[ww520]

The system needs to be architected in certain way to make upgrade without downtime. Something like the Command and Query Responsibility Segregation (CQRS) would work. A update queue serves as the explicit transaction log keeping track of the updates from the frontend applications, while the databases at the end of the queue applies updates and serves as the querying service. Upgrading the live database just means having a standby database with new version software replaying all the changes from the queue to catch up to the latest changes, pausing the live database from taking new changes from the queue when the new db has caught up, switching all client connections to the new db, and shutting down the old db.