Unit, Integration, and Functional Testing: 4 main points of difference

Introduction

Unit, integration, and functional testing are crucial components of software application testing. Each of these employs a distinct and unique process to test the application. However, the most important remains functional testing and functional regression testing. Unit or integration testing cannot replace this, either by itself or collectively. While unit testing involves testing individual modules of an application, isolated integration testing checks if different modules are working together as a group. Finally, functional testing checks whether the system operates the way it is supposed to.

Together, these constitute an application that is bug-free and smooth for the end-user. Functionality is related to integration tests. The latter is performed only when the former has yielded desired results. However, successful automated functional testing signifies that the entire app is running smoothly.

What is Unit Testing?

Unit testing involves testing applications at a unit level. It checks single components. You can optimize ROI (return on investment) if your codebase runs multiple unit tests with fewer integration tests and even fewer functional tests. Since the unit is the smallest part of an application, they are easier to write and quicker to execute. Usually, these tests are written and performed by software developers themselves. The primary aim here is to match the requirements to the unit's behavior. This kind of testing is usually done before integration testing using white box testing techniques. Some of the most popular unit testing tools used for different languages are JUnit of Java Framework, PHPUnit of PHP framework, and NUnit of .Net framework.

Unit testing has several benefits, for instance:

• It checks whether or not there is correct output for valid input. 
• It also checks for failures that occur with invalid input. 
• It finds issues and bugs in the early stages to resolve them faster. It also reduces overall project costs. 
• Since unit testing tests small pieces of code and isolated functions, the issues it spots are independent of other test cases. 
• It makes resolving issues at a later stage easier by simplifying unit test cases and making the very act of testing code easy.
• Unit testing helps to save time and cost and is also easy to maintain.

What is Integration Testing?

Integration testing, as mentioned before, tests if the different parts of a system can work together and how well they work in synthesis. Various modules of such an ecosystem are merged to facilitate integration testing procedures. The purpose of this is to check the overall performance and reliability. This is performed on modules that have already undergone unit testing. It then defines if the combination of these elements can provide the desired output or not.

There are three types of approaches to integration testing as are as follows:

The Big Bang Approach

• Here, the modules are integrated and tested as a whole if the entire system is ready for such an integration. This is different from system testing. While system testing is for the ecosystem, integration testing checks only the modules. The Big Bang Approach lets you test everything simultaneously, thereby preserving resources. However, it could be challenging to identify specific errors.   

The Top-Down Approach

• Here, different modules or units are combined is tested using a top-to-bottom approach. First, the units are tested individually by writing test STUBS. Lower levels are then integrated in sequential order until the last one is compiled and tested. Experts believe that it is the most organic way of organizing a test because it is similar to the way things happen in real-life situations. The primary issue is that functionality is constantly tested at the end. It could mean going back and fixing issues through functional regression testing.

The Bottom-Up Approach  

Modules are tested from the bottom to the topmost level in the bottom-up approach. Simulator programs called DRIVERS facilitate this process. The bottom-up approach helps to detect bugs at lower levels. It enables the team to function as expected.

What is Functionality Testing? 

Automated functional testing is a black-box testing technique that ensures the overall operability of the application. This checks if a desirable output is generated out of a specific input. Test cases are then written based on the requirements and scenarios and then implemented. The number of tests can vary based on the nature of the software. Each test case comprises the test summary, prerequisites, input steps, test data, expected output, and notes (if any).

The two forms of automated functional testing are as follows:

• Requirement-based: The test cases are created based on the app's demands.

• Business scenario-based: The tests are customized to align with the business strategy. 

Functional regression testing tests, or rather re-tests applications when new and improved features are added, and functions are modified for existing applications. It also checks if the older attributes and functionalities work well with the upgrades. While functional testing can assure all operations are working flawlessly, functional regression testing comes into play only when the team has published a new build that aims to fix bugs or debug new updates.

The benefits of functional regression testing include the following:

• It plays an integral role in agile environments where every sprint is crucial. Functional regression testing makes sure that previous and current releases all work together seamlessly.
• It helps to identify bugs in the software. Regression tests make software resistant to discrepancies. Such procedures provide support to testing and QA teams to deliver fast results.
•  It helps retain the integrity of an application despite new additions.
• Regressive testing helps to shorten the overall testing lifecycle. 
• These tests help achieve a higher Customer Satisfaction Index (CSI) and can eventually be considered crucial for expanding business. 
• They help reduce calls for incidents in production. Thus, the cost of the overall project reduces.

Also read: Benefits of Using Automated Functional Testing

The main differences between the three 

The differences between unit testing, integration testing, and functionality testing can be divided into the following categories:

• Purpose: Unit testing checks the most basic unit of the application, each module, individually. Integration testing checks two or more modules combined to perform tasks. Functional automation testing tests the behavior of the application when it functions as a whole. 

• Complexity: Unit testing is simple in its language and is easy to write since it includes the smallest of codes. While integration testing is slightly more complex compared to unit tests, functionality testing constitutes the most complicated of the batch.

• Testing techniques: Unit testing involves white box testing techniques. Functionality regression testing consists of only black-box testing techniques. Integration testing uses both black and white box techniques- also called grey-box testing. 

• Errors covered: Unit tests can cover issues that occur with frequency in different modules. It nullifies the chance of any problem going unnoticed. In the case of integration testing, the errors covered include bugs that occur when integrating various modules. Issue escape is a rare occurrence. For automated functionality testing, issues that hinder the performance of an application are identified. Scenario-based problems are also tested here. There is the most chance of issue escape here because the list of tests to be run is infinite.

