Testing Approaches: Top-down Approach versus Bottom-up Approach

In a large software project, it is impractical to integrate all the modules of the project together and test the software as whole. It should be build and tested in stages. There are different testing approaches.
Test Approach is the Strategy which describes the test team's approch to test the software, both overall and also in each phase.It gives better idea for the team to plan and execute the testing phase with perfection.

Top-down Approach versus Bottom-up Approach

In top-down approach, testing is done from top of hierarchy. Dummy routines called studs that simulate a module are introduced.
Advantage:
• Easy to visualize functionality.
• Sense of completeness in the requirement.
• Easy to show the progress of development.
Disadvantage:
• UI driven approach hence high possibility of redundant business logics.
• Since an UI is readily available no developer would write a Unit test cases.
• No Concrete layer to rely on, as both presentation & Business Logic keep evolving.
• Lack of concrete test suits to ensure one layer is tied up.

In bottom-up approach, testing is done from bottom of the hierarchy. Dummy routines called drivers are introduced that invoke the module.
Advantage:
• Solid Business Logic, hence zero redundancy
• Good Unit test case can be written to validate changes.
• Developer has only option to use unit testing tools to test the Logic.
• Easy to manage changes and modification.
Disadvantage:
• Effort involved to write test cases.
• Progress of implementation cannot be show very effectively.