Learn API Testing - Complete Tutorial with Video

API Testing Tutorial for Complete Beginners

API testing tutorial for beginniers

API (Application Programming Interface) testing is a crucial skill for software professionals. Whether you’re new to testing or an experienced pro, this blog is your go-to resource.

We’ve distilled valuable insights from The Test Tribe’s 4th Virtual Meetup on API Testing with Pricilia into an easy-to-understand blog guide. For the visual learner’s out there, you can watch the complete video below.

Pricilla’s Workshop on API Testing And API Basics from TTT Virtual Meetup

What is an API?

API is an acronym, and it stands for Application Programming Interface. API is a set of routines, protocols, and tools for building Software Applications. APIs specify how one software program should interact with other software programs. Normally, API facilitates the reusability.

For example:

If a user needs to book a room in Hyatt Regency. The user can directly do it on Hyatt Regency website, or through travel booking websites like MakeMyTrip, Trivago, etc.. So, here the Hyatt Regency develops an API and provides specific(read/write) access to the travel agencies via which users can view/book their hotels.

Common Types of API

Types of API
Types of API

Various types of APIs serve distinct purposes, each with its own advantages and drawbacks. The most prevalent API categories include:

Open API (Public API):

These APIs are accessible to all developers and users. They typically have minimal authentication and authorization measures and may limit the data they provide. Some open APIs are free, while others require a subscription fee based on usage.

Private API (Internal API):

Intended solely for internal use within an organization, private APIs remain inaccessible to the general public. They often employ stricter authentication and authorization protocols, granting access to an organization’s internal data and systems for its employees or trusted partners.

Partner API:

Partner APIs are shared exclusively between strategic business partners. These APIs are not open to the general public and require specific permissions for access. They facilitate business-to-business activities, often involving the exchange of sensitive data, and typically employ robust authentication, authorization, and security measures.

Understanding the Client-Server Architecture

three tier architecture
Three-Tier Architecture

Client-server architecture in API testing, within the context of a three-tier architecture, involves the interaction between different components for efficient and organized testing. Here’s an overview of this concept:

1. Presentation Tier (Client):

  • In API testing, the client or presentation tier represents the front-end or user interface through which users interact with an application.
  • Testers may simulate user actions and interactions with the client interface, such as making HTTP requests to the API endpoints.
  • The focus is on ensuring that the client can effectively communicate with the API and process the responses.

2. Application Tier (Server):

  • In the context of API testing, the server or application tier is where the API resides.
  • This tier handles incoming requests from clients, processes them, and provides responses.
  • Testers conduct various API tests here, including functional testing to validate the API’s behavior, performance testing to assess its responsiveness under load, and security testing to identify vulnerabilities.

3. Data Tier (Database):

  • In a three-tier architecture, the data tier, or database, stores and manages the application’s data.
  • While API testing primarily focuses on the interaction between the client and server, it’s important to verify that the API correctly accesses and manipulates data in the database.
  • Testers may perform database-related tests, such as data integrity checks and data consistency validation.

What is API Testing?

API testing is a critical process in software testing that focuses on evaluating the functionality, performance, and reliability of an Application Programming Interface (API). It involves testing the API’s endpoints, request-response mechanisms, and data exchanges.

Steps Involved: Process Used in API Testing

api testing process
API Testing Process

Here’s a detailed explanation of the API testing process outlined:

1. Review and Understanding of API Specifications

Begin by thoroughly reviewing the API documentation and specifications. This step ensures that testers have a clear understanding of what the API is designed to do, its endpoints, input parameters, and expected output.

2. Categorize Entities Based on Flow

Categorize the various entities, such as endpoints, methods, and data flows, based on the API’s functionality. This categorization helps in organizing test scenarios effectively.

3. Define the Parameters

Identify and define the parameters required for each API endpoint. Parameters include inputs, headers, query parameters, and authentication details. Ensure that you understand the purpose of each parameter.

4. Learn How to Send Requests for Different Endpoints

Familiarize yourself with the tools and methods for sending requests to the API endpoints. This may involve using API testing tools, command-line tools, or scripting in a programming language.

5. Frame the Test Cases

Create comprehensive test cases for each API endpoint. Test cases should cover various scenarios, including valid and invalid inputs, boundary cases, and edge cases. Define the expected outcomes for each test case.

6. Add Assertions on the Expected Results

Define assertions to validate the API responses. Assertions are criteria that must be met for a test case to pass. They can include checking response status codes, data integrity, and expected values.

7. Test Execution

Execute the test cases against the API endpoints. Ensure that you follow a systematic approach, covering all defined scenarios. This phase involves sending requests, receiving responses, and comparing the actual outcomes to the expected results.

