What Would You Consider as a Unit in a Software System?

Patrick Burnett

When designing a software system, one of the most important decisions you’ll have to make is how to break down the system into manageable pieces. The way you divide your system will affect everything from development time to maintainability and scalability. In this article, we’ll explore the concept of a “unit” in a software system and what factors you should consider when defining your units.

What is a Unit?

At its core, a unit is simply a discrete piece of functionality within your software system. Depending on the size and complexity of your application, these units could be as small as individual functions or methods or as large as entire subsystems or modules.

Factors to Consider When Defining Units

When deciding how to break down your software into units, there are several factors you should consider:

Functionality: The most obvious factor is what each unit does within the larger context of your application. Ideally, each unit should be responsible for one specific task or set of related tasks. This makes it easier to reason about the code and reduces the likelihood of bugs creeping in due to interactions between different parts of the system.

Coupling: Coupling refers to how much each unit depends on other units in order to function correctly. A highly coupled system can be difficult to maintain because changes made in one part of the code can have unintended consequences elsewhere. Ideally, each unit should be as self-contained as possible with minimal dependencies on other parts of the system.

Cohesion: Cohesion refers to how closely related the functionality within a unit is. A highly cohesive unit performs one specific task and doesn’t try to do too much at once. This makes it easier to understand and modify the code within that unit without affecting other parts of the system.

Size: The size of each unit can also have an impact on maintainability and scalability. Large units can be difficult to understand and modify, while small units can lead to excessive code duplication and make it harder to see the “big picture” of how everything fits together.

Examples of Units

So what might a unit look like in practice? Here are a few examples:

  • A function that calculates the total price of an order
  • A class that handles user authentication
  • A module that provides access to a database
  • A subsystem responsible for generating reports


Defining units is an important part of software design that can have far-reaching implications for your application’s maintainability and scalability. By considering factors such as functionality, coupling, cohesion, and size, you can create a system that is easier to reason about and modify over time.