Database Migrations

Database Migrations

Purpose

Evolve the schema safely: every change a versioned, reviewed, immutable migration; applied in order everywhere; shaped so deploys don't break running code. Moving/transforming data is data-migration; this skill owns schema change mechanics.

When to Use

• Whenever the schema changes, and when establishing the migration workflow.
• Not for backfills/transformations (data-migration) or seed content (seed-data).

Inputs

• Schema change intent (relational-schema-design / document-schema-design output).
• Tooling from the data layer (prisma-relational / drizzle-relational / mongoose-mongodb index builds), deploy flow (../../backend/backend-deployment).

Discovery Questions

• What applies migrations in each environment (deploy step — automated, ordered, once)?
• Is zero-downtime required (drives expand/contract), and what's the table size (locks!)?
• What's the actual rollback story — down migrations, or roll-forward-only with code rollback?

Responsibilities

• Establish the workflow: migrations as versioned files in the repo, generated-or-written then reviewed as code (locks, constraint validity, index build behavior), immutable once applied beyond dev — fixes are new migrations.
• Order migrations with deploys: applied before new code serves traffic (backend-deployment); exactly-once application per environment (migration table/lock).
• Shape changes for zero-downtime where required — expand → migrate → contract:
  • additive first (new nullable column/table/index), old code keeps working;
  • backfill via data-migration (batched, not in the schema migration);
  • constraints tightened (NOT NULL, uniques) only after backfill;
  • destructive contraction (drops/renames) only after no deployed code references the old shape.
• Respect lock behavior on big tables: concurrent index builds where the engine offers them, constraint validation split from creation, batched changes — a migration that locks a hot table for minutes is an outage.
• Define the rollback ruling: schema is usually roll-forward-only (down migrations lose data or lie) — recorded explicitly; code rolls back, schema rolls forward (backend-deployment alignment).
• Prevent drift: CI compares schema-as-migrated vs schema-as-defined; no manual production edits — ever; MongoDB index/validator changes ride the same versioned mechanism.

Required Workflow

1. Express the schema change as migration file(s); review the SQL/operations for locks and correctness.
2. Classify: additive / tightening / destructive → apply expand/contract sequencing if zero-downtime.
3. Coordinate backfills with data-migration between expand and tighten.
4. Verify against a production-like dataset (timing, locks) — staging first.
5. Apply via the deploy pipeline; confirm drift checks pass.

Decision Rules

• Rename = add + backfill + contract, never in-place, under zero-downtime.
• NOT NULL/unique constraints arrive only after data provably satisfies them.
• Long operations (index builds, validations) use the engine's non-blocking modes or run in maintenance windows — chosen, not stumbled into.
• Down migrations exist only where they're honest; otherwise the ruling is roll-forward and everyone knows it.

Rules

• No schema change outside a migration file; no editing applied migrations.
• Every migration reviewed (locks, data safety) before merge (../../code-review).
• Destructive migrations name the approval that authorized them (../../../system/QUALITY_GATES.md).

Anti-Patterns

• db push/auto-sync against shared environments.
• Editing an applied migration "because it was wrong."
• Schema migration + heavy backfill in one transaction, locking the table.
• Dropping a column while old code still reads it.
• Manual production hotfix SQL that no migration records (drift).

Validation Checklist

• Change captured as versioned, reviewed migration file(s).
• Applied-before-code ordering; exactly-once mechanism.
• Expand/contract sequencing where zero-downtime required.
• Lock behavior assessed on realistic data size.
• Rollback ruling recorded (roll-forward vs honest down).
• Drift check in CI; no manual edits anywhere.

Definition of Done

A reviewed, versioned migration path for the change — safely ordered against deploys, lock-aware, expand/contract where needed, with an explicit rollback ruling and drift protection.

Related Skills

data-migration, seed-data, relational-schema-design, document-schema-design, prisma-relational, drizzle-relational, mongoose-mongodb, ../../backend/backend-deployment, backup-recovery, ../../migration-planning.

Related Knowledge

../../../knowledge/ (table sizes, downtime tolerance, environment list).

Related References

../../../references/database/migrations/ (sequencing playbooks, when populated).

Context Loading Guidance

• Requires: the schema change intent, data size, deploy flow, downtime tolerance.
• Does not require: application feature code, full schema history.
• May load: data-migration (backfill leg), ../../backend/backend-deployment (ordering).
• Stop when: the migration files + sequencing + ruling are recorded.

Token Efficiency Guidance

Review the migration diff, not the whole schema. The expand/migrate/contract step list per change is the artifact.