Backend engineer, mostly Postgres and Python. I write the course I wish someone had handed me my first year out of college. Austin, TX.
Priyanka asked me to write this one. SPF, DKIM, DMARC, and why your beautifully-written cold email is landing in promotions tabs regardless.
A 500-million-row backfill is not "a big UPDATE". It's a job that has to run for days, pause gracefully, and resume from where it stopped.
Writing to both shapes during the transition window is how you keep reads consistent without a lock. The failure modes to watch for.
Rachel asked me to write this step because too many founders spend two weeks designing a schema they'll throw out anyway. Here's what to do instead.
Expand the schema additively. Deploy code that tolerates both shapes. Backfill. Flip the reads. Contract the schema. Four phases, clear rules.
Most production databases outlive three generations of application code. Before you invest in zero-downtime machinery, ask whether you actually need it.
