API testing: a critical step in application development and maintenance

Posted: 2023-11-30
Written by: Magdalena Mazur

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.


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What are API tests?

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 methodDescriptionAPI action
POSTCreates new resourceCreate
GETGathers dataRead
PUT/PATCHUpdates existing resourceUpdate
DELETERemoves dataDelete

The response codes for the above methods are as follows.

Response statusResponse
Errors on client’s end4XX
Errors on server’s end5XX

Why is testing APIs so crucial when developing an application?

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.

Invoicing application developed using APIs – A practical example

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.

If you are seeking a technology partner who will take care of the quality of your application – plan the testing process, choose the type of tests, configure the framework, and prepare the test environment, you’ve come to the right place!

We have compiled a set of answers to the 10 questions most frequently asked by our customers about software testing. Should you not find the answers to the ones that concern you, get in touch and set up a free consultation with our software testing experts!

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Written by: Magdalena Mazur,
Software Tester

Functional, manual tester with experience in web, mobile and API application testing. Passionate about cooking and food. Loves traveling and cycling.

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