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API Testing

The Definitive API Testing Tutorial for Beginners

Learn how to perform API Testing and get to know the list of tools you will need by reading our API Testing Tutorial.

The Definitive API Testing Tutorial for Beginners - Blog
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API testing has become increasingly important as cloud apps and interconnectivity platforms have grown in popularity. In fact, many of the services we use on a daily basis rely on hundreds of interlinked APIs. So even if any one of the APIs fails, the service will cease to function. That is why it is extremely important to perform API testing without fail. If you are looking to learn what API testing is and how to do it, you’re at the right place. We will also be covering the different types of bugs and challenges of API testing in this API testing tutorial. But before that, let’s brush up on the basics by seeing what an API is and what it does.

What is an API?

API (Application Programming Interface) is a computing interface that allows two different software systems to communicate with each other and share data. Let’s take a look at a real-time example to better understand the functionality of an API. Let’s assume that you are in a restaurant and that you have decided what to order from their menu. Though you have made a choice, the food will reach you only if your order is received in the kitchen and that is made possible with the help of a waiter who takes your order. So in this scenario, your order is the request, the kitchen is the server, and the waiter is the API.

API Workflow

As shown in the figure, a Web API can also be defined as “a communication interface between a client computer and a webserver.”

Web Services vs Web API:

If you are aware of what Web Services are, you might feel that both Web Services and Web APIs serve the same purpose. Though Web Services (like Web API) are services that allow one computer to communicate with another. However, the main distinction between API and Web Services is that Web Services make use of a network. So let’s take a look at the key differences between the two

S. No Web Services Web API
1 Web Services Are Software Components That Convey Specific Data/Information, Send Or Post Data, Or Update Data Using Web Protocols Such As HTTP And XML APIs Are A Piece Of Software That Lets Two Separate Apps Or Machines Communicate With Each Other Without The Need For Human Intervention.
2 A Web Service Is A Type Of Network Communication Between Two Machines. Eg. Netflix A Web API Is An Interface Between Two Independent Applications.
3 Web Services Are Generally Developed In XML (Extensible Markup Language), Which Increases Their Security. Web APIs Frequently Employ JSON (JavaScript Object Notation), Which Makes Them Easier To Use.
4 Web Services Require A Network To Function. The Functioning Of A Web API Does Not Always Require A Network.
5 Web Service Is A Subset Of Web API. For Example, Web Services Are Based Only On Three Utilizations: SOAP, REST, And XML-RPC. Web APIs Are The More Advanced Version Or Superset Of Web Services. For Example, The Web API Supports All Three Kinds Of Web Services, But It Also Supports Alternative Styles Such As JSON – RPC.
6 In Most Cases, It Communicates Through The HTTP Protocol. It Also Communicates Through SOAP, REST, And XML-RPC. It Uses Any Communication Protocol Such As HTTP/HTTPS To Start The Interaction Between The Applications.
7 It Doesn’t Have A Comprehensive Set Of Requirements, And It Can’t Always Do All Of The Functions That A WEB API Can. An API Is A Collection Of Rules And Criteria That Are Followed To Make Interaction Easier.

What is API Testing?

Given the purpose, API testing can be termed as the process of evaluating a set of application programming interfaces (APIs) individually or as part of an integration test to see if they meet the expected functionality, reliability, performance, and security requirements.

What makes it different from other types of testing?

  • API testing is different from the regular type of testing as it is a black-box type of testing that doesn’t have a Graphical User Interface (GUI).
  • As there is no GUI, you will not employ the conventional input and output mechanisms such as keyboard, mouse, monitor, and so on for your API testing.
  • API testing would require you to use software to make the API calls, receive the output, and record the feedback as well.
  • Since there is no GUI, there is no focus on the look and feel of the app. Rather, it only focuses on the business logic layer of the software’s architecture.

How to do API Testing?

Now that we have seen how API testing differs from the other types of testing, let’s explore the different stages of API testing in this API testing tutorial.

Understanding API Requirements:

Understanding the requirements is the number one step in all types of testing. And when it comes to API testing, it would be helpful if the testers understand the application’s workflow and how it is expected to perform. This step helps the tester define the verification approach as API testing is all about the responses from the database.

Checking the output status:

There is a response code or the response status code in API testing for checking the output status. It enables the tester to know if a test case has passed or failed with ease. So here’s the list of the different types of response codes and their meanings.

  • 1xx (Informative): The request has been received and is being handled.
  • 2xx (Successful): The request was received, comprehended, and accepted successfully.
  • 3xx (Redirection): More action is required to complete the request.
  • 4xx (Client Error): The request has incorrect syntax or is unable to be fulfilled.
  • 5xx (Server Error): A seemingly valid request is not fulfilled by the server.

