Domain-Driven Design

Domain-Driven Design is an approach to software development that centers on the business domain, its elements, and their interactions

It advocates modeling based on the reality of business as relevant to your use cases

When to use

• Dealing with complex business processes / domains
• Large teams with multiple teams, where a common language / understanding is crucial
• Business rules and logic are complex and central to application functionality

Core Layers

Domain Layer

The Domain layer is the heart of your business logic

It represents the business concepts, rules, and logic specific to the problem domain your application is addressing

Components	Description
Entities	Objects that have a distinct identity that runs through time and different states
Value Objects	Objects that describe some characteristic or attribute but are not defined by a unique identity and are typically immutable
Aggregates	A cluster of domain objects (entities and value objects) that are treated as a single unit for data changes Each aggregate has a root and a boundary
Domain Services	Operations that belong to the domain model but don’t naturally fit within a domain entity or value object
Domain Events	Events that are significant within the domain

Application Layer

The Application layer serves as a coordination layer, orchestrating the flow of data and the sequence of operations in the application

It is concerned with the application's overall workflow but not with business rules or domain logic

Component	Description
Application Services	These services orchestrate domain objects to perform tasks They don't contain business logic themselves but coordinate domain layer objects to perform the business logic
DTO	Objects that carry data between processes, often used to encapsulate the data that is transferred over the network
API interfaces	Interfaces that define the operations available to external clients
Command, Query Responsibility Segregation (CQRS)	Separating the read (query) operations from the write (command) operations can be implemented in this layer

Infrastructure Layer

• Implements functionality required by domain layer (persistence)
• Data Access
• Integration with external services