API Testing

What is API?

API (application programming interface) defines the way software components interact with each other; it is a means for a developer to expose functionality for internal or external use, and testing it shows how internal and external users implement the API.

Why is API Testing Important?

API testing determines if the API meets expectations for functionality, reliability, performance, and security. There is a set of testing operations one needs to execute to fulfill the task.

API testing is essential for the whole process of API development, implementation, and maintenance. There are hundreds of various software developed for testing purposes. API testing tools are a necessary means accompanying APIs to make it healthy and ready for its purpose.

API Testing Types

  • Unit testing is defining the compliance of the application units with the design requirements by testing a single endpoint, with a single request to check the performance of a response.
  • End-to-end testing helps to validate the data transfer between a few diverse API connections.
  • Functional testing is evaluating the compliance of a system or component with specified requirements.
  • Runtime error detection - monitoring the execution of automated or manual tests to expose problems such as race conditions, exceptions, and resource leaks.
  • Security testing - validating authentication, encryption, and access control; includes:
    • Penetration testing - begins with gathering information about the target before the test, carries on identifying possible entry points, committing virtual or real attempts for break-in and ends with a complete report on the results;
    • Fuzz testing – identifying a target system, and inputs; generating fuzz data and executing the system using the data; monitoring system behavior, logging defects.
  • Performance testing is conducting to check the speed, effectiveness, scalability, and reliability of APIs, and it also helps to compare data transfer rate and bandwidth.
  • Validation testing is one of the last and most important steps in the development process. Basically, it's a set of simple questions applied to the entirety of the project, including product validation, API behavior on accessing the correct data in the correctly defined manner; and the efficient opportunity for any codebase to be removed or altered to dispose of deteriorations in the general service.

Additional Testing Types for SOAP API

  • Interoperability testing is carrying out for SOAP APIs only to check conformance to Web Services Interoperability profiles.
  • WS (Web Services) compliance testing is implementing for the SOAP APIs only, to check the compliance to WS standards such as WS-Addressing, WS-Discovery, WS-Federation, WS-Policy, WS-Security, and WS-Trust.


Testing APIs is making sure they are up, fast, and performing as expected. Monitoring endpoints gives you a crucial head start to determine and fix the problem before presenting the product to customers, partners, or end-users.