Projection Patterns

Comprehensive guide to building projections and read models for event-sourced systems.

Use this skill when

• Building CQRS read models
• Creating materialized views from events
• Optimizing query performance
• Implementing real-time dashboards
• Building search indexes from events
• Aggregating data across streams

Do not use this skill when

• The task is unrelated to projection patterns
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.
• If detailed examples are required, open resources/implementation-playbook.md.

Resources

• resources/implementation-playbook.md for detailed patterns and examples.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

Example

For OrderItemAdded at position 42, apply the increment and persist checkpoint 42 in one transaction. Replay position 42 and verify no second increment; crash before commit and verify both the row update and checkpoint roll back together.