An API (Application Programming Interface) is a collection of software functions and procedures, called API calls, that can be executed by other software applications. API testing is mostly used for the system which has collection of API that needs to be tested. The system could be system software, application software or libraries. Each API has its different functionality therefore it provides different results on different input. The errors or exception returns may also vary depending upon the API. However once integrated within a product, the common functionality covers a very minimal code path of the API and the functionality testing / integration testing may cover only those paths.
As companies extend software applications by opening up and sharing data via APIs this puts additional demands on software testing teams. This type of testing is technical and requires an understanding of protocols and data formats that go well beyond the usual GUI stuff you run.
API Testing is slightly differ from GUI Testing:
Testing APIs requires understanding of Inner workings: It may possible that some of API interconnected with other to share their functionality. So, there is requirement to understand the internal working properly.
Lack of Domain Knowledge: It is possible that tester may not well versed in using API then requirement of exploring the interface and their usage is needed. This problem can be solved out by involving the tester from starting of programming an application.
Sufficient Programming Skills: Generally, APIs Tests connected with sequence of calls called program that’s why programming knowledge is required for tester to understand each API functionality.
No Available Documentation: This is usually seen, there is no documentation available with API’s that create problem in understanding the purpose of calls, the parameter types and possible valid/invalid values and their return values. Hence, proper documentation will help test designer to test faster.
Access to Source code: The availablity of source code will help test designer in understanding and analysing the mechanism is followed. In the absence of source code, it is problematic for test designer to find out analmolies exist in code.
Time Constraints: It is really time consuming to test APIs thoroughly as there is requirement of resources to develop tools and design tests.
Finally it is concluded that API testing, like other activities owned by the QA team, must consider the ‘full’ functionality of the system, as it will be used by the end user (in this case, another program). This means that API tests must be far more extensive than unit tests, and take into consideration the sorts of ‘scenarios’ that the API will be used for, which typically involve interactions between several different modules within the application.