Database Migration Class Diagram Template

A database migration class diagram maps the relationships between old and new schema versions, migration scripts, versioning controllers, and rollback handlers involved in a zero-downtime schema change. It visualizes how classes like `SchemaVersion`, `MigrationRunner`, `ColumnTransformer`, and `FeatureToggle` interact during a live migration, making it easy to reason about state transitions without taking your application offline. Engineers use this diagram to document the full lifecycle of a schema change — from backward-compatible column additions to data backfilling and legacy column removal — ensuring every team member understands the sequence and dependencies before a single line of migration code is written.

## When to Use This Template

This template is most valuable when your team is planning multi-phase migrations such as the expand-contract pattern, where a new column is added alongside the old one, data is dual-written, backfilled, and then the old column is eventually dropped. It is equally useful for coordinating blue-green deployments, shadow tables, or online schema change tools like pt-online-schema-change or gh-ost. If your migration involves multiple services reading from the same database, a class diagram helps surface coupling risks early — showing which service classes depend on which schema version and where interface contracts must be maintained during the transition window.

## Common Mistakes to Avoid

One of the most frequent errors teams make is treating a database migration as a single atomic event rather than a coordinated sequence of deployable steps. A class diagram forces you to model each phase explicitly, which exposes hidden dependencies that would otherwise cause downtime. Another common mistake is omitting the rollback path from the diagram entirely. Every migration class should have a corresponding rollback or compensating action modeled as a method or associated class, so your team knows exactly how to revert if monitoring detects anomalies post-deployment. Finally, avoid conflating application-layer migration logic with database-layer DDL changes in the same class — keeping these concerns separated in your diagram leads to cleaner, more testable migration code and a safer production rollout.