Imported from jabrena/plinth (skills/055-design-parallel-change/SKILL.md). Install upstream with npx skills add jabrena/plinth --skill 055-design-parallel-change. Copyright stays with the author (Apache-2.0).

Parallel Change Design

Guide Java Enterprise developers through Parallel Change for database migration scenarios. This is an interactive SKILL.

What is covered in this Skill?

• Deciding whether a database migration needs a compatibility window
• Sequencing database changes as expand, migrate, and contract deployable steps
• Preserving old and new application versions during rollout and rollback windows
• Handling column renames, type or data reinterpretation, large backfills, relationship-table changes, and index or uniqueness changes
• Explaining tradeoffs such as temporary dual paths, extra migrations, verification effort, and delayed cleanup
• Routing framework-specific implementation to Spring Boot, Quarkus, or Micronaut Flyway skills after the design approach is clear

Constraints

Use Parallel Change as a database migration design pattern, then hand off implementation details to the framework-specific migration skill when needed.

• MUST read references/055-design-parallel-change.md before applying Parallel Change guidance
• MUST explain expand, migrate, and contract as separate deployable steps when recommending Parallel Change
• MUST decide whether Parallel Change is needed before recommending framework-specific Flyway implementation details
• MUST recommend a simpler single migration when the change is additive, immediately safe, and does not require old and new application versions to coexist
• MUST describe the compatibility, data-preservation, verification, and cleanup tradeoffs of the chosen approach
• MUST use framework-specific Flyway skills for Spring Boot, Quarkus, or Micronaut implementation details after the migration strategy is selected

When to use this skill

• Apply Parallel Change to this database migration
• Use expand migrate contract for this schema change
• Plan a safe column rename migration
• Plan a zero-downtime database migration
• Should this Flyway migration use Parallel Change?
• Design a safe backfill and contract migration
• Decide if this schema change needs a compatibility window

Workflow

1. Classify the Migration Risk

Read references/055-design-parallel-change.md, then identify the schema shape, data meaning, application read/write path, rollout window, rollback expectation, and production-data risk affected by the migration.

2. Choose Parallel Change or Simpler Migration

Recommend Parallel Change when old and new application versions or data interpretations must coexist. Recommend a simpler forward-only migration when the change is additive, small, immediately safe, and does not require a compatibility window.

3. Design Expand, Migrate, Contract

For Parallel Change, separate expand, migrate, and contract into independently deployable steps. Keep the old shape valid during expand and migrate; contract only after all deployed code and data have moved to the new shape.

4. Plan Verification and Operations

Define previous-release fixture checks, row-count expectations, duplicate detection, backfill monitoring, rollback behavior, and cleanup ownership. Prefer production-dialect migration tests and production-like data samples where feasible.

5. Route Framework Implementation

After the strategy is clear, use 313-frameworks-spring-db-migrations-flyway, 413-frameworks-quarkus-db-migrations-flyway, or 513-frameworks-micronaut-db-migrations-flyway for framework-specific Flyway dependencies, configuration, migration locations, and tests.

6. Report the Migration Plan

Report the chosen approach, rejected alternative, expand/migrate/contract sequence or simpler migration, verification evidence, cleanup trigger, tradeoffs, skipped checks, and remaining risks.

Reference

For detailed guidance, examples, and constraints, see references/055-design-parallel-change.md.