Working on APIs is becoming an increasingly important part of the industry, with quality being the top priority of leading organizations. Consequently, API testing is proving to be an integral part of API development, involving not just developers, but quality engineers as well.

These days, whether you build, test, or use everyday applications, you are probably using an API. And it’s not only customers - more organizations and businesses than ever rely on APIs for their products and workflows. APIs are the foundation of communication and data exchange - the interface - between our applications. If this communication breaks or doesn’t work as it should, that threatens and frustrates the whole chain of people and processes that go with the business of building an application. As evidenced in Postman’s State of the API 2021 report, both businesses and developers are focusing on APIs in order to speed up development, reduce failures, and ensure quality and reliability. Making sure your or any API you’re using works properly has become one of the most important parts of application development.

Development priorities for development teams and organizations Source: Postman's 2021 State of the API Report

Development priorities for development teams and organizations. Source: Postman's 2021 State of the API Report

This brings us to API testing. API testing is essentially that: testing the functionalities of an API. But because of API’s role, testing APIs often includes testing an app’s business logic and end-to-end testing.

Usually, when we think of software testing, we’re thinking of UI testing following the traditional waterfall approach - a QA engineer tests the whole application at the end of the development cycle. This can be a very time-consuming and costly process that cannot guarantee that all bugs have been found, fixed, and that quality has been assured - especially when it comes to the middle layer between UI and the database (in other words, the API).

In the age of agile and continuous development, testing early and finding bugs early speeds up the whole development process and produces more reliable code. The sooner the issues are detected, the less effort and cost it takes to repair them. As API can be tested even while UI is in development, many bugs that are usually detected on UI can be found and fixed early on, which ensures that further development runs more smoothly - and ultimately saves time.

Testing is an important part of API development Source: Postman's 2021 State of the API Report

Testing is an important part of API development. Source: Postman's 2021 State of the API Report

API testing is often considered to be the same as unit testing, and as such we imagine that it is the developers’ job to test them. But API testing can be more than that. While unit testing focuses on isolated units or modules, API testing is part of integration testing which validates the business logic of an application and includes multiple components between the UI and the database. As it is the QA engineers and testers who have to have a complete view of how the interface is going to be used, they are the ones who are increasingly performing API testing. Understanding customers’ needs and looking at the system as a whole ensures the quality of the final output of the system, including required business rules and a complete user experience.

“It’s not just developers who work with APIs”: Quality engineers are increasingly working with APIs Source: Postman's 2021 State of the API Report

“It’s not just developers who work with APIs”: Quality engineers are increasingly working with APIs. Source: Postman's 2021 State of the API Report

API testing can be performed both manually and automatically. As APIs lack a GUI, the main idea behind testing APIs is to ensure that the communication between different levels of an application works fine. This communication is established via API endpoints and requests or calls that return certain values. These elements are what testers ultimately focus on. Common API test cases include, among other things, checking API return values based on input conditions, verifying that API is returning any type of data, verifying whether the API triggers some event or calls another API, and verifying if the API is updating relevant data. Bugs that are usually detected during API testing include failure to detect errors, missing functionalities, reliability and performance issues, API response time, incorrect handling of valid argument values, and incorrect response data structure.

When testing APIs, a wide variety of testing practices is used Source: Postman's 2021 State of the API Report

When testing APIs, wide variety of testing practices is used. Source: Postman's 2021 State of the API Report

This blog post outlines what API testing is in general and how focusing on APIs can help improve the agility and reliability of development and ensure the quality of your final product. Stay tuned for the next blog post with an introduction to API testing using one of the industry’s leading tools, Postman.