Design a blue-green deployment system for a platform where some services share a single relational database with others.

Cover: the core challenge that blue-green works cleanly for fully independent stateless services but becomes complex when blue and green versions of a service must share a single database schema simultaneously during the transition — address via backward/forward-compatible schema design (same principle as the migration question above) so both versions can safely operate against the same schema concurrently; database connection routing considerations; and acknowledging that pure blue-green may need to be scoped to the stateless application tier only, with the shared database treated as a separa