# 5. The read switch

[← 4. The dual write](04-the-dual-write.md) · [Next: 6. The NOT NULL trap →](06-the-not-null-trap.md)

Deploy 3 changes exactly one thing from Deploy 2: where reads come from. In
[`CustomerService.findById`](https://ankurm.com/git.app/asmhatre/spring-boot-demo/src/branch/main/db-migrations-expand-contract/src/main/java/com/ankurm/expandcontract/customer/CustomerService.java):

```java
String sql = switch (stage) {
    case 1, 2 -> "SELECT id, name, email AS email FROM customers WHERE id = ?";
    case 3 -> "SELECT id, name, COALESCE(email_address, email) AS email FROM customers WHERE id = ?";
    case 4 -> "SELECT id, name, email_address AS email FROM customers WHERE id = ?";
    default -> throw new IllegalStateException();
};
```

Stage 1 and Stage 2 read the same column: only *writes* changed between them. Stage 3
is the read switch, and it's a separate deploy from Stage 2 for a reason that isn't
obvious until you say it out loud: **the write switch and the read switch cannot be
the same deploy**, because a rolling deploy is not instantaneous. Deploy 2's own
rollout has a window — anywhere from seconds to minutes, depending on fleet size —
where some replicas are still Stage 1, writing only `email`. If Deploy 3's read
switch were bundled into that same deploy, a Stage 3 replica reading
`email_address` alone would return `NULL` for every row a still-live Stage 1
replica had just written. That's not a hypothetical: it's
[chapter 7](07-the-backfill-window-bug.md), reproduced and fixed.

The fix already visible above is `COALESCE(email_address, email)` rather than
`email_address` alone — Stage 3 falls back to the old column for exactly the rows
that predate full dual-write coverage. Stage 4 doesn't need the fallback anymore: by
the time Deploy 4 starts, two full deploys' worth of dual-writing (Stage 2 and Stage
3, run back to back) have guaranteed every row has both columns populated.

## Going deeper

- The bug this switch would reintroduce if it were merged into Deploy 2, captured
  live: [chapter 7](07-the-backfill-window-bug.md).
- All six write/read direction pairs across the three rollouts, proven directly:
  [chapter 8](08-the-rolling-window-proof.md).

[← 4. The dual write](04-the-dual-write.md) · [Next: 6. The NOT NULL trap →](06-the-not-null-trap.md)