| name | db-migrate |
| description | Author or review a Drizzle DB migration for zero-downtime safety — expand/contract phasing, backward-compatibility with the deployed app version, and writing the `-- migration-safe` acknowledgment the check:migrations lint requires. Use when adding/editing files under `packages/db/migrations/` or changing `packages/db/schema.ts`. |
DB Migrate Skill
You make schema changes that survive a deploy without downtime. The check:migrations lint (scripts/check-migrations-safety.ts) is the deterministic gate; you are the judgment that decides whether a flagged change is actually safe and writes the annotation that satisfies it.
The window (why this matters)
A deploy runs the migration, then rolls out the new app image via blue/green. The two are not atomic and cannot be — during cutover the old task set keeps serving against the already-migrated schema. So:
Every migration must be backward-compatible with the app version that is already deployed.
If a migration drops a column the old code still reads, renames one, or adds a NOT NULL the old inserts don't populate, the old code throws until traffic fully shifts — the downtime we're guarding against. You can't fix this by reordering the pipeline; the only fix is discipline.
Expand / contract
Split every breaking change across two deploys:
- Expand (this PR): additive, backward-compatible schema + code that tolerates both the old and new shape.
- Contract (a later PR, after expand is fully deployed): remove the old thing, now that nothing reads it.
Never put expand and contract in the same PR. If this PR both removes the code that used a column and drops the column, the old code is still live during cutover — split it.
Per-operation playbook
| You want to | Do (deploy 1 = expand) | Do (deploy 2 = contract) |
|---|
| Add a required column | ADD COLUMN nullable or DEFAULT; code writes it | backfill, then SET NOT NULL |
| Rename a column/table | add the new name; code dual-writes / reads new-then-old | drop the old name |
| Drop a column/table | stop all reads/writes in code; ship it | DROP (annotate) |
| Change a column type | add a new column of the new type; dual-write | backfill, swap reads, drop old |
| Add FK / CHECK | ADD CONSTRAINT ... NOT VALID | VALIDATE CONSTRAINT separately |
| Index an existing table | COMMIT; breakpoint → SET lock_timeout = 0 → CREATE INDEX CONCURRENTLY IF NOT EXISTS (see packages/db/scripts/migrate.ts) | — |
| Drop an index | COMMIT; breakpoint → DROP INDEX CONCURRENTLY — plain DROP INDEX takes ACCESS EXCLUSIVE on the table | — |
| Backfill data | batched + idempotent UPDATE (keyset/WHERE, bounded) | — |
A CREATE INDEX, ADD COLUMN, or ADD CONSTRAINT against a table created in the same migration is always safe (no rows, no live traffic) — the lint already suppresses those.
Tracking the contract (don't let it rot)
The contract half is deferred to a later deploy — and that is exactly when it gets forgotten, leaving dead columns, orphaned tables, and NOT NULLs that never land. Every deferred contract must become a durable, greppable TODO.
When an expand defers a drop, leave a contract-pending marker on the legacy column/table in packages/db/schema.ts — that is the file you will be editing when you finally do the drop, so the reminder lives where the work happens:
workspaceId: text('workspace_id'),
Format: contract-pending(<precondition>): <what to drop> — <why it's safe once the precondition holds>. The precondition names the PR/release that removes the last reader and must be fully deployed before the contract ships.
The judgment the lint can't do
The lint flags risky shapes; it cannot know whether a given drop is safe right now. For each flagged statement, do the work it can't:
- Is the dependency gone? Grep the app for the table/column: search
apps/sim and packages for the column name, the Drizzle field (camelCase), and the table object. If any live read/write remains, it is not safe — fix the code first.
- Did the expand already ship? The removal of that read/write must be in a deploy that is already out, not this same PR. If it's in this PR, split: land the code change now, do the destructive migration in a follow-up after it deploys.
- Backfills: confirm the
UPDATE/DELETE is batched (bounded WHERE/keyset, not a single whole-table statement), idempotent (safe to replay — a failed migration re-runs unjournaled files from the top), and safe under concurrent writes from the still-live old app.
Workflow
- Edit
packages/db/schema.ts, then cd packages/db && bunx drizzle-kit generate to produce the SQL. If this is an expand that defers a drop, leave a contract-pending marker on the legacy column (see "Tracking the contract"). If this is the contract, delete the marker it resolves.
- Hand-edit the generated SQL where the playbook requires it:
CONCURRENTLY + COMMIT; breakpoint for indexes on existing tables, NOT VALID for constraints, batching for backfills.
- Run
bun run check:migrations (base defaults to origin/staging).
- Verify locally:
cd packages/db && bun run db:migrate against a dev DB.
Hard rule
Never annotate a destructive statement just to make the lint pass. The annotation is a claim that you verified the old code no longer depends on it. If you can't make that claim truthfully, the change belongs in a later deploy — tell the user to split it.
