2.2.2 Integration Testing

Objectives of integration testing

Integration testing focuses on interactions between components or systems. Objectives of integration testing include:

• Reducing risk

• Verifying whether the functional and non-functional behaviors of the interfaces are as designed and specified

• Building confidence in the quality of the interfaces

• Finding defects (which may be in the interfaces themselves or within the components or systems)

• Preventing defects from escaping to higher test levels

As with component testing, in some cases automated integration regression tests provide confidence that changes have not broken existing interfaces, components, or systems.

There are two different levels of integration testing described in this syllabus, which may be carried out on test objects of varying size as follows:

• Component integration testing focuses on the interactions and interfaces between integrated components. Component integration testing is performed after component testing, and is generally automated. In iterative and incremental development, component integration tests are usually part of the continuous integration process.

• System integration testing focuses on the interactions and interfaces between systems, packages, and microservices. System integration testing can also cover interactions with, and interfaces provided by, external organizations (e.g., web services). In this case, the developing organization does not control the external interfaces, which can create various challenges for testing (e.g., ensuring that test-blocking defects in the external organization’s code are resolved, arranging for test environments, etc.). System integration testing may be done after system testing or in parallel with ongoing system test activities (in both sequential development and iterative and incremental development).

Test basis

Examples of work products that can be used as a test basis for integration testing include:

• Software and system design

• Sequence diagrams

• Interface and communication protocol specifications

• Use cases  Architecture at component or system level

• Workflows

• External interface definitions

Test objects

Typical test objects for integration testing include:

• Subsystems

• Databases

• Infrastructure

• Interfaces

• APIs

• Microservices

Typical defects and failures

Examples of typical defects and failures for component integration testing include:

• Incorrect data, missing data, or incorrect data encoding

• Incorrect sequencing or timing of interface calls

• Interface mismatch

• Failures in communication between components

• Unhandled or improperly handled communication failures between components

• Incorrect assumptions about the meaning, units, or boundaries of the data being passed between components

Examples of typical defects and failures for system integration testing include:

• Inconsistent message structures between systems

• Incorrect data, missing data, or incorrect data encoding

• Interface mismatch

• Failures in communication between systems

• Unhandled or improperly handled communication failures between systems

• Incorrect assumptions about the meaning, units, or boundaries of the data being passed between systems

• Failure to comply with mandatory security regulations

Specific approaches and responsibilities

Component integration tests and system integration tests should concentrate on the integration itself. For example, if integrating module A with module B, tests should focus on the communication between the modules, not the functionality of the individual modules, as that should have been covered during component testing. If integrating system X with system Y, tests should focus on the communication between the systems, not the functionality of the individual systems, as that should have been covered during system testing. Functional, non-functional, and structural test types are applicable.

Component integration testing is often the responsibility of developers. System integration testing is generally the responsibility of testers. Ideally, testers performing system integration testing should understand the system architecture, and should have influenced integration planning.

If integration tests and the integration strategy are planned before components or systems are built, those components or systems can be built in the order required for most efficient testing. Systematic integration strategies may be based on the system architecture (e.g., top-down and bottom-up), functional tasks, transaction processing sequences, or some other aspect of the system or components. In order to simplify defect isolation and detect defects early, integration should normally be incremental (i.e., a small number of additional components or systems at a time) rather than “big bang” (i.e., integrating all components or systems in one single step). A risk analysis of the most complex interfaces can help to focus the integration testing.

The greater the scope of integration, the more difficult it becomes to isolate defects to a specific component or system, which may lead to increased risk and additional time for troubleshooting. This is one reason that continuous integration, where software is integrated on a component-by-component basis (i.e., functional integration), has become common practice. Such continuous integration often includes automated regression testing, ideally at multiple test levels.