What is Integration Testing? Comparing with Unit Testing

Software has become an inseparable part of every human being’s life. It is a key ingredient in devices and various other systems.

Now think if you are using software full of bugs and errors. It is not a good sign for the development company if the software or application they are launching is full of bugs.

Therefore, companies use various testing procedures to remove these types of errors. With the help of the testing procedures, the developers not only find out the errors but also works on the errors to fix them to provide their customers with a smooth experience with the software.

There are many types of software testing, and Integration Testing is one of them. This article contains all the details that a person may want to know about this testing procedure.

What is Software Integration Testing?

From the Integration testing definition, we know this is the type of testing where the separate software modules are brought together and integrated with some logic and are tested as a whole. Any software consists of various modules that different programmers prepare.

This Integration Testing is generally used to determine the defects in the interaction between the different software modules when combined to function as a whole.

Moreover, Integration Testing is mainly conducted to evaluate how much the system or the component -complies with some specific functional requirements. The second phase of the complete software testing procedure is performed after Unit Testing.

In this type of testing, the modules tested under Unit Testing are taken as inputs and then grouped in larger aggregates, after which the test process continues according to the integration test plan.

Furthermore, it delivers an output that is then ready for the next phase of software testing, System Testing.

Why do we need to do it?

The process of integration testing is essential in today’s IT and Software development processes market, especially when there are dynamic requirements and the deadline is very close.

Even after Unit Testing, the modules may have some bugs and errors which can be identified and fixed with the help of Integration Testing. That is why integration testing is very crucial. There are a few more reasons behind conducting integration testing:

Combining the Modules into Working Application

Since different developers are coding for the different modules, the individuals will bring their own logic to the development process. It will, in turn, cause some functionality and usability problems when all the modules have been brought together.

Integration Testing helps us to ensure that the units that have been integrated work properly as a single unit and have been aligned with the requirements. It also ensures no errors between the various interfaces or modules.

Incorporate the Changing Modules into the Application

The requirements can often change during testing and development in most real-time application scenarios. At this time, these new requirements may not have been tested under the Unit Testing process, which may lead to some defects or features of the product being missed.

That is where Integration Testing becomes important. Integration Testing can fill these gaps and ensure that new requirements have been included in the final application.

Removing Other Common Problems and Issues

Some modules may interact with third-party application program interfaces and need to be tested to ensure that they are functioning properly. This process may not be performed during the unit testing procedure.

Also, there may be other different issues, such as inadequate exception handling, API response generation, data formatting, error trapping, and many more. All of these issues can be resolved with the help of Integration Testing.

Types of Integration Testing

Now let us check out the different types of Integration Tests. There are various types of integration testing which have been categorized below.

Big Bang Integration Testing

In this type of Integration testing Approach, all the modules have been developed and tested individually under the unit testing procedure. It is considered a convenient approach if the system or testing software is small.

They are integrated and tested together once. The main advantage of this process is that it is very suitable for some smaller systems. Additionally, you can detect any small fault within a few seconds with the help of this software testing type.

You can see some drawbacks to this process as well. The fault in the code cannot be easily pointed out. There are also a lot of delays before the testing finally starts. The critical issues are not given the amount of priority that it needs. Also, it is very difficult to find out the root cause of the problem.

Incremental Integration Testing

This type of integration testing is performed by combining two or more logical modules, after which these modules are integrated and tested successfully. This Incremental Integration testing can further be divided into three types – Top-Down Approach, Bottom-Up Approach, and Sandwich Approach.

Top-Down Integration Testing

The top-down integration testing approach starts with testing the topmost modules and then slowly moving downwards to the lowest modules. This type of Testing takes place from top to down. It follows the control flow of the software system.

As there is a possibility that the lower level modules might not have been developed while top modules are tested, stubs are used instead of those incomplete modules. In simple words, stubs would return the control to the modules superior to them. For complex applications, they would simulate the full range of responses.


  • Finding out the fault in the code is quite easier.
  • The product which is being tested is very consistent.
  • The stubs require a lesser time to write as compared to drivers.
  • Critical Modules are given priority in testing.
  • The major flaws in the design are detected at an early stage.


  • This process requires several stubs.
  • It does not provide proper support for an early release.
  • The basic functionality of the process is tested at the end of the cycle.

Bottom-Up Integration Testing

This integration testing approach starts with the lowest unit of the software and then moves upwards one by one. It is the complete opposite of top-down integration testing. The testing is done from the bottom of the control and then flows upward.

