Unit Testing vs. Integration Testing

To build the most robust, reliable software, unit and integration testing are two of the most crucial checkpoints. Sometimes, however, their roles and scopes intertwine and confuse developers. But there are fundamental differences between the two, along with many unique functionalities and uses. 

In this blog post, we’re on a mission to demystify unit testing vs. integration testing and understand their indispensable roles in the development lifecycle. 

So sit back and let’s begin.  

What Is Unit Testing?

At its core, unit testing means testing individual modules in isolation. 

You strip away the interactions with dependencies and focus on just one component. A unit is the smallest part of an application that you can test, i.e. small pieces of code, individual functions, objects, procedures, or methods. Unit testing is a fundamental step in the agile software development process. 

In simpler terms - unit testing is a piece of code that can verify another piece of code that implements the requirements needed to please the customer. Let’s illustrate this with a no-frills example:

Example: Imagine you’re building a LEGO castle. Before constructing it, you examine the blocks to ensure they’re not broken, bent, or damaged. Similarly, in unit testing, you scrutinize each unit of code independently to verify that it operates correctly on its own.

In the beginning, software developers write the unit test cases, the requirements, and what’s expected of the unit. Then, the developers or white box testers ensure the unit test meets the requirements. They check if the unit does exactly what it needs to do without depending on other functions.    

Developers do unit tests before integration tests and check both positive and negative behaviors. At this stage, via effective bug reporting, any bugs you find will be easy to resolve and won’t impact project costs. If this becomes a standard process and you detect many bugs and defects, you’ll save valuable time and energy.  

You can use a wide range of testing tools in different languages for unit testing, such as JUnit, NUnit, and PHPUnit. 

In the table below, check out the advantages and disadvantages of unit testing: 

Advantages	Disadvantages
Detects bugs and defects early Cheap and easy to do Creates a reusable code Specifies a bug’s exact location Tests can be automated Improves code quality Fixing issues is fast and easy Saves time and energy	Works in isolation, which may conceal bugs Creates too many test cases which then need to be updated Takes time to write and debug the test cases Can get tedious and repetitive Can create a complicated code Maintaining can be difficult and time-consuming

In terms of cost, unit tests are the cheapest option and bring in the highest return on investment (ROI). Consequently, this means your code should include many unit tests—more than integration tests and much more than functional tests. 

What Is Integration Testing?

Integration testing means testing the overall behavior of integrated modules as one unit. 

Integration testing picks up where unit testing left off and checks how the individual units of code work together. We do this to test the interfaces between the modules and their combined behavior. Let’s extend our LEGO building example to integration testing: 

Example: After verifying the quality of each LEGO block, you start building the castle. You combine and interlock blocks to create walls and towers that work as a unit. If the towers stand upright - your castle build is successful. This resembles how integration testing examines how the units of code work together when integrated into a larger system. It ensures everything interacts well and contributes to the functionality of the entire program. 

With integration testing, we verify whether the function meets the requirements and whether the developers have implemented it correctly. Here, we get a clearer picture of the unit’s performance and fix any pressing issues before proceeding. 

However, it’s worth mentioning that integration testing isn’t the last step in the cycle. You need to do integration tests concurrently with development, which makes things more difficult. But alas, it’s an integral part of the testing process because it ensures a high standard of quality and aligns all the units. 

When doing integration testing, you may find that even though each unit works well on its own, they don’t work well as a team. This ultimately leads to failures, defects, or breakdowns. 

There are 3 types of integration testing approaches:

1. Big Bang approach – tests integrated units as a whole at the same time
2. Top-down approach – tests integrated modules from the top levels to the bottom ones, one at a time
3. Bottom-up approach – tests integrated modules from the bottom levels to the top ones, one at a time 

Integration testing is more complicated and demanding than unit testing. Unlike unit testing, it can be both a white box and a black box technique, depending on the data, the database, and the requirements. 

There are many integration testing tools available today, including Protractor, Selenium, Postman, etc. All of them help with a diverse range of programs and functions.  

