API testing is a type of software testing that focuses on verifying the functionality, reliability, performance, and security of Application Programming Interfaces (APIs). An API is basically a set of protocols and tools that allows different applications to communicate with each other.
API testing has been considered the future of software testing thanks to its advantages in the ability to test for core functionality, time effectiveness, language independence, and GUI integration. It becomes a must for software testing projects to ensure product quality.
In recent years, many testers will definitely need to face interview questions when applying for a QA job. The web API testing interview questions below have been collected from the test professionals to help you get ready for a new role. More than just an API interview, this list of content will also benefit both freshers and senior testers who would like to achieve both general and advanced knowledge in web API testing.
An API (Application Programming Interface) is a software intermediary that enables two applications to communicate with each other. It comprises a number of subroutine definitions, logs, and tools for creating application software.
In an API testing interview, you could be asked to give some API examples, here are the well-known ones: Google Maps API, Amazon Advertising API, Twitter API, YouTube API, etc.
Many APIs have a certain limit set up by the provider. Thus, try to estimate your usage and understand how that will impact the overall cost of the offering. Whether this will be a problem depends in large part on how data is leveraged. Getting caught by a quota and effectively cut off because of budget limitations will render the service (and any system or process depending on it) virtually useless.
This is one of the fundamental Web API interview questions. Bellows are four common Web API architectural styles:
Web API can be consumed by any clients which support HTTP verbs such as GET, PUT, DELETE, and POST. Since Web API services do not require configuration, they can be easily used by any client. In fact, even portable devices such as mobile devices can easily use Web API, which is undoubtedly the biggest advantage of this technology.
API testing is a kind of software testing that determines if the developed APIs meet expectations regarding the functionality, reliability, performance, and security of the application.
Learn More: An In-depth Guide To API Testing
In an API interview, they are likely to ask about the advantages of API testing. So be prepared with the significant ones such as:
Many protocols are now available to be used in API testing, such as JMS, REST, HTTP, UDDI and SOAP.
Setting up the API’s test environment is not an easy task, so you should have a ready answer if your API testing interview is coming. The test environment of API is a bit complete and requires the configuration of the database and server, depending on the software requirements. No GUI (Graphical User Interface) is available in this test form.
When the installation process is complete, API is verified for proper operation. Throughout the process, the API called from the original environment is set up with different parameters to study the test results.
The five most important principles of an API test design are:
While there are certainly specialty tests, and no list can be asked to be comprehensive in this realm, most tests fit broadly into the following nine categories that you should remember before attending an API testing interview.
During the API testing process, a request is raised to the API with the known data. This way you can analyze the validation response. While testing an API, you should consider:
The following factors should be considered when performing API testing:
There is myriad different API testing tools available. A few common tools are Katalon Studio, Postman, SoapUi Pro, Apigee, etc. While doing Unit and API testing, both target source code. If an API method uses code based in .NET then another supporting tool must have .NET.
API Testing | Unit Testing |
Conducted by QA Team | Conducted by the development team |
Aimed to assess the full functionality of the system for it will be employed by the end-user (external developers who will use your API) | Used to verify whether each unit in isolation performs as expected or not |
Often run after the build is ready and authors do not have access to the source code | Each of the code modules must be ensured to pass the unit test before being built by developers |
Read More: What is Unit Testing? A Comprehensive Guide
If you can overcome the challenges in API testing, you can be confident in the API testing interview too. They are:
One of the most common Web API testing interview questions is about the testing methods. They are:
API testing is now preferred over GUI testing and is considered as most suitable because:
In fact, according to the State of Quality Report 2024, API service is the second-most automated AUT, following web applications.
Not only API fundamental questions, the interviewer also determine your knowledge and experience by asking about the API errors in a Web API testing interview. So the most common ones are:
The API documentation is a complete, accurate technical writing giving instructions on how to effectively use and integrate with an API. It is a compact reference manual that has all the information needed to work with the API and helps you answer all the API testing questions with details on functions, classes, return types, arguments, and also examples and tutorials.
There are several available API documentation templates that help to make the entire process simple and straightforward, which could be answered in your API testing interview, such as:
APIs, especially modern RESTful APIs, is a nice creation that can certainly simplify and accelerate integration efforts, which makes it more likely you will benefit from them. But APIs can and do change for various reasons, sometimes abruptly, and hence REST APIs do not differ from traditional integration methods in this respect. If an API call is obsolete and disappears, your procedure will interrupt and it is important to understand how often the APIs you depend on change or are deprecated.
REST (Representational State Transfer) is an architectural style for developing web services that exploit the ubiquity of HTTP protocol and uses the HTTP method to define actions. It revolves around resources where every component is a resource that can be accessed through a shared interface using standard HTTP methods.
In REST architecture, a REST Server provides access to resources and REST client accesses and makes these resources available. Here, each resource is identified by URIs or global IDs, and REST uses multiple ways to represent a resource, such as text, JSON, and XML. XML and JSON are nowadays the most popular representations of resources.
Mostly, there are two kinds of Web Services that should be remembered in your next API testing interview:
REST architecture treats any content as a resource, which can be either text files, HTML pages, images, videos, or dynamic business information.
REST Server gives access to resources and modifies them, where each resource is identified by URIs/ global IDs.
REST uses different representations to define a resource like text, JSON, and XML.
XML and JSON are the most popular representations of resources.
RESTful web services use the HTTP protocol as a medium of communication between the client and the server.
Key characteristics of REST are likely asked in a Web API Testing interview. So please get the answer ready in your mind with these 2 ones:
RESTful web services use the HTTP protocol as a communication tool between the client and the server. The technique that when the client sends a message in the form of an HTTP Request, the server sends back the HTTP reply is called Messaging. These messages comprise message data and metadata, that is, information on the message itself.
An HTTP request contains five key elements:
The PUT or POST method should be used to create a resource. GET is only used to request data from a specified resource.
PUT and POST operations are quite similar, except for the terms of the result generated by them.
PUT operation is idempotent, so you can cache the response while the responses to POST operation are not cacheable, and if you retry the request N times, you will end up having N resources with N different URIs created on the server.
In a Web API Testing interview, you should give a specific example for PUT and POST operations to make it crystal clear to the interviewer. Below is an example:
Scenario: Let’s say we are designing a network application. Let’s list down a few URIs and their purpose to get to know when to use POST and when to use PUT operations.
The aforementioned API Testing interview questions are quite common in interviews for software QA engineers and testers positions. To best prepare for your upcoming interview, make sure to read thoroughly those questions, and try to understand the reasoning and knowledge behind them instead of simply memorizing.
For more information, you can have a look at the What is API Testing article we wrote. If needed, you can download our Katalon Studio for free and practice API Testing on the platform to get some hands-on experience. We also have a free course on API Testing with Katalon Studio if you need a more visual and auditory tutorial.
There are also plenty of API Testing tutorials on YouTube, a notable example is this Katalon Studio API Testing video by Automation Step By Step. In addition to all of this, you can better prepare for your interviews with these topic-specific lists of interview questions: