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Explain API testing with an example

API testing is a type of software testing that analyzes an application program interface (API) to verify it fulfils its expected functionality, security, performance and reliability. The tests are performed either directly on the API or as part of integration testing. An API is middleware code that enables two software programs to communicate with each other. The code also specifies the way an application requests services from the operating system (OS) or other applications.

Example: 

Travel booking systems:

For instance, take Kayak the biggest online site for booking tickets. You enter your destination, once you select dates and click on search, what you get back are the results from different airlines. How is Kayak communicating with all these airlines? There must be some ways that these airlines are actually exposing some level of information to Kayak. That’s all the talking, it’s through API’s.