What is Integration Testing?

Integration testing is a process that ensures the smooth interaction of assembled or integrated components within a system. It occurs after unit testing, which focuses on testing individual components, and before system testing, which evaluates the entire integrated system.

Integration testing acts as a test bridge between unit testing and system testing. This article will guide you through integration testing, why it’s important, the top integration testing tools to use, and some prominent techniques you can adopt while running it for software.

Integration testing is a black box testing type where individual units—that are already validated with unit testing—are combined through an interface and tested as a group. This process exposes faults in the interaction between integrated units or modules.

Also Read: What is Black Box Testing?

For instance, the car engine uses several parts like spark plugs, pistons, and crankshafts and runs when these parts are assembled right. It’s not enough that each part passes its own quality test; the entire engine should operate without problems when combined. The same goes for software. Once you ensure the individual module quality with unit testing, you proceed to verify the correctness of their interaction using integration testing.

Also Read: Unit Testing vs. Integration Testing

Here’s a list of top integration testing tools you can borrow:

1. Postman — Postman is specifically designed for API testing and integration testing. It excels in sending requests, validating responses, and handling various endpoints. You can automate test scenarios with Postman, making it efficient for other procedures such as regression testing and continuous integration. It supports multiple execution environments, including integration with CI/CD pipelines.
2. Citrus Framework — Citrus Framework helps validate various data exchange formats (e.g., XML, JSON, SOAP). It supports intricate integration scenarios, including end-to-end testing. Citrus offers tools for both client-side and server-side simulation ensuring rigorous integration testing.
3. Selenium — Selenium is renowned for automating web browsers and is widely used for cross-browser testing. It enables parallel execution of test cases across different browsers, improving testing efficiency. You can seamlessly integrate Selenium with various CI/CD tools.
4. TestNG — TestNG offers flexible test configuration options to run integration tests. You can easily set up test dependencies and execute tests in parallel. It supports data-driven testing, allowing you to run the same test with different input data.
5. Jasmine — Jasmine follows a BDD approach, making test scenarios more readable and expressive. It handles async code well, which is crucial for testing JS applications. Jasmine executes tests quickly, enabling rapid feedback loops during development.  
6. Katalon — Katalon combines web testing (via Selenium) and mobile app testing (via Appium) in one platform. Its intuitive interface simplifies test creation and management. Katalon also provides detailed reports, aiding in test analysis and debugging.

Also Read: Hot Automation Testing Tools

Here are some notable reasons that show the importance of integration tests:

Ensures Correct Code

Integration testing evaluates the consistency and correctness of the code logic. It helps identify functional and usability errors or unexpected behaviors when different modules interact with each other combined through an interface.

Also Read: What is Functional Testing?

Validates Error-free Data

Integration testing confirms correct data flows and transmissions by validating the interaction of APIs, databases, sub-systems, or third-party applications integrated with the system. It ensures that the data flowing across several layers or modules is consistent and helps identify bugs, if any.

Also Read: How to Report a Bug?

Helps Align w/Dynamic Project Specs

During SDLC and STLC, systems undergo numerous code revisions due to frequent scope changes and dynamic project requirements or specs. Integration testing validates that the application does not break in any way when integrated modules undergo code modifications to align with those dynamic project specs.

Also Read: STLC: Getting Started with Software Testing

Errors and Exceptions Handling

Specific errors and exceptions may not be evident or may not arise until system modules are integrated. Testers can identify such errors or bugs to report promptly with integration testing, and developers can ensure the correct handling of errors and exceptions to steer clear of such issues.

Compatibility

Compatibility issues are absolutely critical to identify and resolve while dealing with numerous integrated modules while developing software. Integration testing ensures the compatibility of both hardware and software components.

Ensures Reliable Software Delivery

Systems utilizing CI/CD pipelines during development, testing, and deployment benefit from integration testing as it helps to find bugs early in the development cycle, ensuring that components work together seamlessly. This practice promotes frequent and reliable software delivery, enhancing product quality and faster time-to-market.

Improved Maintainability

Integration testing focuses on verifying the interactions and data exchange between different components or modules. It helps to validate coupled modules by forcing them to interact with each other and identify any issues early on. This allows developers to refactor the code and improve its modularity. This means creating more loosely coupled modules that are more independent and easier to maintain in the long run.

