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The existing solutions for Tomcat session clustering are viable for moderate clusters and straightforward failover in traditional deployments. Several are well developed with a mature code-base, such as Apache's own multicast clustering. But there's no getting around the fact that today's cloud architectures place new and different expectations on Tomcat that didn't exist several years ago when those solutions were being written. In cloud architectures, where horizontal scalability is king, a dozen or more Tomcat instances is not unusual.
In my hybrid, private cloud, first described in my earlier post on keeping track of the availability and states of services across the cloud, I wanted to fully leverage all my running Tomcat instances on every user request. I didn't want to use sticky sessions as I wanted each request (including AJAX requests on a page) to be routed to a different application server. This is how virtual machines work at a fundamental level. Increased throughput is obtained by parallelizing as much of the work as possible and utilizing more of the available resources. I wanted the same thing for my dynamic pages. Not finding a solution in the wild, I resolved to roll my own session manager. The result is the "vcloud" (or virtual cloud) session manager. I've Apache-licensed the source code and put it on GitHub--mostly to try and elicit free help to make it better. You can find the source code here: http://github.com/jbrisbin/vcloud/tree/master/session-manager/
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Over the years there have been a number of connectors developed to enable Apache httpd to communicate with Tomcat that have used a variety of protocols. When searching the web for information on how to do this, it isn't unusual to stumble across some really bad, out of date advice. So first of all the only options you should consider for this are:
All of the other other options have not been supported for a number of years so you should avoid mod_jk2, mod_jserv, mod_webapp and any other module that isn't discussed here.
My experience with providing support to SpringSource customers is that a typical customer is more likely to hit a bug in mod_proxy_ajp than they are in mod_jk or mod_proxy_http. It isn't that mod_proxy_ajp is particularly buggy, I used it myself for 18 months on a production system without a single issue, but that it has a few more bugs than the other two modules. The situation is improving but at the time of writing I would rank mod_jk and mod_proxy_http above mod_proxy_ajp.
In recent blogs, I've been discussing a major change in the IT server applications environment, specifically the transition away from last decades JEE Application Servers and "mega-blob" applications architectures. Increasingly, we see applications being assembled, leveraging collections of services...both local and remote, sometimes even very remotely in "the cloud" (more on that in future blogs).
This time I'll be talking about planning the migration process. Perhaps the most important step when considering migration of applications from one environment to another is planning. While our temptation is typically to jump in and start coding, it is critically important to understand the objectives and benefits, assess the risks and costs, and then make the business decision. Yup, it's a "business decision", mainly because migrating a running application from one infrastructure to another affects so many areas of the IT organization, plus it can impose risks for the users. There are many successful migration projects and one common characteristic I always find is that the up front planning process was completed and socialized before launching the actual project.
In a word, Yes. Tomcat is an "Application Server" for certain types of applications, including a large percentage of the applications being developed today. The answer depends, of course, on your definition of “Application Server”. It also depends on whether you think Tomcat is limited to the Apache Tomcat Project, or whether Tomcat also includes all the add-ons and plug-ins that have been made available through open source or commercially.
In any case, I assert that Tomcat is indeed an Application Server, all by itself. Tomcat becomes a much richer Application Server as you modularly (the only sane approach, IMHO) add functionality as required by your specific application.
I frequently hear the questions “What’s an application server?, “Is (or isn’t) XYZ an application server?”, and “Do I really need an application server?”. These questions also show up in various internet FAQ’s, blogs, etc. Some of the resulting discussions are almost hilarious, with firmly held beliefs sometimes swamping rational thought and one recent discussion at a major client bordered on open warfare. I too have some opinions, though perhaps not as vehemently held, based on years building applications across many architectures and technologies.
I sometimes pine for the days when I just had one server to worry about. I wax nostalgic, remembering how easy my life was when I didn't have servers and virtual machines growing out of my ears. It's almost the same feeling I get thinking about the days when I only had one child and you could just pop them in a carseat and take off. Now I've got kids driving themselves and their siblings to several activities a day and I can hardly keep track of whether I'm coming or going. I feel the same way about my data center.
Blog : Clustering Cloud-friendly Apache Tomcat Sessions with RabbitMQ: Part I
posted by jbrisbin on June 21, 2010 07:11 AM
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Blog : Deciding between mod_jk, mod_proxy_http and mod_proxy_ajp
posted by mthomas on June 16, 2010 07:06 AM
Blog : Migrating JEE Applications to Apache Tomcat: Planning the Migration
posted by avanabs on June 14, 2010 10:14 AM
Blog : Is Apache Tomcat an Application Server?
posted by avanabs on June 9, 2010 09:22 PM
So, what’s an “Application Server”?
Blog : How to Scale Apache Tomcat in the Cloud with RabbitMQ and JMX
posted by jbrisbin on June 3, 2010 10:20 PM
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- graceh commented Apache Tomcat 6.0.37 Released
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