Functional and Non-functional Testing: The Basics You Should Know

Anna Khrupa

4 m read


QA Outsourcing, Testing & QA

Testing is an essential part and success factor of the software development process. If the test management is carried out correctly, tests can help to avoid software errors and ensure a high quality of the product. There are a lot of testing practices to make sure that your solution works correctly and can be used easily across multiple environments and platforms. These methods are usually divided into functional and non-functional tests.

Functional testing and non functional testing: What is that all about?

Functional testing is a type of testing that focuses on the business requirements of the software product. The main idea behind functional testing is to ensure that each functionality of software works as specified in the requirement documents. These types of tests focus only on checking the results of operations, completely bypassing the intermediate states of the system during their execution. Functional tests chiefly involve black box testing techniques, where you don’t peer into internal code structure and rely mainly on specifications.

Non-functional testing is a testing technique that focuses on the evaluation of the non-functional aspects of the system. It checks the parameters that are not tested in functional tests, such as performance, usability, reliability, etc. Non-functional tests are as important as functional tests as they help to understand how the system works. The main goal of this type of testing is to provide the best experience to the end-users of the software product. 

What are the functional testing types?

Since functional testing tests is a methodology that examines various parameters of a software product, different types of functional tests are performed. Let’s have a look at some of them:

  • Unit tests

Unit testing is the first level of testing which is usually performed by developers. This process ensures that individual components of a software are functional at the code level and react as expected. Unit tests can be performed manually, but automating the process speeds deployment cycles and extends test coverage.

  • Integration tests

Integration tests are designed to determine if independently developed components of software work properly when they are connected to each other. Integration tests ensure that communication between different units runs smoothly. Those tests try to uncover different types of errors such as incompatibilities in the message or data formats as well as invalid input or output parameters that can suddenly shut down a function.

  • System tests

System testing is a black-box testing method that evaluates a complete and integrated software product. The objective of system tests is to verify the system’s compliance with the specified requirements. This type of testing is usually performed by a test team other than the development team before the product goes into production.

  • User acceptance tests

User acceptance testing, sometimes called application testing, is the last phase of the functional testing which is performed before the software product is released to the market. The goal of this process is to ensure that the software is ready for the delivery and meets the end-users needs. This, in turn, requires the product to be tested in the “real world” – by end-users through the beta testing process.

What are the non-functional testing types?

The non-functional tests are an effective supplement to the functional tests as they provide important information on the safety, serviceability, and reliability of the system. This type of testing checks how the software product works and includes (but is not limited to) the following types:

  • Reliability tests

Reliability tests check if the software can maintain a certain performance level under certain conditions over a certain period of time. 

  • Robustness tests

Robustness tests are designed to prove that the system works correctly under all conditions, even during unexpected events.

  • Stress tests 

Stress tests monitor the system behavior under not typical circumstances, for example, under a heavy load.

  • Performance tests

Performance tests are conducted to determine how the software works in terms of responsiveness and processing speed under a workload.

  • Load tests

Load tests are the tests that are used to evaluate the behavior of the system under a specific load.

  • Usability tests

Usability tests are designed to verify if the software product can be easily used by end-users.

  • Maintainability tests 

Maintainability tests are conducted to evaluate the software’s ability to meet the users’ requirements and be changed without any issues.

  • Portability tests

Portability tests measure how easily the software can be transferred to another environment, for example, how easily mobile apps can be moved to different operating systems or various devices. 

Functional testing vs non-functional testing – what is the difference?

Both functional and non-functional testing in software it is designed to deliver the product in line with the customer needs. The main difference between functional and non-functional tests is that functional testing is run to ensure that the software meets the particular requirements, while non-functional testing is focused on how well the system works.

This is best illustrated by the following example. Under functional testing, you can verify if the login functionality works as intended. For instance, you can check if entering the username and the password allows you to log into the account on the website. And with the help of non-functional tests, you verify if by entering the same data you can log in to your account within 2 seconds. Checking how many users can log into the system simultaneously is another example of non-functional testing. 

Secondly, functional testing is often aimed at detecting and eliminating bugs in the software so it can meet the business requirements perfectly. At the same time, non-functional testing is concerned with the user experience and behavior but not with “bug hunting”. 

The third difference refers to the criteria on which functional testing and non-functional testing are based. Under functional tests, both valid and invalid operations are performed in order to test the behavior of the software both in desired and unexpected conditions. On the other hand, while running non-functional tests you need to consider the parameters that are not considered while performing the positive and negative functional tests. There is one more distinction between the two types of tests – the functional tests are performed before the non-functional tests.

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