Check out the table below for an overview of the pros and cons of integration testing: 

Advantages	Disadvantages
Detects interface errors Improves the quality and reliability of the software Detects bugs and errors early Reduces cost and time of debugging Ensures the components in the system work well together Starts immediately after the modules become available Can test authentication, encryption, and authorization Facilitates collaboration between developers and testers	Can be challenging and complex Involves multiple components Handles dependencies and scenarios Requires more resources and tools Can become costly to set up, execute, and maintain Demands a high level of coordination and collaboration Depends on the quality and availability of individual components Changes to one component affect others Can be slow

Cost-wise, integration testing is more expensive than unit testing. It also demands more resources to implement and maintain. This means you shouldn’t do them at the drop of a hat, but rather more strategically.

Differences Between Unit Testing and Integration Testing

Now that you understand what unit testing and integration testing do and why we need them, let’s go through the most notable differences between the two. To illustrate the distinctiveness of each, we’ll refer to the following table: 

	Unit Testing	Integration Testing
Test Scope	Tests one small part of a system - a unit in isolation	Tests many parts that work together - many units in collaboration
Test Conductor	Developer/White box tester	Tester
External Dependencies	None	Required
Simplicity	Simple	Complex
Execution Speed	Fast	Moderate/slow
Bugs	Finding bugs and resolving issues is fast and easy	Finding bugs and resolving issues is slower and more complicated
Testing Type	White box testing	Both
Testing Stage	Should be conducted at the initial phase of testing	Must be conducted after unit testing but before system testing
Cost	Cheaper	More expensive
Maintenance	Cheaper and easier	More costly and difficult
Desired Outcome	A detailed visibility of the code	A detailed visibility of the integration structure

Unit Testing and Integration Testing in CI/CD

Both unit and integration testing are vital in a well-constructed CI/CD pipeline. 

As mentioned before, unit testing tests individual parts of the code and can easily be automated. This enables you to validate code quickly and implement new features without looming concerns about bugs or defects. In fact, DevOps best practices may even highlight that automating tests is mandatory. 

Frequent automated tests within the pipeline are crucial to guarantee code changes don’t disrupt individual unit functionalities. Developers can trigger the tests either at regular intervals or on demand. 

As a result, the developer doesn’t have to go back and conduct the tests again manually. So, when the developer creates a pull request, this automatically triggers unit and integration tests that check the functionality of the code and the behavior of the program. 

The incorporation of these testing approaches into the CI/CD pipeline will ensure early detection and resolution of integration-related issues. Hence, this contributes to the quality and reliability of the software. 

After running the tests, the system sends the results to the team leader for confirmation that the pull request is ready to be merged with the main code. 

The more tests you do, the more you ensure your program is stable and your team is on track with development. 

When to Do Unit Testing vs. Integration Testing?

Unit and integration testing provide the foundation for an all-encompassing testing strategy. This means that while there are differences, developers/testers should use the two approaches in tandem. The testing pyramid (or a simplified version of it) conveys this well: 

This model outlines the importance of structuring your testing and defining the order. Of course, unit testing doesn’t have to stop when integration testing starts. But, the latter can’t (and shouldn’t) start before the former has yielded some results. According to AWS, unit tests should make up about 70% of your entire testing strategy. 

The strengths of unit testing are the weaknesses of integration testing and vice versa. For a comprehensive test, include unit testing for safety and precise feedback and integration testing for specific scenarios with real dependencies. 

In the end, you should use both because one cannot replace the other. Use them together as complementary pieces of your test. 

Conclusion 

All in all, developers and testers need to understand the differences between unit testing and integration testing.  

Both tests are pivotal during the development process, but they’re also vital for CI/CD pipelines. Unit tests catch early-stage bugs and integration tests detect more complex issues in unit interaction. 

Use unit testing for isolated functionality checks and integration testing to verify combined operations. Employ them together to fortify software quality, ensure stability at the micro and macro levels, and streamline the development and deployment processes in one fell swoop. 

After this, you’re ready to present the product to stakeholders. 

Need help implementing unit tests and integration tests as part of your software testing strategy—or maybe you want to extend your testing team for wider coverage? Contact us to learn more about our software testing services.