postgresql

Blue-green works for stateless apps because instances are disposable. Databases are shared state, so safe migrations ride on backward compatibility and an explicit rollback boundary.

An idle database with a starved application is the tell. The connections are held, not used. More database capacity does not release leaked connections.

Sharding buys write headroom and bills you in joins, foreign keys, and every future migration. The key choice is a query-pattern bet; the resharding choreography separates operators from diagram artists.

Most of your rows are read rarely and cost the same as the ones read constantly. Tiering pays for itself only when access patterns are honest, which is why the audit comes before the architecture.