A focus on functional vs. unit testing

When examining functional vs. unit testing, it’s crucial to understand their distinct roles within the software testing lifecycle. Unit testing concentrates on testing individual components or functions of the software in isolation. Developers use unit tests to ensure specific parts of the code, such as methods or classes, work as expected. These tests are highly granular, quick to execute and help catch bugs early in development, ensuring the internal logic of the code functions correctly without dependencies.

In contrast, functional testing evaluates the software's performance by testing its functionality in real-world scenarios. It assesses the application's workflow, interactions, and behavior to ensure the software meets user requirements. Unlike unit tests, functional tests simulate user experiences to ensure the software operates as intended.

The difference between functional vs. unit testing lies in their scope: unit testing ensures the correctness of individual components, while functional testing verifies the application's overall functionality. When choosing between functional vs. unit testing, it’s essential to recognize that both testing types are necessary to deliver reliable, high-quality software. However, they serve different purposes at various stages of development.

Functional Testing Vs. Unit Testing Vs. Integration Testing

How Do Unit and Functional Testing Improve Software Quality?

Software quality is paramount in delivering reliable, efficient, and user-friendly applications. Unit and functional testing are critical in enhancing software quality, each contributing uniquely to the development process.

Early Detection of Defects

• Unit Testing: By isolating and testing individual units or components of code, developers can identify and fix bugs at the earliest stage. This proactive approach reduces the cost and effort required to address defects later in the development cycle.
• Functional Testing: This testing ensures the software's functionality meets the specified requirements. Detecting discrepancies between expected and actual outcomes helps prevent functional defects from reaching the end-users.

Improved Code Quality

• Unit Testing: Writing unit tests encourages developers to write cleaner, more modular code. This practice enhances code maintainability and facilitates easier debugging and refactoring in the future.
• Functional Testing: Validating the software's functionality ensures all components work together seamlessly, leading to a reliable application.

Enhanced User Satisfaction

• Unit Testing: Ensuring each component functions correctly contributes to the software's overall stability, providing a solid foundation for user-facing features.
• Functional Testing: Focusing on user scenarios and workflows ensures that the app meets user needs and provides a good UX, increasing user satisfaction and trust.

Risk Mitigation

• Unit Testing: Early detection of issues at the unit level reduces the risk of major failures during later stages of development or after deployment.
• Functional Testing: It helps identify critical flaws that could lead to significant problems if left unaddressed, mitigating potential risks associated with software failures.

Cost Efficiency

• Unit Testing: Catching and fixing defects early in development is less costly than addressing them after deployment.
• Functional Testing: Ensuring the application functions correctly before release reduces the likelihood of costly post-deployment fixes and customer support issues.

Unit Testing and Functional Testing Best Practices

Adhering to best practices in unit and functional testing maximizes their effectiveness and contributes significantly to software quality.

Unit Testing Best Practices

Write Testable Code

Design your code with testing in mind. Use principles like dependency injection to make units of code more testable.

Keep tests independent: Ensure that each test can run independently without relying on the outcome of other tests.

Use mocks and stubs: Mocking frameworks allow you to simulate external dependencies and focus on the unit under test.

Aim for high coverage: Strive for substantial code coverage but focus on critical and complex code paths rather than achieving 100% coverage.

Automate tests: Integrate unit tests into your continuous integration pipeline to run them automatically with each build.

Maintain tests regularly: Update your tests in line with code changes to keep them relevant and effective.

Functional Testing Best Practices

Understand user requirements: Thoroughly comprehend the functional requirements and user expectations to design effective test cases.

Prioritize test cases: Focus on critical functionalities and features that have the most significant impact on users.

Use realistic test data: Employ test data that closely mimics real-world scenarios to uncover potential issues users might face.

Automate where feasible: Automate repetitive and regression tests to increase efficiency and reduce the chance of human error.

Perform end-to-end testing: Validate complete user workflows to ensure all integrated components function seamlessly.

Collaborate with stakeholders: Work closely with developers, business analysts, and end-users to ensure comprehensive coverage of all functional aspects.

Conclusion

To conclude, we must understand that these processes are interconnected and correlated. Unit testing is imperative to ensure that your software is working seamlessly. Unit testing facilitates flawless paths and lines of code. This, in turn, must be followed by integration tests to make sure that separate units can work together cohesively. Finally, all of the above must be followed by functional tests to deliver a polished app. Functional regression testing also needs to be performed if the application in question is an existing software where updates are being implemented. 

FAQs

1. Is integration testing the same as functional testing?

Functional testing is performed to validate all functionalities of an application, while integration testing is performed to validate the interaction across modules and verify that they work well when compiled.

2. Can unit testing be used for Integration testing purposes?

No. Unit testing only tests separate modules in an application at an isolated level, while integration testing tests how the collaboration between all the modules functions together. It is debatable whether unit testing tools will not be sufficient to carry out successful integration testing. 

3. Is Integration testing performed after unit and functional testing?

The order of testing is: 

• Unit testing, 
• Integration testing, and finally- 
• Functionality testing to ensure every development process level is working as desired. 

4. What is mocking in unit testing?

Mocking is used in unit testing when it has external dependencies. Its purpose is to isolate and focus on the code being tested and not on the behavior or state of the mentioned external dependencies.

5. What are the five steps of functional testing?

Functional testing generally follows the following steps:

1. Determining the functionality of the product which needs to be tested.
2. Creating input data for functionalities to be tested according to the particular requirements.
3. Determining acceptable output parameters according to similarly specified requirements. 
4. Executing the test cases.
5. Comparing the actual output of the test with the predetermined desired output values.