But there is a possibility that the higher level modules might not have been developed properly when the lower modules have been tested. In this case, drivers are used which simulate the functionality of those missing modules.

These drivers efficiently perform a wide range of tasks, such as invoking the module under test, passing the test data, or receiving the output data.


  • It allows the development and the testing to run side by side, which in turn helps the product to be more efficient.
  • The creation of the test condition is much easier.


  • This type of Software Integration testing requires several drivers.
  • The testing of data flow is very late.
  • The requirement of the driver makes test data management very complicated.
  • There is no proper support for early release.
  • The defects in the key interface are detected very late.

Sandwich Integration Testing

This testing approach has been designed to avoid the disadvantages and properly use the advantages of the top-down and bottom-up testing approach. The hybrid version of the previous two approaches has been discussed. In this process, the system is considered to have three layers, with the middle layer being the main target layer.

The top-down technique is used from the topmost layer to the target layer. Also, the bottom-up technique is used from the bottom to the middle for the middle or target layer. The big bang approach is used.


  • Both the top-down and the bottom-up testing techniques can be used parallelly.
  • It is useful for larger enterprises with huge projects with some further subprojects.


  • This software testing procedure is very costly.
  • No one can use this method of software testing for smaller systems with huge interdependence between the modules.
  • The testers need different skill sets for different levels of this testing.

Advantages and Disadvantages of Integration Testing


  • Identifying the integration issues between the modules and rectifying them becomes very easy.
  • Integration Testing also helps to ensure that the integrated modules work perfectly before the testing process is shifted to the next phase.
  • Any type of bug, if found, is comparatively easier to resolve than the other stages of testing.
  • It improves the test coverage and provides an extra level of reliability.
  • This is a testing process that can be used in the early as well as later stages.
  • This software testing procedure also allows the modules to interact with API and other third-party tools.


  • It isn’t easy to perform Integration Testing compared to the other types in which the application is considered a black box.
  • The complete integration testing procedure is very time-consuming and requires many resources to test all the interfaces between the different connected modules.
  • It requires some additional effort as there is a need to create stubs and drivers, which, if not created properly, can result in incomplete testing.

How to do Integration Testing?

Since we have gone through the meaning of Integration Testing, we can see that it is quite a straightforward process – combining the unit-tested modules one by one and then testing the functionality of the combined unit.

Usually, the integration testing is performed after unit testing. After all the individuals have been combined and tested, then begins the main process of Integration Testing. The main motive here will be to test the various interfaces between the units or the modules. The integration test procedure includes the following steps:

  • Prepare the complete test plan for the Integration Testing.
  • Decide on the type or the approach of Integration testing you will follow.
  • Prepare the test cases, test scenarios, and scripts in accordance with the type you choose.
  • Bring in the Selected Modules and start the Integration testing.
  • Keep track of the defects and record the results of the test.
  • Keep repeating the steps until and unless the complete system has been tested.

Always remember that you must prioritize the integrated interface links between the modules or the units.

Unit Testing vs. Integration Testing

There are some key differences between the two types of software testing procedures.

  • Unit testing is the method that is used to test the individual units of the code and determine whether they are ready to use. In contrast, Integration Testing is the method for checking the combination and usability of those same units or software modules.
  • In unit testing, each part of the testing is done, which shows whether the individual parts are correct or not. In integration testing, those individual parts are combined and tested as a group to check whether they work the way we want them to.
  • Unit testing starts with module specification, whereas integration testing starts with interface specification.
  • Unit testing is the first step in software testing and can be performed by the developer at any time. But software integration testing is the second step of the software testing process which can only be performed after unit testing.
  • The developer performs unit testing, whereas a quality analyst team performs Integration testing.
  • Unit testing is a kind of white box testing in which it is very easy to find out the errors, whereas Integration testing is a type of black box testing in which it is quite difficult to find out the errors.


What is the main purpose of Integration Testing?

The main purpose of Integration Testing is to find out the faults in the interaction between the interfaces or the integrated unit. It also determines the correctness of the interface.

Is Integration Testing an SDLC?

Integration Testing is a key Software Development Life Cycle (SDLC) Strategy.

Final Words

It does not matter what type of code you are testing with the help of integration testing and how you prepare for the process. The main focus of everyone is on the final product being delivered to them.

All you need is to follow all the steps properly. After completing all the steps in software integration testing, you will see that making the software more functional becomes easier.

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