Differences Between Top Down Testing Vs Bottom Up Testing – Software Testing Stuff

Effective software testing heavily on incremental integration testing. In this approach, testing begins with the integration of multiple modules that are logically related, followed by functionality tests. 

This test ensures that the integrated units work properly as a unit. However, there are two approaches to incremental integration testing. You can execute an incremental integration test by following a top down or bottom up approach. 

As they are both incremental integration techniques, it’s easy to get confused between top down testing vs bottom up testing. Despite the fact that both tests detect integration flaws, they do follow significantly different techniques. 

Additionally, if you don’t choose the method suitable for your specific project, you might not get the desired results. As a result, understanding the differences between them is critical. 

We’ve covered everything you need to know about top down and bottom up testing in this article. So, let’s get started.

What Is Integration Testing?

Integration testing also known as I & T is a software testing technique where different software modules or components are integrated and tested as a single unit. 

Software projects consist of many software modules. These modules are often coded by different programmers. 

Integration testing verifies if the different modules perform as they should when integrated together.

Customer requirements may change at any point during the software development life cycle. These requirement modifications may not reach the developers correctly.

Moreover, the new changes might not be unit tested either hence Integration testing is required to ensure that the software works properly.

It mainly focuses on checking communication between different modules. If integration testing is skipped, many errors may go undetected. Such errors can result in integration failures which are time-consuming and expensive to fix once the software has been released.

Objective Of Integration Testing

Integration testing is a crucial part of the software development industry. Especially since client requirements are extremely dynamic and deadlines are limited. 

As a result, ensuring software quality in a limited time becomes a difficult task for the QA team. The main objectives of integration testing are:

• Ensuring that software modules function properly after integration

• Verify that software features follow client requirements

• Assess all the functional and non-functional features for efficiency

• Check that related modules are synchronized

• Expose interface errors that may arise after integration

• Test communication between separate modules

• Pinpoint weak spots in the software architecture

• Ensure proper fixes for all the exception handling errors

• Prevent errors from leaking to high-level tests

What Is Top-down Testing?

In top down testing technique, high-level modules are tested first, and then lower-level modules are tested. High-level modules are also known as main modules, whereas low-level modules are referred to as submodules.

If the submodule is not ready for integration yet, temporary modules called STUB are used for simulating the real submodule. The main modules are also integrated first and submodules are integrated subsequently to check if the software functionality works properly.

The term “top down” refers to the fact that testing is done from top to bottom, as in from main modules to submodules following the control flow of the software.

Furthermore, while integrating submodules, we must ensure that the module we are integrating is a child of the main module. This testing is primarily performed on software written in a procedure or structure-oriented language.

The prime objective of top-down integration testing is to uncover key design defects early on since the main modules are tested first.

Advantages And Disadvantages Of Top-down Testing

Advantages:

• Critical Modules are tested on priority

• Early detection & fixing of major design defects

• Fault localization becomes easier

• Allows creation of early prototype

• Early prototype makes test case creation easier

Disadvantages:

• Many stubs need to be produced

• Creating stubs for intricate features may become complicated

• Representing test cases in stubs can be difficult

• Observation of test output is more difficult

• Lower-level modules are insufficiently tested

What Is Bottom Up Testing?

In the bottom up testing technique, testing is done from the submodule to the main module. The modules will be added from the bottom to the top, and the data flow will be tested in a similar manner.

Meaning lower-level modules are tested & integrated first to execute certain software functions. If the higher-level module isn’t constructed yet, a temporary program called DRIVERS is used for mimicking the main module. This driver configures test case input and output in order to test the lower modules.

These tested modules are then utilized to help with testing of higher-level modules. While following this upward testing direction, drivers are eliminated as main modules get developed completely. 

Here, the top-level modules are tested at the very last stage. As a result, testers get less time to test the critical modules which can result in major errors being left in the code.

This integration testing is done on applications developed using object-oriented languages.

Advantages And Disadvantages Of Bottom Up Testing

Advantages:

• Detects major flaws that occur at the lower level modules

• Testing can start without waiting for all units to be completed

• Easy to create test conditions

• Test result observation is easier

Disadvantages:

• Driver modules have to be produced

• Early prototype can not be generated

• Critical modules at top get tested inadequately

• Main modules may remain prone to defects

Key Differences Between Top-down And Bottom-up Integration Testing

Learning the key differences between top down testing vs bottom up testing will enable QA testers to make well-informed choices as to which type of integration testing technique will be suitable for their projects.

First of all, the top down approach utilizes stubs as a temporary replacement for lower modules whereas the bottom up approach uses drivers to simulate the higher-level modules.

The main or top modules are given the most importance in the top down approach. The top modules are tested first and through these main modules, other submodules are called. 

On the contrary, higher emphasis is given to the lower modules in the bottom-up approach. Here, submodules are created, tested, and integrated first. 

Another major distinction is that top down technique examines the risk by collecting the internal operational failure impacts, but the bottom-up method evaluates individual processes risk with models’ support.

Moreover, top-down testing goes from major to minor components, whereas bottom-up testing goes from small to significant modules.

Lastly, top down testing is simpler in complexity & data intensity as compared to bottom up testing.

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