Comparison Table: Hexagonal, Clean, Onion, and Traditional Layered Architectures

Comparison Table: Hexagonal, Clean, Onion, and Traditional Layered Architectures.md

Aspect	Traditional Layered Architecture	Clean Architecture	Onion Architecture	Hexagonal Architecture
Definition	Divides the application into layers based on functionality, with each layer having dependencies on the layer below it	Emphasizes separation of concerns and the use of SOLID principles for building scalable and maintainable software	Similar to Hexagonal Architecture, it aims to decouple the application core from the infrastructure layers. However, it adds the concept of concentric layers	Also known as Ports and Adapters Architecture, it focuses on separating the application core from the surrounding infrastructure layers
Dependency Flow	Dependencies flow from higher-level layers to lower-level layers, with each layer depending on the layer directly below it	The dependency flow is unidirectional, with higher-level layers depending on lower-level layers. Dependencies are not allowed to flow in the opposite direction	Similar to Hexagonal Architecture, the dependency flow is inward, with the core defining interfaces and the adapters implementing them	The flow of dependencies is inward, with the application core defining interfaces (ports) that are implemented by the adapters. The adapters depend on the interfaces defined by the core
Dependency Inversion Principle (DIP) Support	May not explicitly enforce the DIP, leading to potentially tightly coupled dependencies between layers	Emphasizes the DIP by using interfaces and dependency injection to invert dependencies, enabling higher-level layers to depend on abstractions defined by lower-level layers	Supports the DIP by using interfaces to define dependencies between layers, allowing for the inversion of dependencies and loose coupling	Strongly aligns with the DIP by relying on interfaces (ports) that are implemented by adapters, allowing the core to depend on abstractions
Deployment and Infrastructure	May have tight coupling with the infrastructure, making it more difficult to adapt or switch deployment environments without impacting the entire system	Is agnostic to the deployment and infrastructure details, allowing for easy adaptation to different environments	Separates infrastructure concerns into outer layers, making it easier to adapt or switch to different deployment environments without affecting the core	Abstracts away the infrastructure, making it easier to switch or adapt to different deployment environments without affecting the core
Flexibility and Adaptability	Can be less flexible and adaptable due to tight coupling, making it harder to change or replace components without impacting other parts of the system	Promotes flexibility and adaptability by enforcing separation of concerns, allowing for independent modification of layers without affecting others	Provides flexibility and adaptability by separating concerns into layers, making it easier to change or replace components without affecting other layers	Offers high flexibility and adaptability by isolating the core from external dependencies, making it easier to switch or upgrade components
Layering Philosophy	Layers are organized vertically based on functionality, with each layer providing services to the layer above it	Layers are organized based on business logic and responsibilities. Each layer has a clear purpose and can depend only on the layer below it	Concentric layers approach: The innermost layer represents the core business logic and the outer layers handle infrastructure and frameworks	Ports (interfaces) and Adapters (implementations) are the core elements. The application core is isolated from the external dependencies
Scalability and Maintainability	Can be scalable and maintainable, but tight coupling between layers can make it more challenging to modify or add new features without affecting other parts of the system	Promotes scalability and maintainability by enforcing separation of concerns and dependency inversion, making it easier to modify and add new features without impacting other parts of the system	Aims to improve scalability and maintainability through its layered approach, allowing for easier modification and replacement of components without affecting other layers	Enables scalability and maintainability by decoupling the application core from the infrastructure, allowing for easier modification and replacement of external dependencies
Testing and Isolation	Can make testing more challenging due to tight coupling between layers, requiring more complex integration testing	Advocates for testability by employing dependency inversion and interfaces, enabling mocking and unit testing of individual layers	Emphasizes testability through the use of interfaces and dependency injection, allowing for isolated testing of the core business logic	Promotes easier testing and isolation of the application core by using ports and adapters, allowing the core to be tested independently of the infrastructure
Use cases	- project requires a simple and straightforward architecture - project involves a small number of external integrations - project requires a high degree of scalability	- project requires a high degree of maintainability and understandability - project involves a large number of business rules and use cases - project requires a high degree of testability	- project requires a high degree of flexibility and maintainability - project involves a large number of complex business rules and domain logic - project requires a high degree of testability	- project requires a high degree of testability and maintainability - project involves a large number of external integrations, such as databases, file systems, or web services - project requires a high degree of flexibility, such as the ability to switch from one technology to another