Tuesday, June 07, 2011

HOWTO: Parallel deployment on tomcat 7

News broke out recently that Tomcat 7 supports parallel deployment.

This feature allows one to deploy a newer version of a web application while maintaining the current one online. The net effect is that new users will be connected to the newest version while already connected users will not be kicked out. Neat uh?
One could even have more than two versions online at any time (if that was necessary).

The details of how this works are explained in the Context Container documentation. Only it doesn't really tell you how to use it, it tells you how they implemented instead.

So what do you have to do to take advantage of this great feature?

First of all upgrade to Tomcat 7 (which btw requires Java 6), but you knew that already, didn't you?

Then simply adopt the following naming strategy when deploying apps: name your war file mygreatapp##version.war (note the ## used to mark the beginnning of the version substring) where version can actually be anything (it is mapped to a String property) but to be safe I suggest using a zero-padded always increasing numeric value (your SVN revision number, perhaps).
Example:

mygreatapp##0001.war

That's it. No configuration, rebuilding or trick hats required.

Note: previous app versions will NOT be removed automatically. You will have to remove them yourself using the manager app when their session count drops to zero or you just can't wait anynmore.

Note2: the app version is visible in the manager app, in a dedicated column right beside path.

Note3: if you plan to use a String for the version make sure that the newer version values are greater (in String terms) than previous ones or you'll end up with new users connecting to an older version of the app instead. That's what zero padding is for.

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