8. Report the Failure

If a test case fails, document the failure with as much detail as possible. Include information about the test environment, input data, and any error messages or unexpected behavior encountered during testing.

Why is API Testing Done?

Why we do API testing?
Why We Do API Testing

In the ever-evolving realm of software development, ensuring the reliability and efficiency of applications has never been more crucial. This is where API testing steps into the spotlight as a game-changer. In this, we explore the compelling reasons why API testing should be an integral part of your testing strategy.

1. Time Efficiency

First and foremost, API testing is a time-saver. Traditional testing methods often involve testing the entire application, which can be time-consuming, especially in complex systems. API testing, on the other hand, allows testers to focus on specific functionalities or endpoints. This targeted approach significantly reduces testing time, allowing for quicker development cycles and faster time-to-market.

2. Cost Reduction

In the world of software development, time is money. By accelerating the testing process and streamlining it with API testing, you’re effectively reducing testing costs. With fewer resources required for testing, your organization can allocate resources more efficiently and effectively, ultimately saving valuable budgetary resources.

3. Language Independence

API testing breaks down language barriers. Unlike some testing methods that depend on specific programming languages or technologies, API testing is language-independent. This means you can test APIs built with different technologies or languages without the need for a deep understanding of each language. This flexibility is a significant advantage in today’s multilingual software landscape.

4. Core Functionality Testing

At the heart of every software application lies its core functionality. API testing specializes in scrutinizing this essential aspect. It allows you to dive deep into the core of your application, testing how its various components interact and ensuring that they perform as expected. This pinpoint accuracy in testing core functions enhances the overall quality of your software.

5. Risk Mitigation

Software development inherently carries risks. API testing acts as a risk mitigation tool. By thoroughly testing APIs before integrating them into the application, you can identify and address potential issues and vulnerabilities early in the development cycle. This proactive approach reduces the likelihood of critical failures in the production environment, ultimately safeguarding your system’s integrity.

Types of API Testing

Types of API Testing
Types of API Testing

APIs (Application Programming Interfaces) are the backbone of modern software, enabling seamless communication between applications and services. To ensure that APIs perform flawlessly and securely, API testing comes into play. Let’s take a closer look at the various types of API testing and their distinct roles in the software testing ecosystem.

1. Validational API Testing

Validational API testing, also known as Schema Testing, focuses on verifying that the API responses adhere to the expected data format and structure. This type of testing ensures that the data exchanged between the API and the application is correctly formatted, preventing potential data-related issues.

2. Functional API Testing

Functional API testing is all about functionality. It verifies whether the API functions as intended by testing its various endpoints and methods. Testers create test cases to assess the API’s behavior, input validation, and output correctness. This type of testing is critical for confirming that the API delivers the expected results under various scenarios.

3. Security API Testing

In an age where cybersecurity is paramount, Security API Testing is indispensable. It involves scrutinizing the API for vulnerabilities and security flaws. This testing type assesses the API’s ability to protect sensitive data, prevent unauthorized access, and resist common security threats like SQL injection or cross-site scripting (XSS) attacks.

4. Load API Testing

Load API testing assesses how well the API performs under different levels of load and stress. It helps determine the API’s capacity to handle concurrent requests and large volumes of data. By simulating heavy loads, testers can identify performance bottlenecks and ensure the API remains responsive and reliable in real-world scenarios.

5. Integration API Testing

Integration API testing evaluates how well the API interacts with other systems and services within an application’s ecosystem. It ensures seamless communication between various components, detecting integration issues that could disrupt the overall functionality of the application.

6. Documentation API Testing

API documentation is the user manual for developers and users who interact with your API. Documentation API testing validates that the documentation accurately represents the API’s behavior. It confirms that developers can rely on the documentation to understand how to use the API effectively.

Common Types of API Protocols

API protocols form the foundation of how data is exchanged and communicated between software systems. In the world of web services and APIs, two prominent protocols stand out: SOAP (Simple Object Access Protocol) and REST (Representational State Transfer). Let’s delve into each of these protocols to understand their key characteristics and use cases:

SOAP (Simple Object Access Protocol):

  • SOAP is a protocol for exchanging structured information in web services using XML.
  • It relies on a predefined set of rules for message formatting and communication.
  • Known for its strict standards and support for complex operations, it’s commonly used in enterprise-level applications.

REST (Representational State Transfer):

  • REST is an architectural style for designing networked applications.
  • It uses standard HTTP methods (GET, POST, PUT, DELETE) and operates on resources represented as URLs.
  • Known for its simplicity and flexibility, it’s widely used for web APIs, including those serving web and mobile applications.

HTTPS Methods You Need to Know About

response codes
Response Codes

When it comes to interacting with web APIs, understanding the core HTTP methods is crucial. Let’s dive into the essential HTTP methods you need to know for seamless communication with APIs:


  • This method is all about retrieval. It requests data from a specified resource.
  • It’s commonly used for fetching information from a server without making any changes to the resource.


