Changes made in the code here do not affect/break the entities or external dependencies such as databases. Thinking of flexibility for elegant architectures, the domain layer is also where domain events could be triggered. Event Sourcing might be implemented here, with events being emitted when the domain entity changes. Notice that our front-end UI code depends only on our domain layer, and not the persistence layer directly.

By following clean architecture, you can write software today that can be easily switched out for different technologies in the future. Clean architecture reduces the database, the web framework, even the dependency injection framework to just a plugin, a plugin that can be changed with a moderate effort. Moreover, you gain unit testability because everything that is expensive is hidden behind a boundary. Last but not least, this is layer is responsible for presenting results and accept incoming request from end-user/external service. It holds all layers, registers and configures them as this is an entry point to our application.

Perhaps they belong in a different era where we were trying to scale client server applications for thousands of users. Layering a load-balanced presentation layer on top of data processing logic made a lot of sense as it helped to make applications more scalable and resilient. It is often described as an onion, presumably in response to the tears that are shed a few years down the line. “Clean” architecture extends this metaphor with a rule stating that dependencies can only point inwards, i.e. from the UI towards a central layer of data entities. Hexagonal architecture prefers a clean separation between the “inside” and “outside” parts of an application. This layer is responsible only for operating the application.

onion architecture vs clean architecture

Each has at least one layer for business rules, and another for interfaces. The domain layer is basically where all the business rules live. It doesn’t know any other layer, therefore, has no dependencies, and is the easiest to test. Even if you change all your application around, the domain is still intact, as all it contains are business rules that anyone can read to understand what is desired by your application. It might also be where you want to start testing. If you don’t have an enterprise, and are just writing a single application, then these entities are the business objects of the application.

Clean Architecture Node Js

Handles the creation, validation and reading of our entities . Note that this should be custom logic and not include the DB implementation of models (e.g. in Mongoose DB ORM, their models should be encapsulated in the DB layer below). The point is that you should have the freedom to pick and choose your abstractions, frameworks and technologies to suit each use case. A “clean” architecture based on layers tends to preclude this. This is one of the more darkly pragmatic benefits of service-based development.

onion architecture vs clean architecture

The web world is a collection of various traditional architectures. Each of these architecture has its pros and cons. We then dependency inject the validation library into the model. Note we use a wrapper for the validation library to make it easier to switch out libraries in future. Business rules don’t know anything about outside world. Libraries and frameworks should be treated as tools and not dependencies.

The reason for that is you can have some logic in the Entity that is common to all of the domain classes. In this example, the common logic is to generate a collision-resistant ID optimized for horizontal scaling, which means for all of our entities, we will use the same strategy. It allows us to be better programmers and prepare for inevitable future change to the project.

Example Application Structure

No code inward of this circle should know anything at all about the database. If the database is a SQL database, then all the SQL should be restricted to this layer, and in particular to the parts of this layer that have to do with the database. We do not expect changes in this layer to affect the entities. We also do not expect this layer to be affected by changes to externalities such as the database, the UI, or any of the common frameworks. In fact your business rules simply don’t know anything at all about the outside world. Start with how we think the outer layers will communicate with the data-access layer and what they should expect to get.

  • This layer is dependent only on Application layer where the interfaces are defined and Common.
  • It is a whole different approach having one place for all the business logic.
  • The interface can be put together with the domain layer, while its implementation stays in the infrastructure.
  • This architecture is made up of several concentric layers interfacing towards the core that denotes the domain.
  • My current focus is on providing architectural leadership in agile environments.
  • Over time any single implementation will struggle to meet the scale requirements of both large-scale data ingestion, tactical reporting and responsive interfaces.
  • And second testing the creation and read only of the model entity.

In the controller config section of a module, you define a factor to provide the implementation of whatever controller is requested . Through this factory method, you would instantiate a controller object, passing the OrderRepository from the Persistence library as an argument to the constructor. Also in this layer is any other adapter necessary to convert data from some external form, such as an external service, to the internal form used by the use cases and entities. By the same token, data formats used in an outer circle should not be used by an inner circle, especially if those formats are generate by a framework in an outer circle.

A Look Into Layers Of Onion Architecture

You should be able to test any significant functionality in isolation. My motivation to revisit this topic is my experience applying clean architecture. It is a whole different approach having one place for all the business logic.

