Ship a risky schema change to a 2 TB Postgres primary with near-zero downtime. How does that actually work?

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.

Grounded in researched DevOps, SRE and platform engineering interview loops, written to a senior-engineer editorial bar, and never padded to hit a word count.