  • POST is all about submission. It sends data to be processed to a specified resource.
  • This method is often used when you need to create a new resource or submit form data to a server.


  • PUT is for updating. It sends data to a specific resource to replace or update it.
  • Use PUT when you want to modify an existing resource entirely, making it a powerful tool for updates.


  • DELETE is the method for, well, deleting a specified resource.
  • It’s used to remove a resource from the server, providing an important way to manage data.

Response Codes in API Testing

In the world of API testing, understanding response codes is akin to reading the language of the digital realm. Here’s a concise guide to the response codes you’ll frequently encounter:

1XX – Informational:

  • These codes provide information about the ongoing request’s status.
  • Typically, they signal that the request is received and understood, but further action may be required.

2XX – Successful:

  • The coveted 2XX codes signify successful request processing.
  • A 200 OK, for instance, indicates that the request was processed without issues, delivering the expected results.

3XX – Redirection:

  • These codes indicate that the client must take additional steps to complete the request.
  • Commonly seen is the 301 Moved Permanently, which redirects the client to a new URL.

4XX – Client Errors:

  • When something goes amiss on the client side, these codes come into play.
  • A 404 Not Found, for instance, means the requested resource couldn’t be located on the server.

5XX – Server Errors:

  • Server errors signal that something has gone awry on the server’s end.
  • A 500 Internal Server Error is a catch-all for various server-related issues.

Best Practices to Follow When Testing APIs

API testing is a vital component of software quality assurance. To ensure robust and reliable APIs, it’s crucial to follow best practices. Here, we explore six key practices:

  • Call Sequencing: After standalone validation, consider the sequencing of API calls. Ensure that APIs interact seamlessly, maintaining data integrity and functionality.
  • Parameterization: Implement parameterization to test various inputs and scenarios, uncovering potential issues and ensuring your API can handle diverse data.
  • Delete Operation Handling: Pay special attention to how your API handles delete operations. Ensure it behaves as expected, and data deletion is secure and controlled.
  • Scenario-Based Grouping: Organize your API requests based on scenarios. This makes testing more systematic, helping you identify and address specific use-case issues.
  • Automation: Whenever possible, automate your API tests. Automation streamlines testing, detects issues early, and accelerates the testing process.
  • CI/CD Integration: Integrate API testing into your CI/CD pipeline. This ensures continuous testing, reducing the likelihood of bugs slipping through to production.

Real Time Challenges in API Testing

API testing brings its own set of real-time challenges. Here’s a quick overview of these hurdles and how to tackle them:

  • Initial Setup: Setting up API testing environments can be complex. Streamline this by using containerization tools like Docker for consistent setups.
  • Documentation: Inadequate or unclear API documentation can slow testing. Collaborate closely with developers to ensure comprehensive documentation.
  • Without User Interface: APIs lack user interfaces, making testing less intuitive. Leverage API testing tools and scripts to interact with APIs directly.
  • Tool Selection: Choosing the right testing tools is critical. Assess your project’s needs and opt for tools that align with your testing objectives.
  • Error Handling: Robust error handling is essential. Test various error scenarios to ensure your API gracefully handles unexpected situations.
  • Drafting Scenarios: Creating effective test scenarios requires careful planning. Understand the API’s functionality and potential use cases to draft meaningful scenarios.
  • Coding Skills: Some testing tools may require coding skills. Invest in training or consider user-friendly tools to accommodate testers with various skill levels.


So, we’ve delved into the essence of API testing, equipping you to elevate software quality. APIs are the linchpin of modern software, connecting applications and services. Understanding the types of APIs, such as open, private, and partner APIs, empowers you to harness their full potential.

You’ve gained insights into SOAP and REST protocols, HTTP methods, and response codes, essential for effective API communication. Follow best practices like call sequencing, parameterization, and automation to streamline your testing process.

While real-time challenges exist, from initial setup to handling APIs without user interfaces, your newfound knowledge ensures you’re ready to conquer them. In a rapidly evolving tech landscape, mastering API testing is your ticket to software excellence.

What to Learn Next?

The next part of this blog (and the workshop had a live demonstration) which you can find in the Postman tutorial blog. Here in you will learn how to do API testing using Postman.

Check related learning courses:

We recreated this blog using the content from Pricilla Bilavendran‘s API Testing Workshop at The Test Tribe’s 4th Virtual meetup. Pricilla is a Postman Supernova, and a long time contributor to the testing community. She is also an instructor to the Postman course that helps you learn API Testing on our learning platform Thrive EdSchool. If you are looking for a solid resource to master the Postman tool for API testing, we urge you to check it out. You can connect her on the social-media by following the links below.

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