So you heard Java EE 7 will be about the cloud, but that didn't mean a whole lot to you. Then it was characterized as PaaS, something in between IaaS and SaaS. And finally it all became clear when referenced as support for multitenancy. Or did it?
When it comes to JPA and persistence is general, multitenancy is defined as the ability to share a database schema among various groups of users (i.e. tenants). This means that there is no database setup or reconfiguration required as the data is co-located in the same database.
EclipseLink 2.3 (the Indigo train release) let's you do just that by supporting tenant discriminator column(s) via annotations or XML with applications providing values for these discriminators via an API or PU configuration. Check out details here.
EclipseLink 2.3 is scheduled to be the default and supported JPA provider for GlassFish 3.1.1. Another nice feature of this release is the ability to extend persistence units on the fly. The GlassFish Podcast has an interview up with EclipseLink's Doug Clarke.
Expect more on multitenancy across the Java EE spectrum as the specification work progresses.
Update: a screencast is now available.