Several integration testing techniques exist, each with its own benefits and limitations. Here you go:

#1 - Big Bang Integration Testing

When all the components or modules in a system are combined and the integration among them is tested to find any faults or errors while they interact, it’s called big bang integration testing.

Benefits

• Simple to run — It’s a straightforward approach with fewer bells and whistles to account for, since it does not include an in-depth and granular integration check.
• Convenient for small systems — Less complex or smaller systems can take advantage of this approach. Integration tests can be performed fast, to identify bugs, speeding up the development and deployment process.

Limitations

• Difficulty in fault localization — The big bang approach can sometimes become a pain due to the simultaneous testing of all modules at once, making it extremely difficult to localize and isolate faults, especially in medium or large systems.
• Increased dependency — It requires a completely ready system with all modules combined, making testers wait until the entire integration is done to start testing. The same goes for developers — giving them limited time to fix issues.
• Time-consuming for large systems — Owing to several integrated units in a large system, the big bang approach becomes time-consuming in such scenarios.
• Resource intensive — Requires all modules to be completed, which can delay testing.

For example, a simple application with five modules (M1, M2, M3, M4, and M5). Once all modules are developed, they are integrated and tested as a group to ensure they work well in unison.

#2 - Top-Down Integration Testing

Top-down integration testing is one of the three incremental testing approaches. It starts by testing the top-level modules, moving gradually toward the lower-level modules. Each module is then tested individually to identify faulty interactions or unexpected behaviors.

Benefits

• Early prototyping — Allows for a demonstration of a partially tested system.
• Easier fault localization — Facilitates easier fault localization and pinpointing defects at higher levels before proceeding.
• Prioritization of critical modules — Critical modules of the system can be prioritized very early for proper validation.

Limitations

• Stub development — Stub development can be time-consuming, which can sometimes delay project completion. Stubs are dummy modules or temporary code snippets that act as replacements until the actual modules are fully developed. They are typically used while testing lower-level modules.
• Delayed testing of lower-level modules — Lower-level modules are tested in later phases, which may delay the discovery of defects at foundational levels.

Example: In a CMS, the top-level UI or front-end modules are tested first. Lower-level modules like database access or query handling are simulated with stubs until they are integrated.

#3 - Bottom-Up Integration Testing

Bottom-up integration testing is the next incremental approach. It begins with the verification of the most lower-level modules. It then progresses to include higher-level modules.

This is facilitated by using drivers, which are temporary codes used typically in bottom-up testing. Drivers simulate the behavior of the functions that call upon stubs. Conversely, stubs represent the functions being called upon. Drivers initiate these function calls.

Benefits

• Early testing of low-level modules — Ensures that the foundational modules are tested thoroughly.
• Lowers dependency — Starting tests from lower modules significantly lowers dependency to wait for a fully integrated system — giving testers and developers enough time to test and debug.

Limitations

• Driver dependency — It requires the development of drivers, which can be an overhead.
• Early prototyping is not possible — Early demonstration of tested modules is not an option — since demoing lower-level modules is not useful at all from a high-level business perspective.
• Late testing of top-level modules — This can delay the testing of high-level functionalities — which can lead to a time crisis in later testing phases.
• Increases risks of missing critical modules — Testing high-level modules later down the road increases the chance of missing critical modules — if they are not strategically prioritized beforehand, which could be a side effect of delayed testing.

Example: Starting with a payment processing module at the bottom, testers gradually integrate and test it with the UI module at the top using drivers.

#4 - Sandwich Integration Testing

Sandwich integration testing, also known as hybrid integration testing, is yet another incremental testing approach. It combines top-down and bottom-up approaches. It tests high-level and low-level modules in parallel and comes somewhere in between the two testing approaches (bottom-up and top-down).

Benefits

• Parallel testing — Allows simultaneous work on different layers of the application, which is especially beneficial for larger systems.
• Comprehensive test coverage — Ensures both high-level and low-level modules are tested early.

Limitations

• Increased complexity — Managing two streams of integration can be challenging.
• Resource intensive — Requires more resources to maintain parallel testing streams.

Example: In a web application, the front-end and back-end are developed and tested in parallel. The middle layer is where both meet and integrate such as the business logic.

Let’s quickly summarize the aforementioned integration testing techniques: