Difference Between Unit Testing and Integration Testing

Unit testing and integration testing are the test methods used to verify the smooth functioning of the software. As the name suggests, unit testing verifies each software unit, component or module in isolation.

However, integration testing takes all the unit-tested components and integrates them to create a software architecture that dictates user-defined design. Let’s first begin with what software testing actually is.

Content: Unit Testing Vs Integration Testing

1. Comparison Chart
2. What is Unit Testing?
   • Types of Unit Testing
3. What is Integration Testing?
   • Types of Integration Testing
4. Key Differences
5. Conclusion

Comparison Chart

Basis for Comparison	Unit Testing	Integration Testing
Definition	It tests each module of the software in an isolation	It tests the unit-tested components when they are brought together
Time	Unit testing is the first test conducted	We conduct integration testing after unit testing but before system testing
Testing	Performed by the software developers	Performed by a separate team of testers
Errors	Identifying errors is easy	Identifying errors is comparatively difficult
Focuses	It focuses on testing the internal functionality of the module	It focuses on testing the interfacing between the components
Performed	Unit testing is an adjunct to the coding step. You can even parallelly test several components of the software	We can perform integration testing only after all the components of the software are developed
Cost	Less costly	Costlier
Speed	Faster	Slower

What is Unit Testing?

Unit testing verifies the components or modules of the software in isolation. It tests the internal control paths of the module to detect errors within the boundary of that particular module. In unit testing, we can parallelly test multiple modules or components of the software.

The complexity of the unit testing and the errors it discovers is low. Because it only focuses on the module’s internal processing logic and data structure.

• We test the module’s interface to ensure that information smoothly flows in and out of the program unit. We have to exercise this test before we initiate any other testing. Because proper data flow across the module interface is necessary for all the data and other tests to be successful.
• Testing independent paths through the control structure ensures that all the instructions in the module have been executed. It uncovers errors that may occur due to erroneous computation, improper control flow, incorrect comparison, etc.
• We test logical data structure to ensure that the module maintains data integrity during all the steps of algorithm execution and that it does not impact the global data during the testing.
• Testing boundary conditions ensures that modules operate efficiently at their boundaries to limit or restrict their processing. Usually, software often fails at its boundaries.

Unit Testing Environment

Unit testing creates an environment to test each component or module of software in isolation. The software has several components, so we must develop a driver or stub software to test each. Each test case arrives with a set of expected results.

A driver is a program that accepts test case data and passes it to the component undergoing the test. Stubs are replacement for missing methods invoked by the components under test. They perform minimal data manipulation, prints verification of components execution, and return control back to component undergoing testing.

But if we observe carefully, these drivers and stubs are testing overheads. It takes effort and time to design/build them, but they are not delivered with the final software. Keeping the design of the driver and stub simple can reduce overhead, but it would not adequately unit test many of the software components.

So, sometimes unit testing is postponed until integration testing. Unit testing is preferable when developers design the component with high cohesion. It immensely reduces the number of test cases, and uncovers the errors easily.

Types of Unit Testing

There are two types of unit testing manual unit testing and automated unit testing.

• In manual unit testing, the test cases are run manually from the end users’ point of view.
• The test cases are run using specialized automated tools in automated unit testing. The quality of automated testing depends on how well the developers design test cases.

What is Integration Testing?

Sometimes, the unit-tested components that work well individually will also work when we put them together. It might happen that the data  lost across the interface, a component puts an adverse effect on another, together they may not produce the desired result, etc.

Integration testing aims to incorporate unit-tested components in a software architecture that accomplishes the customer’s requirement.

Types of Integration Testing

We have three different types of integration testing

Big-bang

In the big bang, there is no prior testing. All the generated components incorporates to form a complete system and then tested to uncover the errors. Error correction is difficult as it becomes hard to identify the source of error in this method.

Top-down

It starts with the main control module, and gradually incorporates rest of the modules  by moving downwards through the control hierarchy. The modules subordinate to the main control module are incorporated in a depth-first or breath-first manner.

Bottom-up

Bottom-up testing begins with integrating and testing the components at the lowest level of the program structure that performs a specific function. Thus, the test integrates the components, moving upwards in the program architecture.

Conclusion

Unit testing and integration testing are the methods to verify and validate the software’s function and uncover its errors. Unit testing focuses on the functional verification of the components. However, Integration testing focuses on incorporating components into the software architecture.