Onion architecture in development: how it’s used

How many types of architecture do you remember? Well, onion architecture is probably the best option to enhance testability and maintainability. It creates a structure of layers, just like an onion, where you could test everything separately.

Let us have a closer look at the core of onion architecture, the way it works, and why it is so important.

Onion architecture in development: how it’s used

What is onion architecture?

Briefly, onion architecture is a pattern that is designed to be used at the solution or system level. It comes with several principles and layers that will be explained below.

Onion principles

The main idea you should understand is group coupling. This is when layers depend on each other to work appropriately. In our case, there should always be an inward dependency. This means that the code should only depend on what’s closer to the center, ignoring “outside” layers.

Onion layers

You might have already guessed, but onion architecture isn’t about one stage. Instead, it consists of four layers:

  1. Domain model;
  2. Repository;
  3. Services;
  4. User interface.

Let’s start one by one. First goes the domain level. It could be called the heart of the whole application. Onion implies an inward dependency. Thus, it is reasonable that this layer depends only on itself. Apart from keeping the domain objects, the core also involves interfaces. Don’t get lost in them!

Next goes the repository layer. It is the one that sets a difference between the domain model and the general logic of the software. Technically, this is where we add features for saving objects, meaning a database. It’s all about data here.

The third layer is the services. It involves multiple features for editing, storing, attaching, and deleting data. It also acts as a mediator between the UI and the repository layers. Finally, the services even involve the entity’s business logic.

The last layer is the user interface. A UI is often also a unit test or web API of the application. Its only connection to the layers located inside is via other interfaces. Now, it is time we moved on to the pros and cons of this type.

Advantages of onion architecture

If you decide to implement onion architecture in your application, you’ll get multiple benefits. Some of them will be explained below.

1. Flexibility

The outer layer is like a separate part of the whole application logic. It doesn’t depend much on the previous layers, so you can easily swap databases and other details. The UI will still work.

2. Independence

The main code is independent. It doesn’t rely on anything, only on itself. However, all the other structures of the software are bound by its settings.

3. Rules

Onion architecture comes with very clear rules about priorities, dependence, and layers. Due to their clarity, you will always be able to understand which part relies on which layer.

Disadvantages of onion architecture

Alas, this isn’t the perfect solution, so it also has some drawbacks. Several of them will be explained below.

1. Navigation

There are multiple interfaces, making it easy to get lost in the whole mix. As a result, you’ll waste a lot of time figuring which interface to use.

2. Core

If you apply a change to the core, it will affect all the outer layers. That’s because the whole application focuses on logic, so any wrong action could break it down.

3. Newbies

Amateurs and newcomers in the area will have some trouble figuring out how the system functions. You’ll need an experienced developer to get the most out of onion architecture.

Onion architecture in development: how it’s used

Why do developers use onion architecture?

Every architecture type comes with its pros and cons. Onion is the best in terms of separation and coupling problems. It’s the perfect solution to create a logical model, although this often turns out to be a pain in the back.

Everything is focused in the middle of the app. This means that external changes won’t have any effect on the core. However, the slightest changes in the core will significantly influence all other layers. So, if you decide to apply some new features too late, you are likely to get into a lot of trouble.

Is onion architecture popular?

Yes, onion architecture is well-known in the programming world, and many applications use its principles. However, it isn’t the ultimate solution. You could say that it’s another approach to solving a business problem.

Each type of architecture has its advantages and disadvantages – that’s the whole point of why different versions are created. The option your app might need completely depends on its area, features, and many other details.

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Ivan Kolesnikov

About the author:

Ivan Kolesnikov

Experienced professional in programming.