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Integration Tests vs Unit Tests: What Are They And Which One to Use?

Imagine you open an e-commerce website, browse around, then place some items in the cart. The next step is checkout, and you decide to use PayPal – only to discover that the website throws an error, making checkout impossible. That is an example of a bug that could’ve been easily caught if an integration test was in place. So today, we will talk about what exactly is a definition of unit vs. integration test and the stages at which they’re used.

There are multiple testing stages that the software product typically goes through before being released to the customers. Unit testing, integration testing, system testing and regression testing are some of them. Test-driven development can ensure your deliverables are perfect.

Unit testing is the most common one and the first testing strategy. It’s so common it’s almost synonymous with software testing for some small size projects. In contrast, integration testing is done in the next phase when the software goes beyond the modular level—more on that below.

In this article, let’s get a clear understanding of unit testing and integration testing. We will start with the definitions and then get into the differences. By the end of this article you will have a clear picture of what unit testing and integration testing are, and which is most suitable when.

Let’s start!

What is Unit Testing?

Unit testing (most commonly done by software engineers) is testing individual components at the modular level. Each unit is tested in an isolated way, which oftentimes includes mocking the necessary data. Unit testing is about testing internal components, and should never include communicating with external components.

Unit testing isn’t particularly useful to anyone else other than software engineers, and it doesn’t show the whole picture. Does it mean it’s not very important? Quite the opposite! Having good unit test coverage will always result in higher quality code, and QA will end up with lesser bugs to find and log. So it’s crucial to have robust unit test coverage, with many companies striving for as high as 90% for any new feature.

Unit tests ensure each small function of code performs intended tasks.

For example, if you are developing a chess game, you might want to ensure each piece can only move as it’s supposed to.

The most important aspect of unit tests is they are executed in isolation. A test is not a unit test:
  • If it includes connecting to a database

  • If it includes an interaction with a web browser

  • If a request is sent over a network

  • It touches multiple modules

Why Is Isolation Good for Unit Tests?

Unit tests are supposed to catch bugs from very specific portions of code. If a unit test fails, we must be able to understand precisely where in the code the malfunction occurred. That’s why unit tests are performed in isolation.

By performing unit tests in isolation, we are taking dependencies like database connections and usage of file systems out of the picture. This not only reduces the cost but also speeds up the test execution. Unit tests are typically the fastest to get the test results and commonly run on every commit before it can be released for further testing.

Unit tests also help in making sure basic functionality is robust. Unless code is changed, tests passing or failing are supposed to exhibit the same behavior, which is only possible in isolation.

What is Integration Testing

We hope that it’s clear to you now what unit testing is about. With integration testing, however, it can get a little bit trickier. Let’s explain.

It determines if multiple modules together can function seamlessly as part of the system.

Essentially, when people talk about integration testing, they most commonly will refer to either of these two options:
  1. (Less common) Tests that use any resource (such as network connection or database connection) that isn’t mocked out.

  2. (More common) Full-on software testing, where various modules of an application under development are tested together.

That’s the fundamental difference we need to identify between the two most common forms of testing.

We can now look at integration testing at a deeper level for a clear definition. Let’s start by understanding why it is important, the best practices, and some tools.

Why is Integration Testing Important?

Even after coming up with the most optimized algorithms and following the clean coding practices out there, no project can escape vulnerabilities, bugs, and functional breakdowns. These problems always exist.

The main reason behind it is that multiple developers collaborate on a single project. Each developer has his own ways of approaching problem solving, and different ideas. No matter how they collaborate, developers often end up accidentally inserting bugs into each other’s features. That’s where integration testing comes in.

A checkout bug we’ve referred to at the beginning of this article wouldn’t have been caught by a unit test, but the correct integration test would easily identify it.

What’re the Main Differences?

Many junior developers often think of unit testing and integration testing as synonyms or closely related testing types, as they are performed in the consequent stages of development.

Let’s explore major differences between unit testing and integration testing to get a clear picture of them and to which is applied and when.

Unit Testing Integration Testing
1 Unit testing is done on basic functions i.e at module level or component level Integration testing is needed when modules are combined
2 Extensive knowledge of the underlying code is mandatory for this type of testing The test engineer may not have complete access to the codebase which handles the integration among modules but the expected behavior of integrated modules is known and the same is tested
3 Unit testing is the first testing stage done at the developer level Integration testing is done after unit testing, so it’s usually the second step in software testing
4 White box testing, typically written by developers Typically gray box testing, typically done by QA people
5 It’s very easy to find bugs as the developer can see the internal code structure It’s harder to catch bugs in integration testing as multiple components are involved
6 Easy to debug More difficult to debug
7 Unit testing is done when any new component is introduced or modified Integration testing is necessary only when we implement multiple components
8 As unit testing is done at the beginning stage of software development, the cost associated in its implementation and bug fixing is the least among all testing types Integration testing is a little pricier because of its timing. Bugs caught during this stage will cost more because of most likely usage of higher resources , external modules, etc
9 In unit testing all test data is mocked In integration testing we will have real world dependencies
10 Unit tests are run before commit to the main branch Integration tests are (typically) run during the next stage

When to Use Which?

We can use the testing pyramid concept to make it easier to digest. This is the widely accepted way of thinking about tests.

Unit tests sit at the bottom of the pyramid, representing the most fundamental and extensive part.

The second third of the pyramid is represented by integration tests.

Finally, the top third is where we will put UI and end-to-end tests. Only these tests will give us a wholesome understanding that the entire software works exactly as intended, and users will get the experience they expect. This is where you would use a test automation tool such as testRigor.

What About the Cost?

Of course, cost is a very important factor when it comes to the efficiency of software testing. Every project starts with a discussion about the ROI of test automation.

Unit tests are the fastest to write as developers implement these at the time of feature development. They are also cheaper to run as they don’t usually require a special environment or access to external resources.

When it comes to the speed of test execution – unit tests are the fastest. The best part is we can run unit tests in parallel as they are completely isolated, saving us a lot of time.

On the other hand, integration tests cost us a bit more time and money, but that does not mean they are less useful. Rather it’s just a different perspective as well as type of feedback.

Respectively, all following testing types such as functional, UI, end-to-end tests cost more than unit and integration and are slower to run. However, they result in giving software teams a final clear picture, and assurance that no bugs have been missed.


We hope that you now have a much better understanding of these testing types, as well as when each type of tests is being run.

And if you’re looking for ways to improve your UI and end-to-end testing, don’t hesitate to give testRigor a try – to experience the future of software testing, with ultra-stable tests and minimal test maintenance. And best yet, no coding skills are necessary to build tests – so anyone on the QA team can do it.

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