Unit Testing vs. Integration Testing: Which to Use and When?

Unit testing and integration testing are both key stages in the process of software development before release. The challenges that crop up when applying both protocols have seen DevOps question whether both testing methods are necessary. 

We’ll take a look at what they are and when to use them.

What is Unit Testing?

Unit testing is a protocol for software testing that allows a developer to target a specific section of code. These sections, or units, do not have system dependencies (or can be isolated from them in mock scenarios). The units are known as single classes or functions.

In essence, unit testing separates the smallest testable parts of an API and confirms that they function properly in isolation.

Unit tests are faster to build than other tests and can be executed with speed and accuracy. 

In application, not all code sections are entirely isolated from underlying dependencies. However, it is possible to swap out dependencies to allow for isolation. This process is known as mock testing.

Unit testing is also at the heart of test-driven development, allowing developers to create specified tests for an application before starting with the implementation. This streamlines the process of development and improves precision. 

Developers primarily use unit tests to check the integrity of their code and to allow them to return to a section of code they previously worked on without having to start from scratch to identify errors. This means that they can build a strong code-base at a lower cost.

It also allows developers to provide proper documentation for higher-level testing protocols, such as integration testing. 

What Is Integration Testing?

Integration testing is a software testing protocol used by testers to verify the inter-functionality of different aspects of a program. In other words, it tests how parts of an application work together as a whole, focusing on the accuracy of the interface rather than the individual modules themselves.

In essence, integration testing is used to check the entire software, its hardware, and networking infrastructure, from the front-end to the back-end database systems.

For example, an integration test could use the connection to a database (a dependency in unit testing) to query and mutate the database as it usually would. The tester would then need to prepare the database and read it out correctly afterward. 

A common error that testers make is to attempt to “mock away” these external resources as if a unit test was being performed. However, mock testing is not required in integration tests and can result in testers obscuring the failures caused by APIs beyond their control. 

It is important to remember that while you can replace external systems, an application ultimately works in an integrated way. That is why integration tests are important and have unique applicability compared to unit tests.

Primarily, testers use integration tests to help find issues that would not be obvious by examining an application’s or a specific unit’s implementation.

Unit Testing vs. Integration Testing: What’s the Difference?

Unit tests and functional integration tests are inherently different. Each protocol tests for separate types of functionality and provides different surety regarding the integrity of the software. 

This is why the question ‘Which is better, unit testing vs. integration testing?’ doesn’t make that much sense. Both are important in unique and complementary ways.

Unit tests

• Performed by a developer.
• The first step in the testing process.
• The person testing knows the internal design of the software.
• Each module of a program is tested individually without dependencies. 
• Allow the developer to understand the functionality that a unit provides.
• It offers the opportunity to refine code and ensure each module works properly.
• Allows testing to start on existing modules without requiring all modules to be completed. 
• Falls into the category of white-box testing.
• Low cost
• Presents code branch and edge case errors.
  
  

Functional integration tests

• Are performed by a tester.
• The second step in the testing process comes after unit testing and before system testing.
• The person testing doesn’t know the internal design of the software.
• All modules that combine to form a program are tested in combination.
• Allows for the evaluation of the performance and functionality of a software application before it is delivered to a client.
• It considers the user’s perspective, allowing the software development team to create test scenarios that represent real-world use scenarios.
• It empowers the development team to meet the requirements of the end-user and the client.
• Enhances the quality of the software product.
• Falls under the category of black-box testing.
• High cost.
• Presents software and application errors.
  
  

Are Unit Tests More Important than Integration Tests?

No, unit tests are foundational to integration tests, but they are not more important.

Unit testing is required to proceed to integration testing, as a development team must be confident that each module functions in isolation before testing for inter-functionality between several modules.

However, without integration tests, it would be impossible to know if the application functions appropriately as a whole. 

Therefore, the question isn’t ‘unit testing vs. integration testing: which is more important, but rather what’s most important about each protocol and how they complement each other.

Can Integration Tests Replace Unit Tests?

Integration tests can’t replace unit tests because they test for different areas of functionality.

Integration tests verify the interface’s functionality between several units (modules) within the application. However, if these individual modules have not been tested for accuracy, it will be impossible to determine why they fail to integrate. 

Therefore, it would be incorrect to think about either protocol in dichotomous terms (unit testing vs. integration testing). Unit testing is an essential first step in the testing process, and one cannot proceed to integration testing without it.

Why Is Integration Testing Harder than Unit Testing?

Integration testing takes longer and is more complex than unit testing due to the nature of the functionality being tested.

Because integration tests check the interface accuracy of several parts of an application, more setup is required to effectively create and run the tests. Integration tests also reveal more defects than unit tests and require the tester to build real-world user scenarios into their testing protocol. 

Other challenges for integration testers include:

• Having to manage a variety of factors, such as database, platforms, environment, etc.
• Testing the integration of a new system with a legacy system or integrating two legacy systems requires multiple changes.
• Testing in scenarios where there is low compatibility between systems developed by separate companies dramatically increases the workload.
• There are instances where too many paths and permutations are required to test for integration.

The Bottom Line: Why You Need Both

When it comes down to it, both unit tests and integration tests are essential elements of the software development process. 

• Unit tests allow developers to catch any errors with their code before submitting it to testers for integration testing. 
• Integration testing allows testers to guarantee that the needs of both the client and the end-user will be met. 

They require different skill sets and focus while providing different types of feedback and levels of surety.

Combined, they streamline the development process and allow for the most efficient use of team resources. It is essential to understand this interplay when optimizing your team systems for success.

Finally, you cannot be sure of the quality and integrity of your software without using both testing protocols. As such, it is essential to be confident in the results of both before presenting the end product to your client.