tasks/CROSS2_schema_evolution/spec.md

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.