Application

What the system does

The Application block defines system behavior by coordinating domain concepts, handling incoming requests, and invoking outbound capabilities.

It depends on Domain and Foundation. It must not depend on Presentation or Infrastructure implementations.

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How it works

The Application block sits between the outside world and the Domain as a behavioral boundary.

UseCase flow:
1. Receives a request through an inbound port.
2. Coordinates domain objects.
3. Invokes outbound ports for persistence or side effects.
4. Returns a Result[OutputType, Error].

A MessageHandler follows the same pattern but reacts to a single message type — commands, events, or queries.

Ports define what the application needs, never how. Outbound ports like RepositoryPort or EventBusPort are contracts. Infrastructure implements them. The Application block composes them without knowing their implementation.

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How to use

Wire up a use case step by step:

1. Define an inbound port for each system capability.
2. Implement it as a UseCase class.
3. Inject outbound ports through the constructor — repositories, event buses, loggers.
4. Return Result[OutputType, Error] so callers handle both paths explicitly.

Keep use cases thin. They orchestrate; domain objects decide. When a use case grows, extract domain logic into value objects or entities. When it needs new I/O, add an outbound port and implement it in Infrastructure.

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Core abstractions

• Use Cases — Cohesive units of application behavior; coordinate domain + outbound
• Message Handlers — React to commands, events, and queries
• Inbound & Outbound Ports — How the system is interacted with and what it depends on
• Application Errors — Structured error types for application-level failures

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What it does not do

• Enforce domain invariants
• Persist data directly
• Handle transport or frameworks
• Implement infrastructure concerns

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Glossary

Use Case

An inbound port representing a cohesive unit of application behavior. Orchestrates domain operations and outbound interactions.

Message Handler

An inbound port that reacts to a single message (command, event, or query).

Inbound Port

An abstraction defining how external actors interact with the application.

Outbound Port

An abstraction representing a capability the application depends on (persistence, messaging, etc.).

Repository Port

An outbound port abstracting access to persisted domain objects.

Unit of Work

An outbound port defining a transactional boundary across multiple persistence operations.