Understanding Application Programming Interfaces(APIs)

Application Programmable Interfaces (APIs) are collections of software functions or procedures that can be used by other applications to fulfill their functionality. APIs provide an interface to the software component.These form the critical elements for the developing the applications and are used in varied applications from graph drawing packages, to speech engines, to web-based airline reservations systems, to computer security components.

Each API is supposed to behave the way it is coded i.e. it is functionality specific. These APIs may offer different results for different type of the input provided. The errors or the exceptions returned may also vary. However, once integrated within a product, the common functionality testing/integration testing may cover only those paths. By considering each API as a black box, a generalized approach of testing can be applied. But, there may be some paths which are not tested and lead to bugs in the application. Applications can be viewed and treated as APIs from a testing perspective.

The distinctive attributes that make testing of APIs slightly different from testing other common software interfaces like GUI testing.
- Testing APIs requires a thorough knowledge of its inner workings: Some APIs may interact with the operating system kernel, other APIs, with other software to offer their functionality. Thus, an understanding of the inner workings of the interface would help in analyzing the call sequences and detecting the failures caused.
- Adequate programming skills: API tests are in form of sequences of calls, namely, programs.Each tester must possess expertise in the programming language that are targeted by the API.
- Lack of domain knowledge: Involve the testers from the initial stage of development. This would help the testers to have some understanding on the interface and avoid exploring while testing.
- No documentation: Without the documentation, it is difficult for the test designer to understand the purpose of calls, the parameter types and possible valid/invalid values, their return values, the calls it makes to other functions and usage scenarios. Hence, proper documentation would help test designer design the tests faster.
- Access to source code: The availability of the source code would help tester to understand and analyze the implementation mechanism used; and can identify the loops or vulnerabilities that may cause errors.
- Time constraints: Thorough testing of APIs is time consuming, requires a learning overhead and resources to develop tools and design tests.