Functional vs Non-Functional Testing: Which Should You Focus On?

One of the biggest challenges in software testing is prioritizing your tests. You can’t test everything all at once, so you have to decide which to prioritize. Like, which should you do more, functional or functional tests?

As usual, the answer is never straightforward.

So in this article, we’ll first consider:

• What functional and non-function testing are.
• How they’re similar to being a food critic.
• When to focus on either based on your SDLC

It’s going to be fun! Let’s dive in.

Functional Testing

Functional testing is all about verifying that an application can do what it’s supposed to based on defined user requirements.

You can think of functional testing like verifying that a meal provides all the nutrients required for optimal body functions. You don’t care about how it tastes, just that it has the right nutrients.

Functional testing is primarily concerned with user interaction and business logic, focusing on verifying that the product successfully performs its expected tasks. When you’re performing functional tests, you’re asking questions such as:

• can the user login successfully?
• can the user create a new account?
• can the user update their account details?

So in one sentence, functional testing verifies that the product works!

Types of Functional Tests

1. Unit Tests: These tests verify the quality of the code and form the foundation of functional testing. They verify that small pieces of code work as they should in isolation.
2. Integration Tests: These tests verify that different parts of your application like database, and authentication service works together to carry out a user’s task.
3. End-to-End Tests: They simulate an actual user’s interaction with the product as a whole not just its components.

These are the foundational types of functional testing. Others include component tests, acceptance tests, system testing, and API testing.

But how about non-functional tests, what do they test?

Non-Functional Testing

If functional testing is similar to verifying that a meal contains the right nutrients, then non-functional testing checks if it’s served at the right temperature, tastes good, and doesn’t make you sick.

Non-functional tests evaluate aspects of a software product that are not related to specific functions or features. Rather they evaluate how well the application performs its tasks based on qualities like:

• Performance: How quickly and efficiently the system responds to user actions.
• Security: The system's ability to protect sensitive data from unauthorized access.
• Reliability: The system's ability to function consistently and without errors.

So in essence, while functional testing validates the “IF”, non-function testing validates the “HOW”.

Let’s consider some types of non-functional tests.

Types of Non-Functional Tests

1. Performance Test: These tests evaluate how well the system performs under various load conditions such as a large number of users using the system concurrently.
2. Security Tests: These tests ensure that the system isn’t vulnerable to exploits that could comprise sensitive data. Penetration testing, vulnerability testing, and even code review are considered part of security testing.
3. Reliability Tests: They evaluate the application’s ability to function consistently over time without crashes or errors.
4. Usability Testing: This tests how seamless it is for a user or customer to use an application.

There are other types of non-functional tests, often referred to as “ilities,” such as maintainability, portability, interoperability, and scalability. However, these are the types carried out on most applications.

Both functional and non-functional tests are integral in ensuring that a product satisfies the user, but which should you focus on more?

Which Test to Focus On

Realistically, you can’t do everything at once, especially in small teams. So let’s consider different phases of your development lifecycle and determine which test is more suitable.

You’re still in the early stages

If you’re still in the early development stages, building new functionalities, focus more on functional tests as they’re the foundation of non-functional tests. There’s no point in optimizing for performance for features that don’t work correctly. This means prioritizing unit tests, or integration tests that verify your core business logic.

You’ve Made Major Changes

When you’ve made major changes to a feature, you should focus on functional tests first, to validate that the feature meets the new requirements or hasn’t broken other features, then you can conduct non-functional tests.

Your Application is High Risk

If you’re building an application in a high risk industry, like finance industry for example where cost of failure can be very high, you should prfioritize non-functional tests like security testing, even in the early stages.

You’re Scaling Your Application

If you’re preparing for increased load or growth, maybe you expect your application to get increased traffic during holiday season, you should definitely focus on non-functional testing such as performance and scalability tests.

There’s No One-Size-Fits-All Approach

There’s no one-size-fits-all to software testing. You need to assess the product and determine what testing approach would achieve the business objective. Here’s what you need to remember:

• Use functional tests as a foundation: There’s no point in doing non-functional tests if your applications don’t work, so your functional tests are the foundation for non-functional tests.
• Focus on high-risk non-functional tests: Every application should be secure. However, if your application is in a high-risk industry, you should integrate security tests throughout your entire development lifecycle.
• Adjust your testing strategy as your project grows.