Testing the API accelerates the overall testing effort by validating the functionality of the lower layer before the UI is ready. This type of level helps to reduce development, testing and support cost.

API (Application Programming Interface) is a collection of software functions and procedures, also called “API calls”. These API calls can be executed by other software applications to fetch or provide data. Developers use existing or create new APIs to make use of their functionality (this is almost like re-useable code). This link is seamless and end-users of the application are generally unaware of using a separately developed API.

Testing at the API level can be done to validate the functions, methods and procedures at early stages in the developments process, even before the GUI (Graphical User Interface) is created/coded). API testing helps testing systems that do not have a presentation layer such as web-services.

During testing, the test application is constructed to simulate the use of the API. These simulations are created in the early stages of development

The following are some typical API testing verifications:

1. Test the variety of parameters within the API. Make sure that the “API calls” verify functionality and expose failures. This includes assigning common parameter values as well as exploring boundary conditions.
2. Generate parameter value combinations for calls with two or more parameters.
3. Determine the content or business rules under which an API call is made. This might include setting external environment conditions (files, peripheral devices, and so forth) and also internal stored data that affect the API.
4. Sequencing API calls to vary the order in which the functionality is exercised and to make the API produce useful results from successive calls.