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You rent cars, send money, and book overnight stays all the time. Did you know mysterious APIs are behind all these transactions? Without testing the API in every possible way, there’s a high chance that your application won’t allow you to perform specific operations. In this article, you will learn what API testing is about and why this step is crucial for future customers and users when developing a system or application.
API (Application Programming Interface) is an interface that allows communication between applications or programs. An API contains a specification that describes the rules and requirements that both parties must meet for communication to be possible and for them to exchange data.
Suppose you embark on a dream vacation and want to rent a car. Those interested in a particular supplier may check their website for prices and availability. A site that gathers offers from several providers is usually a better choice since competition never sleeps. Although such a search engine does not have access to the database of individual car suppliers, it can use each supplier’s API to find the best deals. Testing the API is essential before we can be certain everything works as expected.
API testing is a type of software testing that verifies application programming interfaces. The API is verified for functionality, reliability, performance, and security. GUI (graphical user interface) or application appearance are not focus areas. The most critical thing is to analyze input values and return data.
API testing is carried out using specialized software. As a result, we can send queries (requests) to the appropriate endpoints (links to where API services are located) and then read and interpret the system’s response (response).
Getting a full understanding of how an API works requires knowing what response codes (HTTP codes) are expected for each request, what codes we can expect if a request fails, and what messages we should see in response to a failed request.
For the REST API, requests use standard HTTP methods: GET, POST, PUT/PATCH, and DELETE. These methods enable webpages to work, while in the case of the API, they are used to perform basic operations on resources. These operations are CREATE, READ, UPDATE, DELETE, or CRUD.
| HTTP method | Description | API action |
| POST | Creates new resource | Create |
| GET | Gathers data | Read |
| PUT/PATCH | Updates existing resource | Update |
| DELETE | Removes data | Delete |
The response codes for the above methods are as follows.
| Response status | Response |
| Information | 1XX |
| Success | 2XX |
| Redirections | 3XX |
| Errors on client’s end | 4XX |
| Errors on server’s end | 5XX |
API testing can be done at an early stage of software development so that conceivable bugs can be detected before they hit production. When the GUI user interface testing occurs, this will save time. The API should, naturally, work according to the documentation, which describes the available endpoints, parameters, HTTP methods, etc. API tests can confirm that the API works according to this documentation.
API tests can be automated to be easily repeated after any source code change. Examples of tools for API testing include Postman, SoapUI, Swagger, and JMeter.
Wrapping things up, the API testing process is essential to creating and developing software, ensuring that programming interfaces are functional and that applications are of the highest standard.
An example of an API-based application that uses various data sources is Carnect, a leading global specialist in car rental distribution. Our team was tasked with streamlining the client’s invoicing process and creating a solution that merged their two systems, revolutionizing this problematic process to free up time and resources for more engaging and impactful tasks for business expansion. Testing was integral to the project – we verified the functional requirements, so we know that the most critical features operate as expected.