It makes it easy to test the domain layer, which means you can ensure all of your business rules are being respected and write long term bug-proof code. DDD together with Onion are a consistent way to avoid the cascade effect code, where you change one piece and the side effects are innumerable. Your services, controllers, views, etc don’t even notice anything changed.

At the center of the onion is your domain – your business logic core on which everything in the application depends. The foundation of Onion Architecture is based on the inversion control principle. This architecture is made up of several concentric layers interfacing towards the core that denotes the domain. The unique part of Onion Architecture is – it has zero dependencies over data layers like classic multi-tier architectures.

In my project I called this layer ShenAn.Domain. As you can see in snippet – it has no dependencies. I will try to explain to you how to setup clean app architecture based on my university project with a temporary name ShenAn.


They encapsulate the most general and high-level rules. They are the least likely to change when something external changes. For example, you would not expect these objects to be affected by a change to page navigation, or security. No operational change to any particular application should affect the entity layer. Entities encapsulate Enterprise wide business rules. An entity can be an object with methods, or it can be a set of data structures and functions.

These approaches are all trying to achieve separation of concerns. The architecture is not dependent onion structure upon any specific framework or technology. There is a separation between data processing and the UI.

Clean Architecture Vs Onion Architecture

This layer is dependent only on Application layer where the interfaces are defined and Common. With our class that allows us to have a memory-based persistence layer created, we are now ready to use a mapper/repository approach. To implement our domain layer, we start with a base abstract class called Entity, that other domain classes can extend.

The diagram at the top of this article is an attempt at integrating all these architectures into a single actionable idea.

Each one of them points inwards towards the use cases. You can swap out Oracle or SQL Server, for Mongo, BigTable, CouchDB, or something else. Your business rules are not bound to the database.

The Dependency Rule

Tests do not depend on heavy frameworks anymore in order to test a small use case. You gain the freedom to postpone decisions about tools and frameworks until you really get to the point . MVC or ORM patterns are still valid but live then only within the external plugin, hidden by a boundary. Here you keep all domain entities and interfaces. This layer contains enterprise types and logic.

How Do Autonomous Services Solve The Problem?

The important thing is that isolated, simple, data structures are passed across the boundaries. We don’t want to cheat and pass Entities or Database rows. We don’t want the data structures to have any kind of dependency that violates The Dependency Rule. The outermost layer is generally composed of frameworks and tools such as the Database, the Web Framework, etc. Generally you don’t write much code in this layer other than glue code that communicates to the next circle inwards. Similarly, data is converted, in this layer, from the form most convenient for entities and use cases, into the form most convenient for whatever persistence framework is being used.

This facilitates by protecting your business from undesired dependencies. On the other hand, we can see dependencies and services relied upon each other on the code level. In such cases, you will find numerous dependencies to switch with interfaces. So there is no one place where you can find all the use cases.

Practical Example Of Clean Architecture In Node Js

We don’t want anything in an outer circle to impact the inner circles. The business rules can be tested without the UI, Database, Web Server, or any other external element. The term “Clean Architecture” is just the name of the article. The onion architecture is a specific application of the concepts explained in the article. Today, we will discuss Onion Architecture which is also said to be a cousin of layered and hexagonal architectures.

Then, the outer layers implement these interfaces. Any changes to the inner layer will cascade up and as a result there should be less testing done on the outer layers. E.g. if we were to test changes to the model schema in the driver layer we would need to rewrite our tests here as well. This is an example of a simple CRUD application with layered software and separation of business logic vs technology. Layered applications tend to be based on a very rigid set of common abstractions, e.g. a “controller” must talk to a “service” that must talk to a “repository”. This creates a mock-heavy code base that slows down development by insisting on applying the same solution to every problem.

That would violate The Dependency Rule because it would force an inner circle to know something about an outer circle. Typically the data that crosses the boundaries is simple data structures. You can use basic structs or simple Data Transfer objects if you like. Or the data can simply be arguments in function calls. Or you can pack it into a hashmap, or construct it into an object.

The UI can change easily, without changing the rest of the system. A Web UI could be replaced with a console UI, for example, without changing the business rules. Finally we write our drivers whose only dependency is our data-access layer. The drivers should not communicate directly with the model or db. Note for our MongoDB data-access implementation we depend on the DB from the inner layer.

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