| # CROSS2: Schema Evolution — Cross-Service Compatibility |
|
|
| ## Goal |
| Service A has been updated with a database migration that modifies the shared schema. |
| Update Service B to work with the new schema without breaking either service. |
|
|
| ## Requirements |
| 1. Update `service_b/models.py` to match the new schema (see `service_a/migrations/002_add_columns.py`) |
| 2. Fix `service_b/queries.py` to use explicit column names (no `SELECT *`) |
| 3. Implement `scripts/backfill.py` to populate new columns for existing records |
| 4. Use default values from `service_a/config.py` for the backfill |
| 5. All tests must pass: `pytest tests/` |
|
|
| ## Supporting Documents |
| - `service_a/migrations/002_add_columns.py` — The migration that changed the schema |
| - `service_a/config.py` — Default values for new columns |
| - `shared/schema.sql` — Expected final schema (source of truth) |
| - `service_b/models.py` — Stale ORM models (need updating) |
| - `service_b/queries.py` — Contains SELECT * (needs fixing) |
|
|
| ## Important |
| The migration renames one column and adds three new columns. Service B must be updated |
| to use the new column names. The backfill script must use exact default values from |
| Service A's config — do not invent values. |
|
|
| ## Real-World Context |
| Column renames silently breaking downstream services are a perennial data engineering |
| incident: |
| - **The SELECT * anti-pattern**: When a source table adds or renames columns, any |
| downstream `SELECT *` query changes shape invisibly — no error until a consumer |
| reads a now-missing field. This pattern caused major outages at Uber (2019) and |
| multiple data platform teams documented in Martin Fowler's "Evolutionary Database |
| Design" (2016). |
| - **Expand-Contract migration**: The standard fix — add new column, backfill, update |
| consumers, then drop old column — is documented in every major database migration |
| guide (Liquibase, Flyway, Alembic). Skipping any step causes exactly the failures |
| this task tests. |
| - **ORM stale model bug**: Django ORM and SQLAlchemy models that reference old column |
| names fail silently at query time with `OperationalError: no such column`, not at |
| import time — making this class of bug hard to detect in staging. |
|
|
