Use Cases and Message Handlers¶

Use Case¶

A Use Case represents a cohesive unit of application behavior.

Characteristics:
- Expresses a system capability
- Coordinates domain operations
- Returns an explicit result
- Does not depend on infrastructure

Use cases as coordination

A Use Case orchestrates domain behavior and outbound interactions, keeping policy separate from details. It does not contain business rules itself.

When to use¶

Implement UseCase for any system capability that coordinates domain objects and outbound ports. Return Result[OutputType, Error] so callers handle both outcomes. Keep the class thin — it orchestrates, domain objects decide.

Influence: Ivar Jacobson

The focus on use cases as units of behavior originates from Ivar Jacobson's work. ForgingBlocks provides a structured, type-safe contract for executing them.

Message Handler¶

A Message Handler represents an inbound port for a single message type.

Specializations:
- Command Handler — Handles commands (MessageHandler[CommandType, None])
- Event Handler — Handles domain events (MessageHandler[EventType, None])
- Query Handler — Handles queries and returns a typed result (MessageHandler[QueryType, ResultType])

When to use¶

Implement a MessageHandler when a single message type needs dedicated processing. Use the type aliases — CommandHandler, EventHandler, QueryHandler — to signal intent at the type level.

Message-driven architecture

Message Handlers enable CQRS, event-driven, and message-driven patterns by decoupling message types from processing logic.