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What is API Testing in Software Testing? End To End Guide

By January 30, 2021September 20th, 2023No Comments6 min read
API Testing: What, How & Why?

Testing is one of the most vital phases for the development of any product or software and has been practiced in order to deliver better solutions. But every product might demand a different type of testing, for example, though functionality testing does the job for a website, security testing is still required to be done. With major developments in Software Development Life Cycle approaches, we have various types of testing that assure the best quality products through all endpoints and API testing is one such type.

Multiple reasons like Security, Quality, cost-effectiveness, fulfillment of customer’s requirement, understanding the performance, hitting a rock with unknown feature along with how testing helps in enhancing the while development process makes the testing key phase. 

What is API Testing?

A type of testing that involves point testing of Application Programming Interface and is responsible for regulating the requirements for multiple functions in software. Here, we don’t provide any user input but rather a set of calls hit the API and receive the response. Now, to simply explain to you what API is, it is a set of programming commands and tools that programmers can use to write code on multiple levels. Every time we do a simple product search on Amazon, we use APIs written by the developers.

So, API testing is to examine these APIs and improve them. Functions like Performance, Security, Reliability, etc. are properly tested. While most of the other testing consist of examining through User Interface, API testing is done on a different level where the design of the product is of no relevance and core business logic is inspected and improved. 

How Does an API Testing Work?

Now that we’ve learned what API testing is and how important is it, let us look into how it is done. Basically, API testing is planned after builds are ready and it is crucial you understand the functions of the API that you intend to test. A suitable test environment is established in accord with parameters, and databases and servers are made ready for proper tests.

Multiple questions are asked and answered as a process to have a strong argument through API testing, questions like Target Consumer for API, how the API works normally, Problems that might appear, what will decide the outcome of the test, etc. 

There is a number of tests that fall under API and testing these aspects assure the working of API. Tests like Functionality, Reliability, Load, Security, Proficiency, etc cover most parts of the software on the ground of API testing. One of the most interesting tests is Negative Testing, which is to predict what possibly could go wrong, set up a test, and make sure that even if it goes wrong as predicted, it won’t impact the functionality of the whole.  

Environment: Setting up the environment for testing can be complex as it does not involve any GUI, and all you do is play with parameters hit the database, and invoke APIs.

Test Cases: Multiple test cases are written like what would be the return value, what if there is no response or a null one, what if another API is triggered – whether relevant or not, etc.

Output: With different test cases, different out will be expected, and later it is important to compare the actual output with the expected output. 

Moving on, the errors that can be successfully detected in testing are security and performance issues, handling of unlike conditions, end-to-end working of a single functionality, structure of Response Data, ignored or absent functionalities, etc. These results assist in improving the API for further builds.

Learn Also: What are the Types of API Testing?

What Are the 4 Methods of API Testing?

There are several methods for conducting API testing, each serving specific purposes in evaluating API functionality. In this blog section, we will explore four common methods of API testing.

1. Unit Testing

Purpose: Unit testing focuses on testing individual components or functions of the API in isolation. It verifies that each unit of code performs as expected.

Implementation: Developers write unit tests to check the smallest building blocks of the API, such as functions or methods. These tests help catch bugs early in the development cycle.

Tools: Popular unit testing frameworks like JUnit (Java), NUnit (.NET), and pytest (Python) facilitate writing and running unit tests.

2. Integration Testing

Purpose: Integration testing ensures that different parts of the API work harmoniously together. It validates the interactions between various components, such as modules, services, or APIs.

Implementation: Test cases are designed to examine the interfaces where these components connect and exchange data. It helps uncover integration issues, such as data mismatches or communication problems.

Tools: Tools like Postman, SoapUI, and RestAssured are often used for conducting integration tests.

3. Functional Testing

Purpose: Functional testing evaluates the API’s functionality from an end-user perspective. It verifies that the API delivers the expected results for various inputs and conditions.

Implementation: Test cases are designed to simulate real-world usage scenarios, checking the API’s responses, error handling, and data validation.

Tools: Postman, Newman, and RestSharp are commonly used tools for functional API testing. Additionally, automation frameworks like Selenium can be integrated to test API functionality within a user interface.

4. Load Testing

Purpose: Load testing assesses an API’s performance and scalability under heavy loads. It helps identify bottlenecks and ensures the API can handle concurrent requests without degradation in response time.

Implementation: Test scenarios are created to simulate a large number of concurrent users or requests, pushing the API to its limits.

Tools: Tools such as Apache JMeter, Gatling, and are used to conduct load testing on APIs.

Why API Testing is Needed?

Basically, a simple reason for why API automation is required is to make sure the final product is of the best version, ensuring the API implemented functions as intended in multiple factors like security, performance, etc. Every software has a single-core API, so it is essential to make sure it’s working. APIs now being one of the key parts of an application, serving the business logic, makes API testing vital. On the other hand, most tests involve testing the GUI parts and do not bother with the functioning logic of the software.

One of the major aspects of API Testing is to test the capability of the API to perform under pressure. Here, what we refer to as Load Testing is somewhat similar to performance testing, where you have a number of test scripts, test cases, and multiple operations or functions performed, hitting the APIs and receiving response. Every hit to API and its response is recorded, along with time consumed, and performance is evaluated.

Furthermore, testing can be done in two ways like most other tests, which are Manual Testing and Automated Testing. Recent Developments in software enable you to set up a tool for automated testing, while on the contradict manual testing requires you to write test scripts and follow up on each step.


API automation testing is to make sure that the application functions the way it is made to, assurance of every function working properly. It certifies the product of its handling capabilities, security features, and compatibility across platforms. 

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