How would you add a required column or split a column in production without breaking the app?

Plan a safe schema migration without downtime

Explains expand-and-contract migrations, backwards-compatible schema changes, backfills, rollout order, and rollback thinking.

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I would avoid making a breaking schema change and code change at the same instant. For a required column, I would first add it nullable or with a safe default, deploy code that writes it, backfill existing rows, then add NOT NULL after the data is complete. This expand-and-contract approach keeps old and new application versions compatible during deployment.

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Schema migration safety, expand-and-contract, backfills, compatibility, deployment order, locks, and rollback.

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Schema migrations are where database design meets deployment reality. This question checks whether a candidate can change data safely while the app is live.
I would avoid making a breaking schema change and code change at the same instant. For a required column, I would first add it nullable or with a safe default, deploy code that writes it, backfill existing rows, then add NOT NULL after the data is complete. This expand-and-contract approach keeps old and new application versions compatible during deployment.
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For a column split or rename, I would use a similar staged approach. Add new columns, dual-write or derive new values, backfill, read from the new shape after validation, then remove the old column in a later deployment. I would consider lock duration, table size, indexes, foreign keys, and whether the migration can run online. Rollback plans matter because code rollback may need the old schema to remain available.

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Adding a NOT NULL column without default or backfill on a large live table.
Renaming or dropping a column in the same deployment that changes code.
Running an unbounded backfill during peak traffic.
Having no rollback plan for code and schema compatibility.

How would you add a required column or split a column in production without breaking the app?

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