
Subject Re: [users@httpd] Apache 2.2.11 mod_proxy returns 503 (service unavailable) under load
Date Fri, 16 Jul 2010 08:01:39 GMT

 Since in  your apache logs you can see a  timeout while connecting to the 
backend :

  "[Wed Jul 14 15:59:46 2010] [error] (110)Connection timed out: proxy:
HTTP:  attempt to connect to (

  it is normal that there not to have any logs at that time. 

  The problem here is that your backend may not support a spike load or can hove 
some capacity problems.

From: pratyoosh sharma <>
Sent: Fri, July 16, 2010 7:10:34 AM
Subject: [users@httpd] Apache 2.2.11 mod_proxy returns 503 (service unavailable) 
under load

We are running a high volume portal that uses Apache 2.2.11 mod_proxy
to reverse proxy content from downstream apache-> application servers,
we recently started observing 503 errors on the Apache which happen
sporadically throughout the day during activity on random requests.

The requests returned with 503 don't appear on the backend apache
access logs at all, there are no 503 failures on backend
apache.Network traces of connectivity between frontend apache & the
backend don't reveal anything either.

An interesting pattern is the failed requests always take ~3-4 seconds

Frontend Apache:
Error log:
[Wed Jul 14 15:59:46 2010] [error] (110)Connection timed out: proxy:
HTTP: attempt to connect to (
[Wed Jul 14 15:59:46 2010] [error] ap_proxy_connect_backend disabling
worker for (
Access log:
"USER13" - [14/Jul/2010:15:59:43 -0400] "GET
/myapp/web/css/ext-all.css HTTP/1.1" 503 1030  3041412

This request never reached the backend server & doesn't shows up in
the access logs of backend server

All the failing requests like the above take ~3 seconds so i guess its
like a timeout but i can't find any such configuration.

Is there some known Apache bug for this behavior? We have retry=0 &
disablereuse on for all proxy pass directives

What kind of starvation should i look for in Apache we have mod_status
enabled, file descriptor limit set a 64K, CPU is almost always idle &
25 Gig memory free on the server.


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