How would you deploy a .NET API change that requires a database migration without downtime?

Migration-safe deployment for a .NET API

Explains how to deploy database changes safely with .NET APIs using backward-compatible steps.

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TL;DR

I would avoid destructive schema changes in one release. First, I would review the generated SQL and understand table size, locking, and compatibility. If the change is risky, I would use an expand-and-contract approach: add new schema safely, deploy code that supports both old and new shape, backfill data, switch usage, and remove old schema later.

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Migration safety, expand-and-contract, compatibility, rollout order, rollback planning, and production risk.

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Why this question matters
Database migrations are one of the main reasons deployment safety is hard. The code and schema must be compatible during rollout.
I would avoid destructive schema changes in one release. First, I would review the generated SQL and understand table size, locking, and compatibility. If the change is risky, I would use an expand-and-contract approach: add new schema safely, deploy code that supports both old and new shape, backfill data, switch usage, and remove old schema later.
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Renaming a column directly can break old instances during a rolling deployment. I would add the new column first, deploy code that can read the old shape and write both values where needed, backfill in bounded restartable batches, switch reads after verification, and remove the old column only after every running and rollback version no longer depends on it. Index creation, constraints, and data conversion receive the same compatibility treatment. The temporary complexity is deliberate: it keeps mixed versions functional and preserves a recovery path while the data transition is still in progress.

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Details

Renaming or dropping columns in the same release that needs them.
Auto-running risky migrations at app startup.
Not reviewing generated SQL.
No backfill or rollback plan.

How would you deploy a .NET API change that requires a database migration without downtime?

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