I've been thinking about Zero Downtime Deployment for the past few weeks. I even raised it as a discussion topic in our LSCC Roundtable. Here are the key points discussed. Obviously the feasibility/suitability completely depends on the application and platform architecture. Also this is not an attempt to provide a solution but more to record the thoughts and ideas:

• Switch over to backup upgrade the primary and then switch back, and then update the backup site.
• Have a HA configuration, take hosts out upgrade then switch gradually.
• Allow for parallel runs, so you can switch over to a parallel when updating.
• Develop application to run on previous and new version of the database. Hot deploy the app and "hot" migrate the database.
• Use dynamic features in your platform to allow for hot deployment.
• Use semi structured datastore so data migrations are minimal.
• Provide targeted releases, i.e. if a particular area of the application changes than only that area is updated and not the whole application.
• If eventual consistency across partitions is acceptable then deploy in partitions and update partitions gradually, i.e. like Facebook, Google etc.