12. Use Domain-Driven Design tactical patterns

Date: 2019-07-01

Log date: 2019-11-05

Status

Accepted

Context

We decided to use the Clean Architecture (ADR #10) and create Rich Domain Models (ADR #11) for each module. We need to define or use some construction elements / building blocks to implement our architecture and business logic.

Decision

We decided to use Domain-Driven Design tactical patterns. They focus on the Domain Model implementation. Especially we will use the following building blocks:

• Command - public method on Aggregate (behavior)
• Domain Event - the immutable class which represents important fact occurred on a special point of time (behavior)
• Entity - class with identity (identity cannot change) with mutable attributes which represents concept from domain
• Value Object - immutable class without an identity which represents concept from domain
• Aggregate - cluster of domain objects (Entities, Value Objects) with one class entry point (Entity as Aggregate Root) which defines the boundary of transaction/consistency and protects business rules and invariants
• Repository - collection-like abstraction to persist and load particular Aggregate
• Domain Service - stateless service to execute some business logic which does not belong to any of Entity/Value Object

Consequences

• We need to define entities and value objects
• We need to define aggregates boundaries
• We need to add repositories for each aggregate
• We can invoke only public methods on Aggregate Roots, everything else should be hidden
• Developers need to be familiar with DDD tactical patterns