How would you test an EF Core schema change before a rolling production deployment where old and new application instances may overlap?

Test database migrations and rolling compatibility

Verifies migration SQL, data preservation, production-provider behavior, mixed application versions, backfills, and expand-contract releases.

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I would generate and review the migration SQL, then apply it to an isolated database using the same provider and a schema from the previous release. The fixture would contain representative existing data, including nulls, duplicates, long values, and relationships affected by the change. After migration I would verify schema shape, preserved data, constraints, and normal reads and writes through the new application. An in-memory provider cannot prove SQL syntax, locking, conversion, or provider-specific behavior.

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Migration review, production-provider fidelity, upgrade paths, data preservation, expand-contract compatibility, backfills, rollback, and lock risk.

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A migration can compile and still lose data, block a large table, or break the older application instances that remain alive during deployment. The test target is the transition between versions, not only the final empty schema.
I would generate and review the migration SQL, then apply it to an isolated database using the same provider and a schema from the previous release. The fixture would contain representative existing data, including nulls, duplicates, long values, and relationships affected by the change. After migration I would verify schema shape, preserved data, constraints, and normal reads and writes through the new application. An in-memory provider cannot prove SQL syntax, locking, conversion, or provider-specific behavior.
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For a rolling release I would test the compatibility sequence. An additive migration runs first, the old application continues to read and write, then the new version starts using the new shape. Backfills are restartable and can run in bounded batches. Code handles the temporary state where both old and new representations exist. Renames, required columns, and splits use expand and contract so destructive cleanup happens only after old instances and rollback windows are gone.

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Testing migrations only by creating a fresh empty database.
Relying on EF Core InMemory or SQLite for SQL Server migration behavior.
Dropping or renaming a column in the same release that changes the code.
Ignoring the time and locks required for a large backfill.

How would you test an EF Core schema change before a rolling production deployment where old and new application instances may overlap?

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