# 9. The contract migration

[← 8. The rolling window proof](08-the-rolling-window-proof.md) · [Next: 10. What happens if you drop too soon →](10-what-happens-if-you-drop-too-soon.md)

Deploy 4 is split into two parts on purpose, the same way Deploy 1 was schema-only
and Deploys 2/3 were code-only:

- **4a — code.** A rolling restart to Stage 4, which never reads or writes `email`.
  The old column is still physically present; Stage 4 code simply ignores it.
- **4b — schema.** Once every replica is confirmed on Stage 4 code, and only then,
  [`V3__drop_email_column.sql`](https://ankurm.com/git.app/asmhatre/spring-boot-demo/src/branch/main/db-migrations-expand-contract/src/main/resources/db/migration/V3__drop_email_column.sql)
  runs, with zero app restarts:

```sql
ALTER TABLE customers DROP COLUMN email;
```

[`ContractSafetyTest`](https://ankurm.com/git.app/asmhatre/spring-boot-demo/src/branch/main/db-migrations-expand-contract/src/test/java/com/ankurm/expandcontract/ContractSafetyTest.java)
confirms the happy path: Stage 4 reads and writes keep working, both for a row that
existed before the drop and for one created entirely after it —

```
-- Stage 4 read, after V3 dropped the email column --
Customer[id=1, name=Annie Easley, email=annie@example.test]

-- Stage 4 create + read, entirely after the drop --
Customer[id=2, name=Mary Allen Wilkes, email=mary@example.test]
```

Full transcript:
[`docs/output/09-contract-safety.txt`](https://ankurm.com/git.app/asmhatre/spring-boot-demo/src/branch/main/db-migrations-expand-contract/docs/output/09-contract-safety.txt).

Why not drop the column in the same deploy as the code change? Because "every
replica is confirmed on Stage 4" is a statement about the *fleet*, and a rolling
deploy makes it true only once, at the very end of the rollout — never at the moment
the deploy starts. Running 4b before that point is dropping a column a live Stage 1,
2, or 3 replica might still need. What that actually looks like, captured directly,
is [chapter 10](10-what-happens-if-you-drop-too-soon.md).

## Going deeper

- The DDL Deploy 4b runs is not, on this specific database engine, quite as free of
  side effects on concurrent traffic as "just a schema change" suggests — see
  [chapter 14](14-the-ddl-lock-window.md) for what H2 actually does while this
  statement executes.

[← 8. The rolling window proof](08-the-rolling-window-proof.md) · [Next: 10. What happens if you drop too soon →](10-what-happens-if-you-drop-too-soon.md)