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What is API testing?

Today let’s talk about what is API testing, and what exactly do we test when we perform API testing?
To start from the very beginning lets understand what API is.

 What Is API?

API – Application Programming Interface.
This is a functional piece of a system that sits usually between a data source and a front end presentation to the user. In other words if we take a simple example of a web application the nice looking web site will be using API calls to retrieve data from the back-end database.
It is important to understand that API can mean different things in different systems, for example for web application an API can be a SOAP or a REST web service, but for a desktop application API can be a set of known functions that allow retrieve or modify data within that application.

 What Do We Test With API?

When we talk about API testing we talk about testing some specific functionality that is described in the requirements for that API, and our verification points might change drastically based on what required, but we can group our verifications for all APIs into 3 main categories:

  •         Verify that the expected data is received in response
  •         Verify that the expected action was performed
  •         Verify that additional APIs functions were triggered (nothing was performed yet but a job might have started to execute action in a future)
At this point we usually differentiate between a specific API testing and an End-To-End API testing.  
During the specific API testing we make sure that the API can be called in every possible way and that the proper results are being received or if not then a proper error is returned. For example if we have an API that return weather information for a specific city we will try if it can give information for several cities at once, no city at all, and a city that does not exist in addition to a proper city name.
During the End-to-End API testing we verify that a proper series of calls is trigged by calling some entry point API, so for example if we have online registration form submitted that should generate a temporary password and email that to customer, we will call a form submission API, that in turn expected to call a password generation API and both results will be passed over to an email generating API  

 What Tools We Use?

There is a lot of popular tools that are being used to test different types of APIs but this is beyond the scope of this article. What is important to mention at this point is the fact that we cannot test APIs without a proper tools or an environment setup. In difference to a GUI testing this type of testing requires a setup of a tool and an environment to access the component that is under test as direct access without special coding and tools is usually not possible


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