If you thought Tomcat could not get any better, you thought wrong. Tomcat 7 introduces what is called Parallel Deployment.
Simply put, parallel deployment is the ability to deploy more than one version of your web application in parallel, making all versions available under the exact same URL.
Think about this for a minute. If you have a new version of your application, you can simply drop it into the Tomcat that is running the old one and it will Just Work™. In fact, they will both work. Tomcat handles all session management and traffic routing between application versions. No need to restart Tomcat. No need to stop processing requests. No need to talk to your boss about downtime. No need for your boss to talk to any customers about downtime.
The people who created this feature chose a surprisingly easy solution for how to tell Tomcat what is an alternate version of what. All you have to do is tack ## onto the WAR’s file name. Simple and effective, if a bit odd-looking.
There are a few things to consider when you want to start using versioned WAR files with your Tomcat server. So before you go off and change the deployment strategy at your company, check off the list below.
1) Internal caches should be write-through and expire quickly
2) You need sessions to be enabled
3) Where does logging go?
4) Disk files and directories need to be sharable
5) No TCP socket listeners
6) Your apps must be able to undeploy
For the complete walk-through and details on the items on the checklist, read more on http://java-monitor.com/forum/showthread.php?t=1288