What is API testing?

API testing ensures the API is behaving as expected. Testing typically involves sending requests to the API and then monitoring its responses. This is crucial because APIs serve as the main interface for application logic and a graphical user interface is difficult to maintain.

If your organization is deploying APIs, you should definitely be testing them. API testing is more widely available and faster than UI-based testing, and it serves as the first line of defense against errors or vulnerabilities. UI testing is also essential but typically should happen later in the process.

API testing can be automated and incorporated into your DevOps process, while UI testing is difficult to automate. API testing can also quickly pinpoint bugs in the system within your development timeline.

Benefits of API testing

Testing earlier: With APIs, you don’t have to wait for teams to finish their work or applications to be built.

Easier testing maintenance: REST APIs are described in OpenAPI specification files. As a result, it is easier to automate and maintain an API test suite.

Faster time to resolution: An API test failure lets you see where the defect is and where the system broke. Because you know exactly where the problem is, you’ll save time triaging bugs between builds and integrations.

Language-independent: Backend systems and applications can use proprietary data formats and communication protocols, but with REST APIs, all that is hidden from a client. The communication and data formats are usually standardized and well understood.

Types of API testing

There are several types of API testing including:

• Functional testing: Checks that the API returns the right response from a request.
• Load testing: Gauges how an API handles a large volume of requests during a short period of time.
• Security testing: Assesses how an API responds to various security threats like cyberattacks and other threats.
• Penetration testing (pen testing): Allows penetration testers to assess the threat from an outside perspective.
• Fuzz testing: Checks how the API responds when a large number of random requests are sent to it.

What’s needed for API testing?

Before you begin API testing, you need to know two distinctions of web service for the web API: SOAP and REST. Here’s what you must be aware of:

API output status needs to be clear! The most important indicator is the response status code. Having your ducks in a row is part of API testing.

Read more about API testing strategy: Who’s testing your API-driven product?

Where is API testing performed?

API testing is typically performed in three separate layers: the business layer, the user interface layer, and the database layer.

What bugs does API testing detect?

API testing can detect:

• Error conditions
• Flags that are not utilized
• Reliability issues
• Security issues
• Missing or duplicate functions
• Multi-threading and performance issues

API testing best practices

To keep testing running smoothly, keep these best practices in mind:

• Always begin with categorizing test cases by type.
• Develop your tests in parallel as you develop your APIs.
• Develop your API test suite based on business-level understanding of your API’s functionality, not on your understanding of the code.
• Cite what names the APIs should be called.
• Have certain criteria mentioned for the APIs.
• Rank the API function calls.
• Keep the tests independent of one another (self-contained).
• Pay close attention to the different processes — well-executed tests are best in the long run.

API testing outline

To work efficiently, API testing includes testing APIs in isolation. This also applies to integration testing. Many transactions take place in the testing phase. This includes several kinds of endpoints such as databases, web services, mainframes, and more.

API testing is all about discovering whether the API returns the right response for effective use with a desirable level of security and performance.

API mocking takes this process one step further by testing the API interface (also called contract) while the API is under active development.