Unit Testing vs. Integration Testing: 5 Key Differences and Why You Need Both

What Is Unit Testing? 

Unit testing focuses on testing individual units or components of a software. The purpose is to validate that each unit of the software performs as designed. A unit is the smallest testable part of any software—it usually has one or a few inputs and a single output. The term ‘unit’ in this context can refer to a function, procedure, method, or module within an application.

Unit testing is typically conducted by developers themselves using a unit test framework. This type of testing helps to uncover issues and bugs at the earliest stage of the development process, making it easier and less costly to fix. Additionally, unit testing serves as a form of documentation, providing a clear and concise description of the software’s behavior.

It’s important to note that in unit testing, the focus is on the internal design of the software, not the behavior of the system as a whole. As such, external dependencies such as databases or network interfaces are usually mocked up to ensure that the unit being tested is isolated from the rest of the system.

What Is Integration Testing? 

Integration testing focuses on combining individual units and testing them as a group. The aim is to expose faults in the interaction between integrated units. It typically comes after unit testing and before system testing in the testing lifecycle.

In integration testing, different modules or services that have already been unit tested are brought together to test their collective behavior. For instance, in a web application, this could involve testing the interaction between the database, server, and client.

Integration testing can be performed in different ways. The ‘big bang’ approach involves combining all units and testing them all at once, while the ‘incremental’ approach tests two or more units together, gradually adding more until all units have been tested together.

While integration testing is more complex and time-consuming than unit testing, it’s essential in ensuring that the different parts of the system work together cohesively. It’s particularly important in modern software development, where applications often consist of multiple services that need to interact seamlessly.

Unit Testing vs. Integration Testing: Key Differences 

Now that we’ve laid out what unit testing and integration testing are, let’s delve into the key differences between them.

1. Granularity

One of the main differences between unit testing and integration testing lies in their granularity. Unit testing is fine-grained, focusing on individual units or components of the software. It’s the first level of software testing and is carried out during the development phase.

In contrast, integration testing is coarse-grained. It involves testing multiple units or components together and is performed after all units have been individually tested.

2. Focus

Unit testing focuses on the internal design of the software, ensuring that each unit functions as intended. It’s a white-box testing method, meaning it’s concerned with the internal logic and structure of the code.

On the other hand, integration testing is more focused on the interaction between different units or components. It’s a black-box testing method, focusing on the functionality of the system without considering the internal workings of the individual components.

3. Complexity

Integration testing is generally more complex than unit testing. While unit testing isolates a single component, integration testing requires a deep understanding of the entire system and the interactions between different components.

4. Dependency

Unit testing is independent and can be done in isolation. Each unit is tested separately, and external dependencies are typically mocked up.

integration testing, however, depends on the units that are being integrated. It requires that all units be tested and functioning correctly before it can be performed.

5. Running Speed

Unit tests are typically fast to run, as they only involve a single component. Integration tests, however, can take a longer time to run due to their complexity and because they are a more realistic test of software behavior. In addition, there is often a need to set up a testing environment.

Use Cases for Unit Testing 

Let’s explore some of the use cases of integration testing.

Single Component Verification

Unit testing verifies the functionality of a single component. Whether it’s a function, a method, or a class, unit tests ensure that each part of your code behaves as expected under different circumstances. This level of granular testing can help you unearth bugs and flaws that might be overlooked in higher levels of testing.

Regression Testing

Unit testing plays a crucial role in regression testing. Any modification in the code can potentially introduce new bugs. Writing unit tests for all functionalities can help pinpoint the exact location of a regression after code is modified, making bug fixes more efficient.

Test-Driven Development

Unit testing is the backbone of Test-Driven Development (TDD). In TDD, developers first write the unit tests for a feature before writing the actual code. The code is then refactored until it passes all the tests. This approach promotes clean code and ensures that the final code meets expectations.

Early Bug Detection

One of the biggest advantages of unit testing is early bug detection. It encourages developers to test their code as they write it, which means bugs can be detected and fixed at an early stage before they snowball into larger issues.

Related content: Read our guide to unit testing tools (coming soon)

Use Cases for Integration Testing 

Let’s look at the main use cases for integration testing.

Verification of Interaction Between Multiple Components

The primary purpose of integration testing is to verify the interaction between multiple components. It helps identify issues with data communication among modules and checks if the system works correctly when the units are combined.

System Integration

Integration testing is essential when connecting a new system to existing ones. It ensures that the new system can interact seamlessly with the old systems, thereby maintaining overall functionality and performance.

Interface Contract Verification

Integration testing is used to verify the interface contracts between different modules. It checks whether a module correctly uses the output of another module and whether it provides the correct output for subsequent modules.

Performance and Load Testing

Integration testing is also used for performance and load testing. It measures the system’s performance by putting it under different loads and helps identify bottlenecks that can affect the software’s speed and efficiency.

Unit Testing and Integration Testing: Importance of Using Both 

Why You Shouldn’t Have Unit Tests Without Integration Tests

Small to medium-sized companies often develop applications that have only unit tests without any integration tests. This lack might be due to:

  • Predominance of junior developers who are familiar only with unit tests.
  • Previous integration tests were discarded because they were more trouble than beneficial.
  • A complex application environment that makes testing challenging, leading to features being tested directly in production.

While there are exceptions, setting up test environments is now easier than ever due to technology like Virtual Machines (VMs) and containers. It is now possible to automatically provision the required environment, with no manual effort, before testing the application.

Integration tests are vital as they detect issues that unit tests can’t, especially concerning databases, connections to other modules or systems, performance verifications, and more. With the rise of microservices, the importance of integration tests has increased, because they ensure the seamless interaction of various services, especially if they are managed by different teams.

In essence, while unit tests are crucial for basic logic verification, integration tests are indispensable because they show how different components of an application interact and perform together.

Why You Shouldn’t Have Integration Tests Without Unit Tests

In larger enterprises, there’s a common anti-pattern of relying solely on integration tests, neglecting unit tests. Some developers perceive unit tests as futile, citing past pressures from management to boost code coverage, leading to trivial unit tests.

  • Integration tests require many more scenarios than unit tests for full coverage. Yet, often, developers will opt for a “representative” subset of integration tests. This method overlooks corner cases, which unit tests can handle efficiently.
  • Unit tests are considerably faster than integration tests.
  • When an integration test fails, it’s harder to pinpoint the problem because it tests multiple components. In contrast, unit tests can specifically highlight problematic areas.

While theoretically, one might operate with just integration tests, practical considerations emphasize the need for both unit and integration tests. The two complement each other and should be jointly implemented for efficient and comprehensive software testing.

Supercharging Continuous Integration and Unit Testing with Codefresh

Codefresh has comprehensive support for all different types of testing and is very flexible about how and where you can test your applications. More specifically you run tests:

  • Before compiling the code
  • After compilation
  • During container building
  • Before a deployment
  • After a deployment (smoke tests)

At the same time you can use Codefresh both with mocked services as well as real services. You can launch service containers inside a pipeline and attach required services such as databases, queue, caches in the service that is under test.

Finally Codefresh integrates with the Allure testing framework for producing not only test reports, but also historical trends for your testing results.

This means Codefresh covers all testing needs from all software phases in the most effective way.

Learn more about Codefresh

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