Multiple Apache mod_jk servers pointing to the same Tomcat worker?

Question

We have a single Tomcat app server and a single front-end web server running Apache and mod_jk. Is it possible for us to add a second front-end web server, that points to the same app server and also runs Apache and mod_jk?

We don't want to do this for reasons of load balancing. Rather, it's for reasons of migration. The new web server will be an entirely different OS and will use a different SSO authentication module. We want to be able to stage and test it, switch a DNS entry to make it live, and decommission the old server.

The current workers.properties file looks like this:

worker.list=worker1
worker.worker1.type=ajp13
worker.worker1.host=10.x.x.x
worker.worker1.port=8009

Can I just duplicate that config onto a second web server?

I have no experience or background whatsoever with any of this, but have been asked to assist with a server migration and am trying my best to help.

I have been reading the documentation for mod_jk and Tomcat. I have found all sorts of documentation on pointing a single mod_jk instance to multiple Tomcat app servers, but nothing describing the opposite of that, which is what I'm trying to accomplish?

Is this possible?

Edit: I should note that we do not have any access to the Tomcat server, as it is managed by a third-party vendor. I'm sure they'd make changes if we asked them, but we don't have the ability to log into it ourselves.

Answer 1

Yes, of course you can. I have done it several times, even just to change the static files served by Apache (js, images, css) and test the Tomcat application with a different "skin".

Usually when building a high availability system, not only the Tomcat's or any other back-end servers get replicated, the front-end Apache or IIS or whatever is used gets replicated too.

As you said, it should be fine just copying the workers.properties file and the mapping rules in the Apache httpd's *.conf files.

An also, check with tomcat management team that the incoming connections to Tomcat's AJP port are not limited by net rules or firewalls, making just the old Apache able to reach the Tomcat.

Answer 2

Yes - duplicating will be easiest. Most important** is keeping the worker name the same.

One gotcha is making sure Tomcat has enough connections available to be handled by both web servers. The normal defaults are typically high enough, but if you try to stress test, the tomcat server may need the sum of workers available on the webservers. But if you don't have enough, Tomcat does write a warning to the logs.

** Most important - Ok - Not that important since you are not using sticky sessions. But could be confusing later if you try this experiment with 2 tomcats when switching between web servers.

Answer 3

Can I just duplicate that config onto a second web server?

Yes sure, since you want to hit the same Tomcat server so you can simply copy your worker.properties from Apache instance 1 to Apache instance 2. If you have only those 4 properties then nothing but if you have some properties like worker.worker1.cachesize=10 or worker.worker1.cache_timeout=600 and you want to play around then change it. But bottom line is that since you want to hit same Tomcat instance so you can simply copy.

Understanding it in non-tomcat way - you can have more than 1 HTTP web server like Apache intercepting all the requests and then forwarding it to same application or web server. However this is not common because common thing is a web server load balancing the requests for back end application servers.

I have been reading the documentation for mod_jk and Tomcat. I have found all sorts of documentation on pointing a single mod_jk instance to multiple Tomcat app servers, but nothing describing the opposite of that, which is what I'm trying to accomplish?

Is this possible?

You couldn't find in any of the reading because what you are trying is a corner case, generally people configure multiple Tomcat workers to serve servlets on behalf of a certain web server in order to achieve load balancing, virtual hosting etc.

You mentioned that you are doing all this in order to test the Apache running on different OS and using different SSO, I am assuming there is no hardware load balancer sitting in front of your web servers (Apache) so how you are gonna hit your new Apache? I think you need to do this explicitly because your current URL will be pointing your first Apache, so in order to hit second/new Apache you need to give your testers/users the URL containing end point (IP:port) of second/new Apache. Even if you are doing all this locally still you need to have your second Apache listening on different port, or may be different IP but that's not common.