What is API Testing?

What is an API?

API is an acronym for Application Programming Interface. API is responsible for communication and data exchange between two separate software systems. A software system implementing an API contains set of routines, protocols and functions/sub-routines which can be executed by another software system.

What is API testing?

The API Testing is performed for the system, which has a collection of API that ought to be tested. During Testing, a test of following things is looked at.

  • Boundary conditions are exposed to ensure that the test harness varies parameters of the API calls in ways that verify functionality and expose failures.
  • Generating more value added parameter combinations to verify the calls with two or more parameters.
  • By considering the external environment conditions such as files, peripheral devices etc. behavior of the API is verified.
  • Sequence of API calls is verified and check is made if the API’s produce useful results from successive calls.

Why API testing is important?

API’s are real off-screen heroes because they are the ones who make our phone smart. More stress should be given on API testing because if an error goes undetected and some API breaks due to it then it could not only break a single application but a chain of business processes hinged to it.

API testing is one of the most challenging parts of the whole chain of software testing and QA testing because it works to assure that our digital lives run in an increasingly seamless and efficient manner. While developers tend to test only the functionalities they are working on, testers are in charge of testing both individual functionalities and a series or chain of functionalities, discovering how they work together from end to end.

Difference between API testing and GUI testing


There are some distinctive attributes that make API testing different from GUI testing.

  • Testing API requires deep knowledge of its inner working:

Some APIs may interact with the OS kernel, other APIs, with other software to offer their functionality. Thus if tester has through knowledge of internal working it would help him analyze the call sequence and detect cause of failure.

  • Adequate programming skills:

API tests are generally in the form of sequences of calls, namely, programs. So tester must have deep knowledge and understanding in the programming language(s) that are targeted by API. This would help the tester to review and scrutinize the interface under test when the source code is available.

  •  Lack of Domain knowledge:

If the tester is lacking the domain knowledge i.e. not trained in using the API then lot of time would be wasted in exploring the interfaces and their usage. This problem can be overcome by involving tester right from the start of development of application. This would help the testers to have some understanding on the interface and avoid exploring while testing.

  •  No documentation:

Most of the time it is seen that there is hardly any proper documentation available for the API developed. 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 having proper documentation would help test designer design the tests faster.

  • Access to source code:

With access to source code it will be easy to understand and analyze the method of implementation and mechanism used. It can also help in tracing the loopholes or vulnerabilities that may cause errors. Thus if the source code is not available then the tester does not have a chance to find anomalies that may exist in the code.

  • Time constraints:

Thorough testing of APIs is very tedious and time consuming process, requires a learning overhead and resources to develop tools and design tests. Keeping up with deadlines and ship dates may become a nightmare.

Advantages of API Testing:

Putting more effort into API testing leads to a much healthier final product. Ensuring that all data access (read and write) goes only through the API significantly simplifies security and compliance testing and thereby certification, since there is only one interface.

Ensuring that all the required business rules are being enforced at the API tier allows time for much more complete user-experience tests once the UI is released, and not having to concentrate on testing every single business rule and path through the application near the end of the project. Ensuring that the API offers complete functionality allows for easy future expansion of the application as new business needs arise.

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