Golang DDD CQRS

Use this skill to split application logic into clear commands and queries without turning Go code into ceremony-heavy enterprise scaffolding.

Start Here

• Use CQRS when a service has meaningful write-side behavior, mixed read and write models, or application services that are hard to reason about.
• Skip or minimize CQRS when the service is mostly simple CRUD or login-like flows with little business behavior.

Workflow

1. Split the use cases into writes and reads.

• Commands mutate state and may return errors.
• Queries return data and should not perform business mutations.

2. Name them in business language.

• Prefer ScheduleTraining, CancelTraining, ApproveReschedule, AvailableHours.
• Avoid default CRUD names unless the business really speaks that way.

3. Introduce separate handlers when it improves clarity.

• Use one command handler type per command when the logic or dependencies differ.
• Use one query handler type per query when read concerns differ.
• Keep interfaces narrow and owned by the handler that consumes them.

4. Keep handlers orchestration-only.

• Domain rules belong in the domain layer.
• Transport mapping belongs in ports.
• Database or external API details belong in adapters.

5. Shape the ports around CQRS.

• HTTP or gRPC ports may call commands directly.
• If a create command needs a follow-up read, prefer 204 No Content plus content-location when practical, or execute an explicit query after the command.
• Keep port-specific error translation at the edge.

6. Test the application layer as orchestration.

• Mock repositories and outbound services with tiny handwritten mocks.
• Keep business-scenario assertions in domain tests unless the behavior truly belongs to application orchestration.

Guardrails

• Do not create separate command and query packages if the service is still trivial.
• Do not let commands return large read models by default.
• Do not hide business logic in command handlers just because they are convenient.
• Do not keep one giant application service if separate handlers would shrink interfaces and tests.

Use These References

• Read references/cqrs-guidelines.md for naming, packaging, and tradeoffs.
• Read references/application-tests-and-errors.md for handler testing and transport-agnostic errors.

Deliverables

• command and query boundaries that match the use cases,
• business-oriented names,
• narrow handler-owned interfaces,
• thin transport code,
• handler tests focused on orchestration instead of domain internals.