Thanks to many who had off-line discussions with me on this topic. Some quick points about Eucalyptus upgrade (mostly related to "Live Upgrade")
1. We realized that there are no clearly defined industry standards for "rolling upgrade", "live upgrade", and "online upgrade", especially for clouds. Without theorizing too much, we decided to focus on "live upgrade" for the near term, and "rolling upgrade" for the longer term, with the following description customized for Eucalyptus.
2. Live Upgrade refers to the process where the instances in Eucalyptus will remain live.
2a. In other words, instances in Eucalyptus that are running prior to the beginning of the upgrade, will continue to run during the upgrade and beyond.
2b. Live Upgrade does not guarantee that the instances that are live during the upgrade process will also be accessible by the end users during the upgrade process. There will be a downtime on the connectivity, but the goal will be to keep this minimal.
2c. There are two different types of down-times we need to distinguish between - one, the downtime experienced by the cloud management services; two, the downtime in access to the live instances.
2d. Live Upgrade will focus on minimizing both types of downtime, but priority is given for reducing the downtime in access to the instance
3. "Rolling Upgrade" refers to the process where Eucalyptus components can be upgraded one at a time while keeping all the services and instance access intact. This is a tall order since this will involve running and testing Eucalyptus clouds with multiple versions of Eucalyptus at the same time. We will get to this. This is NOT in scope for this feature request.