You will find there's JBoss set up in cluster with Apache HTTP+mod_jk like a load balancer. Do we have to configure anything on Apache side additionally to setting up compression in JBoss configuration for connector?

In standard JBoss gzip compression might be enabled for HTTP connector, although not for AJP. AJP connector can be used between Apache HTTP server and JBoss.

Make it possible for gzip compression on Apache HTTP server side add following lines to mod_jk.conf before </VirtualHost>:

AddOutputFilterByType DEFLATE text/html text/plain text/xml text/css text/javascript application/x-javascript application/javascript

This can enable gzip compression for specified mime types by way of mod_deflate output filter

Also uncomment following line in httpd.conf to show on mod_deflate:

LoadModule deflate_module modules/