Updating deployed EJB definitions

Question

Hi, all,
Imagine the following scenario. You develop an EJB webapp and launch it. A few months go by and you enter a significant amount of data into the app's entity beans -- data that is mission-critical and must not be lost in the future under any circumstances.
Then suddenly the need strikes for you to modify or expand your entity EJBs' class definitions -- say, change some of their fields' types or add totally new fields... How do you then upgrade your EJB class definitions without losing the data that has already been entered into the old definitions, but rather migrating that old object data to your new class definitions? Has anyone had to deal with such a situation in practice?
To give you an example, suppose our web app has an entity bean class named "ContactPerson" but a few months after deploying the application and creating hundreds of ContactPerson instances we realize we need to associate a new attribute with each one of them -- like their Internet telephony ID. We want to append this new attribute to every existing and future ContactPerson bean instance -- without losing any contact person data the application already contains. Any ideas on how to do this?
Thanks for any suggestions,
D. P.

Answer 1

1) changing and recompiling the EJB classes (to make
them include new fields),This shouldn't be a problem. It's the usual stuff. The only thing you need to ensure is that all the old fields and their functionality doesn't go off during changing something.
Still the major part would be the actual DAO stuff... EJBs would not be containing all the backend logic, or is it? Of course, you will need some change in the front-end too, if they are displaying something from the new fields.
2) updating the back-end database in a way that
reflects the new object structure but still contains
the old dataThis can be done by means of loader scripts. And there shouldn't be a problem if you aren't removing any columns or changing their datatypes.
3) making the new EJB's recognize and work seamlessly
once again with the new database.This is a question of some testing. I bet you wouldn't be testing on a production box... so why worry about that right now?
Cheers!
***Annie***