Unit Testing Vs Integration Testing – Important Differences

The software development lifecycle consists of many stages to ensure that the final deliverable meets the requirements. Testing software is one such integral part of the development process. The two most popular tests for any software are – unit testing and integration testing.

On a high level, Unit testing allows us to test a single component of code in isolation which is usually a single method or class. On the other hand, integration testing enables us to test the connection or flow between different components. These components can be several pieces of code, services, APIs, database connections, etc.

In this article, let’s explore the differences between unit testing and integration testing.

Unit Test and Integration Test – Overview

What is Unit Testing?

Unit testing is a testing method that focuses on a single piece or unit of code. We use them to test a methodology in isolation. It determines the logical integrity of a method or a class. Simply put, we use them to determine whether a piece of code does what it’s expected to do.

For example, consider an application that books airline tickets. There might be several services and APIs in the code, each of which might have several methods/functions with their own purpose. There might be a service that allows users to log in, another that allows them to input details such as source and destination, etc. Suppose, in a service, we have a method that filters flights within a specific price range.

In this case, if you want to test this method, you need to ensure that, based on a given price range, the actual output has the same number and description of flights that are expected. This is called unit testing.

What is Integration Testing?

We have learned that only testing the isolated pieces of code is not sufficient to determine the software’s integrity. In such a case, it’s better to test how different parts of the application interact and work together. This process is called integration testing.

Unlike unit testing, integration testing considers the side effects of the code from the beginning, which may even be desirable at times. Let’s consider the example that we used before. Suppose there’s a service that fetches the user’s details from the database and auto-fills it. If we want to test this logic, there’s a dependency on the database here. In these cases, developers need to prepare, query, and mutate the database. They often mock these external dependencies, just like in unit testing.

Integration testing helps us to spot not-so-obvious issues or bugs that might have been caught by examining a specific unit’s implementation. It figures out bugs in the interplay of multiple components, which are often difficult to reproduce or track.

Ideally, developers prepare and execute unit test cases first to validate the logical integrity of specific methods. After that, they prepare integration tests to ensure that several methods or components interact with each other as they are supposed to. In the above example, an integration test here would run the same test cases against a real database as opposed to unit test cases that use mocked data. “Integration testing” is a general term and confines any tests where we involve multiple components.

Check here- Integration Testing Tools

Differences Between Unit Testing and Integration Testing

Aspect	Unit Testing	Integration Testing
Definition	Testing individual components or functions in isolation.	Testing combined parts of an application to ensure they work together.
Purpose	Verify that each unit of the software performs as expected.	Ensure that integrated components function correctly as a group.
Scope	Narrow focus on a single unit or module.	Broader focus on the interactions between multiple units.
Level of Testing	The first level of testing in the software development lifecycle.	Second level, following unit testing.
Features	-Fast execution,  -High granularity	– Tests interactions – May involve multiple systems – Can use real data
Tools	JUnit, NUnit, Mocha, pytest	Testsigma, TestNG, Postman, JUnit (for integration), Cypress
Limitations	– Doesn’t test interactions between units – Misses integration issues	– More complex setup – Can be time-consuming – May miss individual unit issues
Benefits	– Catches bugs early – Simplifies debugging – Facilitates refactoring	– Identifies interface defects – Validates combined functionality – Ensures data flow between modules
Types	– Manual Unit Testing – Automated Unit Testing	– Top-down Integration Testing – Bottom-up Integration Testing – Big Bang Integration Testing – Sandwich (or Hybrid) Integration Testing
Execution Time	Generally quick to run.	Takes longer due to the complexity of integrated components.
Testers	Typically performed by developers.	Often involves testers and developers working together.

Similarities Between Unit Testing and Integration Testing – Why do People Mix them Up?

There are a lot of similarities between unit testing and integration testing, which may confuse developers and testers. Fundamentally, both these tests are functional tests. We use them to catch issues and bugs during the early development phase. We do both tests to ensure good quality and code coverage before release.

Ultimately, this will reduce the time needed for debugging in case of any errors or issues after release. Moreover, these tests help you reduce the time and effort required to maintain the code. Also, if any changes or refactoring are done to a piece of code, you can verify if that piece of code still holds the same functional value. In this way, both tests are correlated to each other. These tests play a vital part in the complete testing process of your project.

Check here- Functional Testing vs Integration Testing

Unit Test vs Integration Test:  When to Use Which?

Unit Testing

What It Is:
Tests a single piece of code (like a function) to ensure it works correctly.

When to Use:

• During Development: Use them while writing code to catch errors early.
• Frequent Changes: When a piece of code is likely to change often, unit tests help ensure it still works.
• Debugging: To find and fix issues in specific functions.
• Example: Testing a function that calculates the total price in a shopping cart to make sure it gives the right result.

Integration Test

What It Is:
Tests how different pieces of code work together.

When to Use:

• After Unit Testing: Once individual pieces are tested, check how they interact.
• Multiple Components: When different parts of the application need to work together (like a front end and a back end).
• Complex Workflows: When you want to ensure that the flow of data between parts is correct.
• Example: Testing the entire process of a user making a payment through a website to ensure it connects correctly to the payment processor.

Check here- Android Integration Testing

Final Thoughts

Unit testing and integration testing are both essential in software development. It’s important to use both strategies to ensure bug-free software. Understanding their differences is key to deciding which one to use. Happy testing!

Frequently Asked Questions

What is the difference between integration testing and system testing?

Integration Testing vs System Testing

Integration Testing: Focuses on verifying interactions between multiple components or modules within a system to ensure they work together correctly. It is typically conducted after unit testing and before system testing.

System Testing: Involves testing the complete and integrated software application as a whole to validate that it meets specified requirements. It checks the end-to-end functionality of the entire system in an environment that mimics production.

Check here- Integration Testing vs End to End Testing

What is the difference between a unit test and an interface test?

Unit Test vs. Interface Test

Unit Test: Tests individual functions or methods in isolation to ensure they work correctly, focusing on specific pieces of code without dependencies. It is usually written and maintained by developers.

Interface Test: Validates the interactions between different software modules or systems, ensuring that the interfaces work as intended and data is correctly passed between them. It typically focuses on the boundaries between components.