mod_proxy_ajp: too many (unused) connections to backend

[Bastian Voigt <post@xxxxxxxxxxxxxxxx>]

Hi,

We're using apache httpd 2.2 with mod_proxy / mod_proxy_ajp as  
frontend for our Glassfish Java Application Server. We have multiple  
(four) ProxyPass rules in our virtual host configuration to forward  
different URIs to Glassfish, while other static content is served by  
apache directly.

Our apache configuration:
- MPM worker
- MaxClients 100
- 25 threads per child process

As the traffic on our website is growing we need to scale the number  
of parallel connections up. However when we did this, we ran into  
problems with too many glassfish worker threads being forked until we  
eventually hit the limit of 250 threads. Wondering what this might  
mean, I used the following command to watch the number of connections  
between glassfish and apache:

netstat -nap | grep VERBUNDEN | grep 8009 | grep apache2 | awk -F' '  
'{print $7}' | sort | uniq -c

(VERBUNDEN is german for CONNECTED).

And to our surprise we discovered that some apache child processes  
hold more than 25 connections to the backend server:

     29 18146/apache2
     35 18188/apache2
     23 18455/apache2

And it seems that some of these connections are "dead", i.e. they are  
no longer used for forwarding requests to glassfish.

So I have a number of questions:

1. Is it true that each ProxyPass rule uses its own connection pool to  
the backend? And if so, is there a way to unify this so that only one  
connection pool is used per child?

2. When an apache thread is in "K" (keepalive) status, does it still  
block backend connections from the pool ?

3. How long does apache keep idle connections to the backend before  
they are closed?

4. Is there a way to make apache show the status of its AJP backend  
connections, similar to the mod_status page ?


I am looking forward to your insightful replies!

Cheers
Bastian



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