In integration testing, what best describes the Top Down and Bottom Up approaches?

Introduction / Context:
Integration testing focuses on verifying that modules or components of a software system work correctly when combined. Two classical strategies for integration testing are the Top Down approach and the Bottom Up approach. Each defines a different order in which modules are integrated and tested and requires different supporting test artefacts such as stubs and drivers. This question asks you to choose the description that correctly explains both approaches together.

Given Data / Assumptions:

• The system is decomposed into modules or components with clear interfaces.
• Some modules depend on others, forming a call hierarchy or layered architecture.
• We want to test interactions between modules incrementally rather than in one big bang.
• Test stubs and drivers can be created to simulate missing modules during early stages of integration.

Concept / Approach:
In Top Down integration testing, testing begins with the top level module or main control logic. Lower level modules that are not yet implemented or integrated are replaced by stubs, which simulate their behaviour with simple implementations. As integration proceeds, stubs are gradually replaced by the real modules until the full system is integrated. In Bottom Up integration testing, the process starts at the lowest level modules, which may be utility or leaf modules. Drivers, which simulate calling modules, are written to invoke these low level components. As higher level modules are developed, they replace the drivers, and integration moves upward until the full system is assembled. Both approaches aim to find interface defects early, but they differ in direction and tooling.

Step-by-Step Solution:
Step 1: Recall that Top Down begins with high level modules and uses stubs for lower level calls. Step 2: Remember that Bottom Up begins with low level modules and uses drivers to simulate higher level callers. Step 3: Identify that both approaches are about integration order, not about test types such as performance or security. Step 4: Compare the options and see that option a explicitly mentions high level to low level with stubs and low level to high level with drivers. Step 5: Conclude that option a correctly summarises both Top Down and Bottom Up integration testing approaches.

Verification / Alternative check:
Testing textbooks and standards often illustrate Top Down with a tree diagram where the root module is tested first and stubs replace children until they are ready. Bottom Up is illustrated inversely, starting from leaves with drivers simulating parent modules. These materials explicitly reference stubs for Top Down and drivers for Bottom Up. This aligns perfectly with the description in option a and does not support the claims made in other options.

Why Other Options Are Wrong:
Option b incorrectly associates Top Down with performance testing and Bottom Up with security testing, which are separate concerns; either integration strategy can be combined with various test types. Option c claims that Top Down requires no test harnesses and Bottom Up requires no test data, which is not realistic; both approaches rely on test harnesses, test data, and support code. Option d suggests that Top Down and Bottom Up are the same strategy, which ignores the clear directional difference in how modules are integrated.

Common Pitfalls:
A common pitfall is choosing an approach that does not align with the system architecture, resulting in excessive stub or driver creation and increased maintenance cost. Another mistake is to perform integration without a clear strategy, effectively turning it into big bang integration, which makes debugging more difficult. For exam questions, remember the core idea: Top Down proceeds from high level to low level using stubs, while Bottom Up proceeds from low level to high level using drivers, as summarised in option a.

Final Answer:
Top Down and Bottom Up integration testing can be described as follows: Top Down starts testing from higher level modules and progressively integrates lower level modules using stubs, while Bottom Up starts with low level modules and integrates upward using drivers.