Here’s a hard truth: These days, any application you are working on needs to be flawless.
It needs to do what it’s supposed to do without any hiccups or glitches.
If the application isn’t perfect, it will impact your business. You can’t afford for your customers to feel like their experience is anything less than stellar. There are many ways you can test an application to make sure it’s ready to go live and serve customers. Unit testing and integration testing are two methods that you can use.
The difference between unit testing and integration testing is that unit testing is a software testing method that verifies individual units of source code. On the other hand, integration testing is a type of software testing that checks how different units of source code work together.
Integration tests and unit tests provide different benefits and advantages for developers and testers, but which one should you use? Here, are some ways to decide which type of test is right for your next project.
What is unit testing?
Unit testing is a process of testing code at the level of small units, small individual parts of a larger program, usually individual functions or methods. Unit testing allows the programmer to identify any defects in the code. In addition, each unit test reveals whether the small unit of code performs the intended function.
For example, if you develop a calculator application, you might want to ensure that the sum function works. One way to test this would be to take two numbers and see if the output of the sum function is correct.
The critical concept is that unit tests run in isolation: meaning that they will not access any external dependencies (such as databases, file systems, and other external resources) besides the tested code. Running code in isolation is beneficial because it allows you to isolate what inputs are required for your code to work correctly, reducing the complexity of testing.
If you need to replace a database or a network call, you can use test doubles to achieve the isolation.
What is integration testing?
Integration testing is a type of testing that can test the interactions between different modules of the application. It is the type of testing that you usually perform when the software has gone through unit testing. The primary purpose of integration testing is to test at the system level. The integration test will help eliminate any potential issues that could otherwise occur during the deployment by checking to see if different system parts work well together.
Integration testing helps to detect problems in the code base, tighten the software design, and improve its stability.
Integration tests are more expensive tests to write, maintain and are prone to error. However, they are valuable for testing the integration of software modules, since you can’t properly test that with unit tests.
Integration tests vs unit tests
Tests at the unit level are usually specific and check a limited set of inputs and outputs within one module. Integration testing is when the individual parts of the system are assembled and tested as a whole. The following table brings an overview of the similarities and the differences.
Comparison
| Compared by | Unit tests | Integration tests |
| Functionality | You test one isolated unit of code | You test how individual components work combined |
| Speed | Fast to write and execute | Slow to write and execute |
| When do you write them | During the development | At the end of the development of a feature |
| Environment | The code is isolated from outside dependencies using stubs and mocks | You need to have a test database, fake API server, and/or 3rd party accounts. |
| Maintenance cost | Low | High |
| Type of testing | White-box testing | Black-box (Grey-box) testing |
| Time of execution | Can be executed at any time | Usually executed after unit tests pass |
When to use which?
You can use unit tests during the development as a way of getting feedback. Since they are fast to execute, you get immediate feedback on whether your feature works or not. You also get feedback from failed tests if your change has introduced a bug in the system. If you use test driven development (TDD), a passing test signals that you have finished the current implementation. If you are not familiar with the TDD, it is a development process where you write a test case method before writing a piece of implementation code.
On the other hand, it’s not practical to run integration tests often during the development of a feature. Usually, they take longer to execute, and you get delayed feedback. I say “usually”. If your integration tests are fast, then, by all means, run them as often as possible.
The one place where you would use integration tests instead of unit tests is when you are just starting with the test coverage of the legacy code. Legacy code is complex, and it’s impossible to write unit tests for it in most cases. So instead, you start by writing characterization tests. These are the integration tests that cover the existing functionality. Once you have characterization tests, you can refactor your code, without any fear, towards better testability.
Why is integration testing harder than unit testing?
Integration testing is harder than unit testing because you can execute unit tests much simpler and quicker. However, when it comes to integration testing, it requires much more time and effort. Also, you need to invest more time and effort into writing them in the first place and maintaining them over time.
Integration tests are also more challenging to write because they require additional knowledge about the system under test and the interactions between its components.
However, if you don’t do any integration testing, then errors may not show up until the code is deployed and used by your customers. The best approach would be to use both types of tests to validate that your software works correctly before releasing it into production.
What are functional tests?
Functional tests are a type of software testing that test the functionality of a software application as a whole. You execute them to ensure that an application or system works as planned. In addition, these tests ensure that the software is meeting its requirements and any user stories set out by stakeholders.
The testing team usually performs functional testing, that’s why this testing is also known as QA testing. You can also automate these tests with Selenium or another similar testing tool. This testing is performed by simulating user input and testing the output to see if it matches the expected. One example of functional testing is when a user logs in to the application, and the login process should be completed successfully without any problems.
Unit, integration, and functional tests represent a testing pyramid: a model that represents in which measure you should use which tests. It consists of three levels, with the largest number of unit tests at the base, followed by a smaller number of integration tests. Finally, the top of the pyramid consists of functional tests.
The basic idea of the testing pyramid is that you start by writing unit tests that test code paths in an area of the application. Then, once these tests have been written and any bugs found and fixed, you move on to integration testing, and more expensive and time-consuming levels of testing (system testing, performance testing, security testing).
Conclusion
Software testing is a critical component of software development. When it comes to testing, there are many different types of tests that you can perform. Integration tests and unit tests are two widespread ones. However, it can be challenging to know when to use them and the difference between the two.
In this guide, you have learned about integration tests and unit tests, their roles, and why they are essential for any software application.