One of the most requested features since OpenStack began (for components other than Object Storage, which tends to "just work"): easier upgrades. From Grizzly onward (and significantly improved in Havana), internal messaging communication is versioned, meaning services can theoretically drop back to backward-compatible behavior. This allows you to run later versions of some components, while keeping older versions of others.
In addition, a lot of focus has been placed on database migrations. These are now better managed, including the use of the Turbo Hipster tool, which tests database migration performance on copies of real-world user databases.
These changes have facilitated the first proper OpenStack upgrade guide, found in Chapter 18, Upgrades, and will continue to improve in Icehouse.
