checking-migration-safety

name	checking-migration-safety
description	Use when a planned change touches persisted data shape — ORM models, migration files, schema definitions, serialized formats, or message contracts — in /change, /refactor, or /implement on an existing codebase. Routes schema changes through expand-contract and proves reversibility before any deploy.

Checking Migration Safety

Schema changes are the brownfield edits that cannot be reverted with git checkout. Code rolls back in seconds; a dropped column does not. The discipline: every schema change ships as a reversible migration, and any change that existing code cannot tolerate runs as expand-contract, never in one step.

The Iron Law

NO DESTRUCTIVE SCHEMA CHANGE IN THE SAME DEPLOY AS THE CODE THAT REQUIRES IT

Step 0 — Does this change need a migration?

Run this check when the planned diff touches any of: ORM models/entities, migrations/, schema.*, *.sql, serializer/DTO field definitions, or queue/event message shapes. If none are touched, exit this skill and proceed.

Process

1. Classify the change.

   Class	Examples	Risk
   Additive	new nullable column, new table, new index (concurrent), new optional field	Safe — old code ignores it
   Destructive	drop/rename column or table, narrow a type, add NOT NULL to existing column	Breaks old code or old data
   Transform	split/merge columns, change units or encoding, backfill derived values	Breaks both directions until complete

2. Additive: generate the migration, confirm the down-migration exists and actually reverses it, proceed. (An index on a large table must be created concurrently/online — a blocking index build is an outage, not a migration.)

3. Destructive or Transform: expand-contract. Plan and execute as separate, independently deployable steps — each step leaves old AND new code working:

   • Expand: add the new column/table/field alongside the old. Backfill in batches (bounded, resumable — never one giant UPDATE). New code writes both, reads old.
   • Migrate reads: switch reads to the new shape behind a flag or config toggle. Old shape still written.
   • Contract: only after the new path has held in production and no consumer reads the old shape, drop the old column/field in its own later migration. The contract step never ships in the same release as expand. If the lane is /change with a single deploy, deliver expand + migrate-reads only, and file the contract step as explicit follow-up work — do not "save a deploy" by contracting early.

4. Prove reversibility. Run the migration and its down-migration against a seeded copy of the schema (local DB or test container) before marking the work done: up → verify shape + data, down → verify the original shape returns. A down-migration that exists but has never run is documentation, not a rollback path. If the operation is genuinely irreversible (dropped data), say so explicitly in the migration's docstring and in your report — never imply rollback that does not exist.

5. Check old-code compatibility (N/N+1 rule). During a rolling deploy, the previous code version runs against the migrated schema. Ask: "does yesterday's code still work on tomorrow's schema?" If no — the change is Destructive; return to step 3.

6. Cross-service coordination. If the schema is read by more than one service (check specs/brownfield/architecture-map.md and the code graph for readers), every consumer must tolerate the expand state before any contract. List the consumers in your plan; an unlisted consumer found later is a blocked contract step, not a surprise outage.

Common Rationalizations

Excuse	Reality
"It's just a rename"	A rename is a drop plus an add — the most common destructive change. Expand-contract it (add new, dual-write, contract later).
"The table is small, I'll do it in one step"	Table size changes nothing about old code reading a dropped column during the deploy window.
"The down-migration is obvious, no need to run it"	Untested rollback paths fail exactly when you need them. Run it once on seeded data.
"No one else reads this table"	Prove it from the code graph and architecture map, then write the consumer list down. "I believe" is not evidence.
"The ORM auto-generates the migration, so it's safe"	Auto-generated migrations happily emit blocking index builds and NOT NULL on populated columns. Read what it generated.
"We can backfill in the migration itself"	A backfill inside a schema migration locks the table for its duration. Batch it as a separate data migration.