I'll take a look...

In general, upgrading from 2.2 to 2.4 is explain in this article

http://httpd.apache.org/docs/2.4/upgrading.html

(thus for instance "Allow from all" is wrong in your configuration)

But I assume the problem is elsewhere. Looking at the logs:

attempt to connect to 192.168.216.3:27233

27233 is this the right port? AJP connector usually uses port 8009. Try switching from AJP to HTTP connector to see if that works.

• O.K, If it is Wildfly 9, you should use attribute proxies, see my answer on the topic of having advertising disabled.
• furthermore, each host (i.e. Wildfly instance) has to have instance-id="name_of_the_worker" set. I'm not sure whether you won't end up with multiple "slave" node names with your current setup
• define <dynamic-load-provider> as it is supposed to be by default
• verify that you have LoadModule cluster_slotmem_module modules/mod_cluster_slotmem.so loaded, because the name of the shared object changed between m_c 1.2.x and 1.3.x
• and the most important thing of all:
  • share your httpd configuration files, especially those of mod_proxy and mod_cluster; the mod_cluster snippet you provided does not show which modules you are loading.
  • share httpd error_log on LogLevel Debug; the short snippet is cut out of its context
  • tell us what is in the Wildfly log; are there Exceptions coming out of the modcluster subsystem about contacting the balancer?

Cheer up, it most definitely works fine; we will find the conf error in your setup

I'm sorry Fran, the error_log is insufficient. I need to see the CONFIG message to find out where 27233 comes from. Please:

• stop the Apache
• stop the Wildfly instance
• delete all logs in logs/
• delete all httpd cache files in cache/
• start Apache
• start Wildfly
• give it ~30 seconds
• stop Wildfly
• stop Apache
• archive and send the error_log

So the ports in the CONFIG message are 8009 and 18009 -- which is in sync with the privided configuration. There is no mention of port 27233 which seems like it was part of some experiment. I am suspecting misconfiguration on a different level such as selinux, routing, firewalld/iptables started again after a restart, etc. Unless Michal sees something specific.

Hmm, I tried to mimic your environment, but I cannot reproduce this thing in your log:

[client 192.168.216.3:34365] AH00944: connecting ajp://192.168.216.3:27233/ to 192.168.216.3:27233

[client 192.168.216.3:34365] AH00947: connected / to 192.168.216.3:27233

(111)Connection refused: AH00957: ajp: attempt to connect to 192.168.216.3:27233 (192.168.216.3) failed

furthermore, I even unloaded shm module and loaded lbmethod heartbeat so as to get the same error on startup as you have:

     [lbmethod_heartbeat:notice] [pid 28385:tid 140592057104448] AH02281: Failed to lookup provider 'shm' for 'slotmem'. Maybe you need to load mod_slotmem_shm?

It looks like mod_proxy remembers a mapping to worker on IP:PORT 27233 and that is weird. Could you check whether it is always the same port? What does lsof | grep 27233 say during your test?

What looks like this in your log:

proxy_util.c(2020): AH00942: ajp: has acquired connection for (192.168.216.3) 
proxy_util.c(2072): [client 192.168.216.3:34365] AH00944: connecting ajp://192.168.216.3:27233/ to 192.168.216.3:27233 
proxy_util.c(2194): [client 192.168.216.3:34365] AH00947: connected / to 192.168.216.3:27233 
(111)Connection refused: AH00957: ajp: attempt to connect to 192.168.216.3:27233 (192.168.216.3) failed 

proxy_util.c(2020): AH00942: ajp: has acquired connection for (192.168.216.3) 
proxy_util.c(2072): [client 192.168.216.3:34367] AH00944: connecting ajp://192.168.216.3:27233/ to 192.168.216.3:27233 
proxy_util.c(2194): [client 192.168.216.3:34367] AH00947: connected / to 192.168.216.3:27233
 (111)Connection refused: AH00957: ajp: attempt to connect to 192.168.216.3:27233 (192.168.216.3) failed 

is always the following with mine:

