API testing ensures that your APIs do what they're supposed to: run seamlessly and securely to allow communication and data exchange between different software components.

That said, it can be difficult to determine the types of tests you should perform for a given API. We'll help narrow down your options by introducing you to 6 common API tests and outlining when it makes sense to use each.

Unit testing

APIs have lots of small moving components. Unit testing allows you to test these small parts of your application, typically individual functions or methods in your API. The goal is to validate that each software module performs as designed. The process is highly granular and focused on the correctness of the internal logic.

Consider an API for an e-commerce application. One function of this API might be calculating the total price of items in a shopping cart. A unit test for this function would supply inputs (e.g., item prices and quantities) and verify that the function correctly calculates the total.

Unit testing is at the core of test-driven development. As such, it's most effective during the development phase, where quick and frequent testing of small code segments is necessary. It's less suitable for validating the application's functionality or the interactions between different components.

Integration testing

While unit testing is very limited in scope, integration testing combines individual units of an application and tests them as a group.

Integration testing checks how the different parts of your software work together. The focus is on the interface and interaction between these multiple units of your application, ensuring that they work together as intended.

To build on the previous example of an e-commerce application, integration testing would check how the calculate total price function interacts with other components, such as the database (retrieving item prices) and user interface (displaying the total price).

Use integration testing to check if your API works well with different parts of your software, such as web services, databases, or third-party applications. This type of testing is less relevant if your API is simple, stable, or isolated (which is rare unless it's an experimental tool) from other components. In such a case, you may only need to perform unit testing.

Related: A guide to performing API integration tests

Performance testing

Performance testing evaluates how well an API meets expected performance criteria, such as response time, throughput, or availability, under different conditions and loads.

Performance testing involves simulating real-world traffic by sending multiple concurrent requests to the API and observing its behavior and metrics. There are different types of performance testing, such as load testing, stress testing, scalability testing, and stability testing, and each type serves a different scope and purpose.

As an example of performance testing, consider an API for a social media application that handles user posts. A performance test on this API would involve establishing benchmarks for response time, error rates, and throughput, and then systematically simulating high-traffic scenarios to measure how the API performs against these benchmarks. This includes testing for response consistency under varying loads and monitoring error rates to ensure they stay within acceptable limits.

Conducting performance testing before releasing an API into a production environment is highly recommended to ensure your API can handle expected traffic. Performance testing is less relevant during the initial development stages when you're focused on building features and the API is not yet stable.

Security testing

Security testing protects your API from data breaches, injection attacks, unauthorized access, and other security vulnerabilities by systematically assessing the API for potential vulnerabilities and implementing measures to prevent or mitigate them.

If you have an API that handles user authentication, security testing would involve attempting to log in with incorrect credentials, executing brute-force attacks, or checking for session management vulnerabilities.

Conduct security testing before production deployment and regularly during production to ensure your API remains secure. It's important to note that while security testing complements functional testing by enhancing security measures, functional testing focuses on ensuring the API's intended operations work correctly.

Related: 5 tips for performing API integration security

Interoperability testing

Interoperability testing assesses how an API interacts with other APIs or systems when different protocols and components are involved. While this interaction is essential for better application functionality, its specifics can vary depending on the API and its integration requirements.

Interoperability testing is especially useful for APIs that provide web services, such as RESTful or SOAP-based APIs that communicate with other servers or clients.

Suppose you want to test a web service that provides weather information for different locations. The web service uses a RESTful API that accepts requests in JSON format and returns responses in XML format. The web service also interacts with other APIs or systems, such as geolocation services, map services, or weather data providers. In this case, interoperability testing could involve testing scenarios such as valid and invalid requests, different locations, different data formats, and different network environments.

You'll want to use interoperability testing for cross-browser compatibility, such as if you have a web-based API that needs to be accessed through various web browsers.

Performing interoperability testing is less important if an API adheres to widely adopted and well-tested standards, such as REST, SOAP, and OAuth.

Validation testing

Validation testing ensures that the API returns the results you expect.

Validation testing is high level, so you're looking at the software as a whole and asking questions such as "Is this API the best solution for the problem we're trying to solve?," "Is the API accessing the data as we defined it to?," and "Is it optimized in the best way possible?"

Imagine you have an API endpoint /getCustomerDetails that retrieves customer details from a database. A validation test case might involve sending a request with a valid customer ID and verifying that the API returns the correct customer details in the expected format (JSON, XML, etc.).

Similar to user acceptance testing, validation testing typically happens at the testing stage of application development when your API is stable and ready for production. Validation testing is not recommended when the API is still under development or undergoing frequent changes, as it may not reflect the current state of the API and may result in false positives or negatives.

Related: What is an API sandbox?

Final thoughts

API testing ensures your API is reliable, efficient, secure, compatible, and user-friendly.

From unit testing for small components to security testing to safeguard sensitive data, you've learned the best scenarios for when you need to perform a particular type of testing.

Are you also looking to build product integrations? Learn how Merge can help you add hundreds of integrations to your product through a single, unified API by scheduling a demo with one of our integration experts.