Functional vs. Non-Functional Testing in Software Development
Software testing is one of the most important parts of the development process. When it comes to more specific methods, testing processes usually fall under either functional or non-functional. Both categories aim to evaluate different areas, but work together to ensure operational and performance success.
What is Functional Testing?
As the name would suggest, functional testing and its subtypes (which we’ll cover in more detail below) primarily focus on validating the actual functionality of a system. Using predefined requirements, these tests check that the software’s features operate as expected, such as user authentication, interface behavior, and more. Think of functional testing as verifying that every input (both valid and invalid) results in the expected output.
Advantages & Attributes
- Confirms that software meets its business requirements and aligns with expectations
- Functional testing is performed earlier in the development cycle, so defects are caught sooner
- Improves user experience by rigorously testing user-facing features and inputs
Examples
- Login Authentication: Verifying that users can access their account with valid credentials and are denied with invalid credentials.
- File Upload: Reviewing whether a document management system allows uploads of and correctly displays specific file types.
- Form Validation: Checking that a form rejects invalid entries (such as phone number formatting or lengths) but submits successfully when valid.
What is Non-Functional Testing?
On the other hand, non-functional testing evaluates the attributes of an application, like how quick it is, how secure it is, and how scalable it is. Instead of testing whether the software produces the correct output, non-functional testing measures how well the software performs under different conditions and in different scenarios.
Advantages & Attributes
- Verifies that a system can handle increased user load, enhancing reliability
- Bolsters security and reliability by identifying vulnerabilities and potential threats
- Further improves user experience by checking responsiveness and stability
Examples
- Data Load Speed: Measuring how quickly an analytics dashboard and data visualizations load for users.
- App Launch: Analyzing how a mobile app responds to a simulated spike in users to prepare for its launch.
- Healthcare Vulnerabilities: Attempting to exploit vulnerabilities in a healthcare platform to identify weak points in patient data encryption.
Functional Testing vs. Non-Functional Testing: Differences Summarized
As we’ve shown, functional testing focuses on the black and white correctness of application features, while non-functional testing evaluates a wide range of other factors important for user experience and long-term growth..
|
Area |
Functional Testing |
Non-Functional Testing |
|
Focus |
What the system does |
How the system performs |
|
Manual/Automated |
Can be automated or manual |
Usually automated |
|
Example |
Login functionality, order processing, payment gateway |
Response time, scalability, data encryption |
However, these testing categories are not mutually exclusive — in fact, it’s highly recommended to use both functional and non-functional testing to comprehensively evaluate your product. Using them together results in a more well-rounded and reliable system that users will be more likely to adopt. Think of these both working together to ensure that your software is correct and efficient.
For example, a payment gateway would have to correctly process transactions (functional testing) as well as handle traffic influxes around major sales without crashing (non-functional testing). Another case might be an airline booking system, which needs to correctly show available seats for each flight (functional), but also securely process customer data without being vulnerable to cyberattacks (non-functional).
There are further nuances within each category, with some testing types being better than others for particular situations or steps in the development lifecycle:
Functional Testing Types
The functional testing part of a testing methodology is typically broken down into four components — unit testing, integration testing, system testing, and acceptance testing — usually executed in this order. Each of them is described below:
Unit Testing
The unit testing part of a testing methodology is the evaluation of individual software modules or components that make up an application or system. These tests are usually written by the developers of the module and in a test-driven-development methodology (such as Scrum or XP) they are actually written before the module is created as part of the specification. Each module function is tested by a specific unit test fixture written in the same programming language as the module.
Integration Testing
Integration testing assesses different modules and components that have been successfully unit-tested when integrated together to perform specific tasks and activities (also known as scenario testing, End-to-End testing, or E2E). This analysis is usually done with a combination of automated functional tests and manual testing, depending on how easy it is to create automated tests for specific integrated components.
System Testing
The system testing step involves checking the entire system for errors and bugs. This test is carried out by interfacing the hardware and software components of the entire system (that have been previously unit-tested and integration-tested), and then testing it as a whole. This testing is listed under the black-box testing method, where the software is checked for user-expected working conditions as well as for potential exception and edge conditions.
Acceptance Testing
The acceptance testing part of a testing methodology is the final phase of functional software testing. It ensures that all the product and project requirements have been met and that the end users and customers have tested the system (User Acceptance Testing) to make sure it operates as expected and meets all their defined requirements:
This is a vitally important activity, because the customer or organization may not accept the software and sign off on it until you have completed UAT. It’s therefore critical to have an effective and intuitive tool that makes this stage of the testing process as easy as possible. For example, make it easy for end users to follow the script, interact with the system under test in the same window, and record results and screenshots without unnecessary complexity.
Non-Functional Testing Types
On the other hand, compared to functional testing, non-functional testing involves testing the application against requirements called technical qualities. These are also known as the “-ilities” because they all end in “-ility” — for example, vulnerability, scalability, and usability. Some examples of non-functional testing are described below:
Performance, Load, Stress Testing
There are several different subsets within performance testing, but they all have a similar purpose — to push a system to its limits and assess the impact of this. For example, performance testing can be used to measure how a system behaves under an increasing load (both numbers of users and data volumes). Load testing verifies that the system can operate at the required response times when subjected to its expected load. Then, stress testing can find the failure point(s) in the system when the tested load exceeds what it can handle.
Security & Vulnerability Testing
Historically, security was something that was tested at the very end of development after other testing efforts. With the rise in cybercrime and the awareness of the risks associated with software vulnerabilities, application security is now something that needs to be designed and developed from the start in tandem with the desired business functionality. Security testing checks the software for confidentiality, integrity, authentication, availability, and non-repudiation factors and weaknesses.
Usability Testing
The usability testing stage analyzes the end-user usability aspect and intuitiveness of the software. In other words, the focus is on how easily a user can pick up and use the product. Usability testing looks at five main aspects — learnability, efficiency, satisfaction, memorability, and errors. These form the basis to verify whether your software is too convoluted and should be simplified or otherwise improved to help users understand the interface and system.
Compatibility Testing
Compatibility testing checks whether the product or application works on all of the necessary operating systems, hardware platforms, web browsers, mobile devices, and other third-party programs (like browser plugins). Compatibility tests ensure that the product works seamlessly across all the different hardware and software combinations and that all functionality is consistently supported.
Data-Driven Testing
If you’re performing automated compatibility testing, a key feature you consider investing in is data-driven testing. This allows you to create sets of different test configurations that need to be tested, which are automatically generated by a system:
For example, you might provide all of the relevant browsers and platforms, and it can create an N by N matrix of all possible permutations. Then, you can quickly and easily run all your tests through all the combinations to ensure there are no missed edge cases.
Use Both Functional & Non-Functional Testing for Comprehensive Coverage
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