YDB is an open-source Distributed SQL database available under Apache 2.0 license. It’s easily scalable across thousands of nodes and is known to be always available. Version migration is a natural process for every software system. In majority of use cases YDB is used as a mission critical OLTP database that cannot afford maintenance windows and must remain available during version migrations. In this talk we will briefly describe YDB layered architecture and share some tricks to minimize database unavailability during minor and major versions migrations from YDB server and applications point of view.