Organizing the API:

A testing project could contain dozens, if not hundreds, of APIs to be tested. For better test management, testers are strongly advised to classify them into categories. It is also critical to organize and size an API’s data, feedback, and goals in order to deliver an API that is easy to use and does not overload consumers.

Test Cases for API Testing

Positive test cases:

  • Check whether the API takes the provided inputs and provides the desired output as defined in the requirement.
  • Check the answer status code in the output status as mentioned in the previous part of this API testing tutorial.
  • Provide an input by filling just the bare minimum of the required fields and by also filling all the fields.

Negative test cases:

  • When the expected output does not exist, make sure the API responds appropriately.
  • Conduct an input validation test.
  • Use different levels of authorization to test the API’s behavior.

Automating the tool:

Using automation as much as feasible and as early as possible in the API testing process is very crucial as it can save a lot of time. It also makes it possible to save the test execution history. Since API testing is considered black-box testing, automation should be used extensively in order to maximize test coverage. It should also be conducted at an early stage of the SDLC.

API Testing Tools:

When running an API test, developers may either design their own framework or utilize one of several ready-to-use API testing solutions. API testing tools provide user-friendly interfaces with low coding requirements, making it possible for less-experienced engineers to deploy tests. So next up in our API testing tutorial, we’re going to take a look at the most useful API testing tools. Being a pioneer API testing Service provider, we always prefer to use open-source tools and have suggested the same here too.


SoapUI is one of the world’s most popular API testing tools that is designed to verify API functioning in both SOAP & REST APIs and web services as well. It is a great choice if you’re looking for an easy-to-use and fast tool to build & run automated functional, regression, and load tests.

Apache JMeter:

The Apache JMeter is a great open-source option that can be used to perform load testing for your API and see how well it performs at different load conditions. Though it was initially developed with the focus of testing web applications, its scope of usage has now widened.


Apigee is an API development and management platform that can be used as an API performance testing tool as well. It has an intuitive UI that works well for both technical and non-technical users. But what makes Apigee great is that it provides security, rate limiting, analytics, and so on by fronting the backend service with a proxy layer.

REST Assured:

As the name suggests, REST Assured is an API Testing tool that can be employed to test REST APIs. Like Apache, REST Assured is also a Java-specific tool and its biggest advantage is that it supports Behavior-Driven Development (BDD) syntaxes.

Swagger UI:

Swagger UI is also an open-source program that allows users to see and interact with the API’s resources without requiring any implementation logic. It simplifies the testing process by generating a web page that lists the APIs that have been used.


It is an end-to-end solution that enables users to perform API testing , verification, and automation. You can quickly explore, debug, & test your APIs and even design complicated API requests in HTTP, REST, SOAP, GraphQL, & WebSockets.


Katalon is a great choice if you are looking to improve your test coverage. It is also highly dependable as it follows the Data-Driven test approach. Despite being an open-source tool, it can be used in projects of all sizes all thanks to its productivity-focused features and user-friendly UI.

Types of Bugs in API Testing

It goes without saying that any type of testing would reveal the numerous bugs in a product. So we are now going to see the most commonly found bugs in our API testing tutorial to help you get an idea of what to expect when performing those steps.

  • Fails to effectively handle the incorrect conditions.
  • Overlooking the flags that have never been used before.
  • Duplicated or missing functionality.
  • It faces issues with Reliability Connecting to API and receiving a response is difficult.
  • Security Concerns
  • Bugs that arise due to multithreading.
  • Performance issues such as API response time.
  • Incorrect errors/warnings being sent out.
  • Inability to handle valid argument values.
  • The data in the response isn’t organised properly (JSON or XML).

Challenges of API Testing

Like every other type of software testing, API testing also has its own share of challenges.

1. When it comes to Web API testing, the 3 most common issues are parameter combinations, parameter selection, and call sequencing.

2. Since it is a black-box type of testing, the lack of a GUI makes it impossible to provide input values during the testing process.

3. API testing is also a little difficult for testers to validate and verify the results in a separate system.

4. The necessity to test the exception handling function is also a challenge.


Since the business logic layer is represented by a set of classes, functions, and methods in an API, it goes without saying that API testing plays a very crucial part in ensuring a product’s success. And you will need to use the correct testing methodology and tools to ensure the best results. We hope this API testing tutorial has provided you with a systematic approach and a lot of useful insights about API testing as well. Being a top Quality assurance company, we will be posting more insightful content in the future and highly recommend you to subscribe to our newsletter to make the most of it.


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