We're currently running Plone 3.3.6 on one instance of Zope with 4 threads behind Apache 2.2. We have a number of subsites all managed from one Plone site.
I'm currently testing a new set-up running 2 Zope instances, 2 threads each, and load balanced by Apache. We have a tool that spiders our sites to check for errors and this is reporting a number of 502 proxy errors. It runs fine on our live sites and usually also on our test sites.
As expected we see a 502 error in the Apache access log.
10.10.11.1 - - [04/Feb/2012:14:34:46 +0000] "GET /family-strengthening HTTP/1.1" 502 345 "-" "Mozilla"
We also get an entry in the Apache error log
[Sat Feb 04 14:34:46 2012] [error] [client 10.10.11.1] proxy: error reading status line from remote server dev:18082
[Sat Feb 04 14:34:46 2012] [error] [client 10.10.11.1] proxy: Error reading from remote server returned by /family-strengthening
Sometimes Zope access log shows the request with 200 response but with 0 bytes served
./instance2-Z2.log:10.10.11.4 - Anonymous [04/Feb/2012:14:34:47 +0100] "GET /VirtualHostBase/http/test.street-children.org.uk:80/sos/soschildrensvillages_org_uk/street-children_org_uk/VirtualHostRoot/family-strengthening HTTP/1.1" 200 0 "" "Mozilla"
Other times the request does not show up in the Zope access log.
There are no errors in the Zope instance logs.
When I request the same pages through a browser I never receive the error. Running our spider again results in a similar number of 502 errors but for different pages.
I think our Apache configuration for our sites is pretty standard:
ProxyPreserveHost On
<Proxy balancer://zopes>
BalancerMember http://dev:18081
BalancerMember http://dev:18082
</Proxy>
RewriteRule ^/(.*) balancer://zopes/VirtualHostBase/http/%{SERVER_NAME}:80/sos/soschildrensvillages_org_uk/%{ENV:SOS_PLONE_FOLDER_SHORTNAME}/VirtualHostRoot/$1 [last,proxy]
I don't know where to start trying to debug this so any help would be much appreciated.
