API Testing Tutorial: A Complete Guide To Beginners

Testsigma Inc.
5 min readMay 4, 2023

Today, we live in a world where almost all programmers or software engineers use the API-centric Approach as it accelerates apps. The majority of apps we see today were built using the Agile methodology.

In Agile development, testing happens more often and continuously as features are added. Therefore, It is essential to test your API and do it correctly.

In this article, I will be walking you through API testing. This is a complete guide for beginners and anyone seeking in-depth knowledge on the best approach to API testing.

What is an API?

API is an acronym for Application Programming Interface. A computing interface allows different software or applications to communicate and share data. Simply put, API is a set of functions that enables the software to access data from other software or servers.

What is API Testing?

An API is an interface for accessing an application’s internal functions. Developers commonly use APIs to create apps dependent on another app’s internal functions. An API tester is responsible for ensuring that the API is reliable and can perform the functions it is supposed to at a set speed. When an API is tested, one of the most important aspects of development, it ensures that any bugs are caught and can be fixed before it goes live.

The primary goal of this testing is to ensure that programming interfaces are secure, fast, and dependable. API testing tools, such as Testsigma, automate API calls, get responses, and record them. API testing does not focus on the looks and feels of a program. Its focus is on the business logic layer of the software architecture.

What API testing entails

Below are key topics in API testing I will cover with you in this article.

  • Shift Left Testing
  • Types of API test
  • API Testing test cases
  • Methodology of Testing
  • Best API Testing Practices
  • Challenges faced in API testing
  • API testing Tools

Shift Left Testing:

Shift Left Testing has become one of API testing interviews’ most common quizzed topics. Shift-left testing is a method of testing in Software development in which testing is performed earlier in the development life cycle.

As stated earlier, Agile methodology includes testing as an integral part of the development cycle.

Before Shift Left testing was introduced, testing was performed in the latter part of the software development cycle. This approach was abolished as it caused a last-minute rush to meet deadlines and significantly decreased the quality of the final output.

Type of API Test

Various tests can be run to make sure the API performs as expected. They span general to specialized software analysis. Here are some common API test types

  1. Validation Testing: This test checks the final performance, behavior, and other functions of the API
  2. Functional Testing: This test examines specific methods inside the codebase to ensure that the API operates within the specified parameters and can handle errors when the results are outside those parameters.
  3. Load Testing: This test checks the ability of the API to withstand heavy loads and the number of calls it can handle.
  4. Security testing: This test ensures the API is safe against threats. It includes checking what authentication is required and whether sensitive data is encrypted over HTTP.
  5. Fuzz: This test is performed by injecting large volumes of random data, also known as noise or fuzz, into the API. This is to create negative behavior, such as a forced crash.
  6. Usability Testing: This test ensures that the end user can use all the API functionality and features without facing any challenges.

Methodology of Testing

In API testing, a lot of methods are used to assist in a better understanding of the testing approaches and features of the API. In this section, I will explain five critical methodologies to consider in writing test cases for your tests.

Before we go there, let’s see what test cases are.

Test case:

Test cases are predefined variables or functions written by testers to determine whether a particular API performs as expected or responds appropriately.

Before writing test cases, it is imperative to consider the following strategies:

  • Know the available endpoints for testing and their corresponding outputs. Endpoints define locations where APIs can access resources. It guarantees the kind of resources/outputs to expect in each test case.
  • Consider and briefly describe the input parameters for an endpoint.
  • Understand the functionality and scope of the API.
  • Test cases must be written using the best approaches. Such approaches include equivalence classes, boundary value analysis, and error guessing.
  • Finally, compare the expected outputs with the actual outputs after carrying out each test scenario.

Best API Test Practices

These are recommended practices for carrying out a good test on API.

  1. Group test cases into distinct and well-defined categories for easier management.
  2. Make use of Positive and Negative tests to validate inputs and desired outputs.
  3. Do not use test chaining in your testing process. Test Chaining is a technique whereby you programmatically make several API calls to carry out a specific function, with each request feeding subsequent requests.
  4. Know the API requirements, such as the workflow and its purpose. This will help in developing good test cases and proper validations.
  5. For more accurate, reliable outcomes, use both manual and automated testing.
  6. Create API test cases for all potential API input combinations to guarantee complete test coverage.
  7. Make sure all test cases are self-contained, if possible, and free from external dependencies.
  8. Parameters used in test cases should be stated and clearly defined.
  9. Use Load Testing to test stress on API before it goes to production.

Load testing involves constantly making multiple requests to cause the traffic you anticipate seeing in production to observe how the program responds before it goes live.

Challenges faced in API testing

These are the most common challenges in API testing:

  1. The lack of a graphical user interface (GUI) for testing the API makes it difficult to provide input data.
  2. Choosing the best Testing tool to aid testing and automation.
  3. API testing requires testers to have coding experience.

API Testing tools

There is a wide range of API testing tools available to choose from. Each of these tools has its unique features and functionalities. These are some popular tools mostly known;

  • Testsigma
  • Katalon Studio
  • SoapUI
  • Postman
  • Apigee
  • Tricentis
  • Assertible

API-centric applications have become the order of the day. This makes API most likely considered the engine of today’s application. It is there expected to perform without any flaws or bugs.

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Testsigma Inc.

Testsigma is a completely cloud-based codeless test automation tool that lets you create stable and reliable tests for web, mobile & APIs — all from one place.