This is a big picture future roadmap kind of thing.

Assume the AS develops the ability to host two versions of the same webapp concurrently. Imagine a workflow like this:

1) Version 1 is running, handling requests
2) Version 2 is deployed, is allowed to receive a low workload to validate that it works properly.
3) Version 2 is shifted to allow full workload
4) Version 1 is put into disabled mode to drain sessions
5) Version 1. is undeployed.

Obviously making this work requires a lot of currently non-existent stuff in the AS. Point of this JIRA is think about how to deal with it at the mod_cluster level.