Multi-tier architecture, also known as n-tier architecture, is a design pattern used to divide an application into separate layers or tiers, each with a specific responsibility. This separation improves scalability, maintainability, and flexibility by isolating different functionalities within distinct components. Layers are a way to separate responsibilities and manage dependencies. Each layer has a specific responsibility. A higher layer can use services in a lower layer, but not the other way around.

The tiers in n-tier architecture

An n-tier architecture typically consists of at least three layers:

Presentation layer (UI): This is the user interface, often implemented with front-end technologies like Angular, React, or Vue.js. It handles interaction with the user and displays the data fetched from the backend.

Logic layer (API/Service): This layer is the business logic layer or service layer, where the core functionality of the application resides. It processes requests from the presentation layer, interacts with the data layer, and sends the necessary data back. In your example, this is handled by the ASP.NET Core API.

Data layer (Database): The data layer is responsible for storing and managing data. It can involve one or more databases, such as Redis for caching and MongoDB for NoSQL storage in your application.

Example: ASP.NET Core API with Redis, MongoDB, and Angular UI

Let's consider an example application where it contains API (C#), UI (Angular) and DB (SQL). The app has a front-end UI built with Angular, an API built using ASP.NET Core, and it interacts with one database: SQL. Each component represents a separate tier of the architecture.

Here’s how the flow works:

1. The user interacts with the Angular UI (presentation layer).
2. The Angular app sends HTTP requests to the ASP.NET Core API (logic layer).
3. The ASP.NET Core API handles the requests, applies the business logic, and interacts with the database (SQL).

Benefits of n-tier architecture

• Scalability: Each tier can be scaled independently. For example, if there’s a high load on the API, you can scale that tier without affecting the others.
• Maintainability: Separation of concerns makes the code easier to maintain. Each layer is responsible for a distinct function.
• Security: By isolating the layers, security measures can be enforced differently for each tier. For example, sensitive data can be protected better in the data layer.

When to use this architecture

N-tier architectures are typically implemented as infrastructure-as-service (IaaS) applications, with each tier running on a separate set of VMs. However, an N-tier application doesn't need to be pure IaaS. Often, it's advantageous to use managed services for some parts of the architecture, particularly caching, messaging, and data storage.

Consider an N-tier architecture for:

• Simple web applications.
• A good starting point when architectural requirements are not clear yet.
• Migrating an on-premises application to Azure with minimal refactoring.
• Unified development of on-premises and cloud applications.

N-tier architectures are very common in traditional on-premises applications, so it's a natural fit for migrating existing workloads to Azure.

Considerations

• N-tier architectures are not restricted to three tiers. For more complex applications, it is common to have more tiers. In that case, consider using layer-7 routing to route requests to a particular tier.