[proxy:debug] [pid 16706] proxy_util.c(2020): AH00942: ajp: has acquired connection for (rhel7gax86-64) 
[proxy:debug] [pid 16706] proxy_util.c(2072): [client 127.0.0.1:50027] AH00944: connecting ajp://rhel7gax86-64:8009/ to rhel7gax86-64:8009 
[proxy:debug] [pid 16706] proxy_util.c(2194): [client 127.0.0.1:50027] AH00947: connected / to rhel7gax86-64:8009
[:debug] [pid 16706] mod_proxy_cluster.c(1413): proxy_cluster_try_pingpong: connected to backend 
-- 
[proxy:debug] [pid 28390:tid 140591534900992] proxy_util.c(2020): AH00942: ajp: has acquired connection for (192.168.122.172)
[proxy:debug] [pid 28390:tid 140591534900992] proxy_util.c(2072): [client 192.168.122.172:44673] AH00944: connecting ajp://192.168.122.172:9109/ to 192.168.122.172:9109
[proxy:debug] [pid 28390:tid 140591534900992] proxy_util.c(2194): [client 192.168.122.172:44673] AH00947: connected / to 192.168.122.172:9109
[:debug] [pid 28390:tid 140591534900992] mod_proxy_cluster.c(1413): proxy_cluster_try_pingpong: connected to backend

Did you erase /var/cache/mod_cluster (or wherever you have the *.shm) files after Apache restart?  Is the error you experience intermittent or always deterministically reproducible?

With this exact message (port number) each restart or the port changes?

(111)Connection refused: AH00957: ajp: attempt to connect to 192.168.216.3:27233 (192.168.216.3) failed

Dear Fran, the configuration file you provided was tampered with either before or after your test.

It is invalid not only with respect to Allow from directives, but there is apparently some portion of it missing because config elements aren't properly closed: Expected </Directory> but saw </IfModule>.

Help us help you and do not provide configuration files that weren't used during the test runtime and therefore do not match the logs.

Lemme suggest these next steps:

• backup, if it's not your sandbox/testing environment
• yum remove httpd, remove all files in /etc/httpd, /var/log/httpd, /var/cache/httpd and any possible stray files in /usr/lib64/httpd/modules
• yum install httpd, i.e. vanilla httpd 2.4 from RHEL7 repos
• configure bare minimum, i.e. IP addresses, ServerName, Listen...
• In /etc/httpd/conf.modules.d/00-proxy.conf  comment: #LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
• use attached /etc/httpd/conf.d/mod_cluster.conf (change IP address for your own...) and your mod_cluster binaries OR see the RPM paragraph at the bottom
• either temporarily disable SeLinux and Firewalld or use semanage port -a -t http_port_t -p tcp 6666, semanage port -a -t http_port_t -p tcp 8009, semanage port -a -t http_port_t -p udp 23364 ....etc and firewall-cmd --add-port=23364/udp ..and so on in order to configure the system properly
• run your tests

Regarding the mod_cluster installation: You could also rebuild mod_cluster 1.3.1.Final RPM from Fedora on your RHEL7 box and use it for installation:

• you need build essentials (gcc, make...), rpmbuild, httpd-devel, autotools, apr-devel, apr-util-devel and a few things it might shout at you as missing deps
• grab https://karm.fedorapeople.org/mod_cluster/mod_cluster-1.3.1-1.fc22.src.rpm and rpm -i install it  (Don't worry about its origin, it's from my Fedora profile, so for testing it's good enough )
• edit your ~/rpmbuild/SPECS/mod_cluster.spec and set %define with_java 0
• in ~/rpmbuild/SPECS/ run: rpmbuild -ba mod_cluster.spec
• yum install ~/rpmbuild/RPMS/x86_64/mod_cluster-1.3.1-1.el7.x86_64.rpm

These are community bits, provided as is, usual disclaimer applies, read ~/rpmbuild/BUILD/mod_cluster-1.3.1.Final/JBossORG-EULA.txt.