What is a three-tier application architecture?

A three-tier application architecture divides an application into three key parts:

1. The presentation tier hosts the front end that users see.
2. The application tier handles internal data processing.
3. The data tier stores any persistent data that the application requires.

In a three-tier architecture, you implement each of these application components separately. This makes three-tier apps different from monoliths, in which all functionality runs as a single process.

You can choose to separately deploy each part of a three-tier app. For example, you could run the data layer using a database hosted on one server, while running the presentation and application layers on other servers and having them interact with each other via a local network. This approach may offer some security benefits because it enables you to create a level of isolation between each application layer.

Alternatively, you could deploy all parts of a three-tier app on the same server while using different processes to run each layer. There's no isolation in that scenario, but the upside is that you don't have to rely on the network to connect the application layers. So, network performance trouble won't harm internal application performance.

What is a microservices architecture?

Microservices is an application design strategy that breaks the application into multiple discrete parts: the microservices.

There are no hard-and-fast rules about how many microservices to include in an application. Typically, you'd have significantly more microservices than three, which is what makes microservices different from a three-tier architecture. For example, your application interface alone could depend on multiple microservices. If you use a micro front-end approach, there could be one microservice for each major page or type of content in the app. Your application's internal logic would typically involve multiple microservices, too, and the data backend may require different microservices for different aspects of data handling.

Similarities between three-tier architectures and microservices

Three-tier apps and microservices apps are similar in that both make applications more modular than they would be under a monolithic architecture. Both provide similar benefits, including the following:

• The ability to separate an application codebase into distinct parts, which simplifies development and source code management.
• The ability to deploy each application component independently of the others, ensuring a simpler deployment and update process.
• Some level of separation between the various parts of the application. Separation offers security benefits by reducing the chances a breach in one part of the app impacts other components. It can also improve performance because an issue, such as a memory leak, with one part of your app won't necessarily cause the entire app to fail.

In short, both architectures enable a higher degree of application flexibility and agility than a monolith.

Likewise, three-tier apps and microservices apps both involve complexity. Building, deploying and managing applications require a dev team to work with multiple parts at each step of the process.

Major differences in three-tier apps vs. microservices

The main difference between three-tier apps and microservices apps is that, with microservices, your application is more modular.

Microservices apps double down on both the benefits and challenges described above. With microservices, you get a higher degree of separation between each part of your application, which leads to even more flexibility and agility than you'd get from a three-tier app.

Because there are so many individual parts, microservices apps are more complex to create and manage. You have more components to deploy and track. Plus, you have more dependencies between microservices to manage. In fact, microservices apps are so complex that they often require the use of special tools, such as orchestrators and service meshes, to help manage them. You typically don't need such tools for a three-tier app.

Which to use when

So, is three-tier architecture or microservices architecture best for your app? Choose by considering the following factors:

• How complex is your application's functionality? Apps that do just a few things -- serving basic Web content, for example -- are a good fit for a three-tier architecture. Microservices work best with complex apps that need to do many different things because you can create a separate microservice for each area of functionality.
• How much do you need the app to scale? Both architectures are more scalable than monoliths, but microservices apps can scale efficiently. Since there are more individual components to scale, you scale up and down in a more granular way.
• Where will you deploy your application? If your host environment consists of just one or a few servers, a three-tier architecture might make more sense because you won't be able to spread your services across a large cluster, which maximizes scalability and resilience -- two of the areas where microservices shine brightest.
• How sophisticated are your software development processes? If you have an advanced CI/CD pipeline capable of developing multiple services simultaneously, microservices might be right for you. Teams that want simpler development operations benefit more from three-tier apps, which don't introduce as much complexity to the software delivery pipeline.
• How ready is your IT team to support a complex application? Since three-tier apps are simpler to deploy, monitor and manage, they are a better fit for organizations with IT teams that are small or unaccustomed to a complicated architecture.