What is API testing?

Mini Vasudeva
5 min readApr 30, 2021


API testing is a type of software testing where application programming interfaces (APIs) are tested to determine if they meet expectations for functionality, reliability, performance, and security.

In simple terms, API testing is intended to reveal bugs, inconsistencies or deviations from the expected behavior of an API. Applications have three separate layers:

  • Presentation Layer or user interface
  • Business Layer or application user interface for business logic processing
  • Database Layer for modeling and manipulating data

API testing is performed at the most critical layer of software architecture, the Business Layer. It is in the business layer, business logic processing is carried out, and all transactions between User Interface(UI) and database happen. So, making sure that, API offers complete intended functionality allows for easy future expansion of the software product.

Why API testing is required?

These days, organizations are moving towards a microservices standard for their software for better quality. What does this mean for your organization? Different sections of their apps might have different datastores and controls to interact with these sections. Microservices allow your enterprise to deploy faster due to APIs. This is where API testing comes in. It’s more effective and has a faster turnaround because it’s more readily dependable than UI-based tests. UI tests tend to be slower and more expensive. But this testing brings forth a rapid turnaround.

Benefits of API testing

  • Language Independent: Data is exchanged via XML and JSON, so any language can be used for automation, independent from the languages used to develop the application.
  • GUI Independent: API testing can be done to check the core functionality of the application to expose small errors and to evaluate a build’s strength. Also, we can test the APIs without a user interface.
  • Faster Releases: The test cases that take about 8–10 hrs when executing UI regression testing take only about 1–2 hrs with API testing. This allows companies to release builds faster with API testing.
  • Improved Test Coverage: Most API’s have features, that allow a tester to create automated test with high test coverage including functional testing and non-functional testing. We can rarely find test cases that can’t be automated.
  • Reduce Testing Cost: With API testing we can find minor bugs before the GUI testing. Usually, these minor bugs will become bigger during GUI testing. So finding those bugs while performing API testing will be cost effective.

How to perform API testing?

API Specification Review:

The first step is documenting the API testing requirements. What is the purpose of the API? What is the workflow of the application? Which integrations are supported by the API? What are the features of the API? Documenting all these API testing requirements is the first thing you need to do. This will help you in planning API tests throughout the testing process.

Setting Up Test Environment

Next step is setting up a testing environment, with the required set of parameters around the API. This involves configuring the database and server for the application requirements.

Integrating Application Data

In this step, you need to combine your application data with the API tests to ensure that the API functions as expected against all possible input configurations.

Deciding Type of API Test

After you’ve created the testing boundaries and requirements, you need to decide what you want to test your API for. There are different types of API tests like functionality testing, validation testing, load testing, security testing, end-to-end testing, fuzz testing and many more.

Text Execution & Reporting

Once you have decided on what to test the API for, the next step obviously is to create test cases around that requirements and execute them.

API Testing Types

  • Functionality Testing — Functional testing is simply a test of specific functions within the codebase. These functions in turn represent specific scenarios to ensure that the API functions within expected parameters, and that errors are handled well when the results are outside of the expected parameters.
  • Reliability Testing — To check if API can be consistently connected to and lead to consistent result.
  • Validation Testing — Helps verify the aspects of product, behavior, and efficiency of an API
  • Load Testing — Load testing generally occurs after a specific unit or the whole codebase has been completed. This technique checks if the theoretical solutions work as planned. Load testing monitors the app’s performance at both normal and peak conditions.
  • UI Testing — It involves testing the user interface for the API and other integral parts
  • Security Testing — To test that the API is secure against all possible external threats; consists of authentication, permission, access control and validation of encryption methodologies.
  • Penetration Testing — To detect vulnerabilities of an application from an attackers perspective
  • Fuzz Testing — In fuzz testing, a vast amount of random data (referred to as “noise” or “fuzz”) will be input into the system to detect any forced crashes or negative behaviors. This technique tests the API’s limits to prepare for the “worst-case scenarios.”
  • Integration Testing — Ensure all the APIs connected to each other and communication properly by validating the flow of data and information between the API connection.

For successfully performing API testing, will need a tool to structure and manage your test cases. Here are some of the top API testing tools that can be used for Rest API and Soap API:

SoapUI: It is the most widely popular open-source tool for API testing. With SopaUI you can perform functional testing, performance testing, security testing, and data-driven testing. It will provide you with the reports for testing and will allow you to export the data.

REST-assured: It is an open-source Java Domain-specific language (DSL) tool that makes testing REST service simple. Supports XML and JSON requests. This tool simplifies things by eliminating the need to use boiler-plate code to test and validate complex responses.

Postman: With more than 4 million users worldwide it is one of the most popular API testing tools. It runs on Mac, Windows, Linux & Chrome Apps. Allows you to set up all the headers and cookies your API expects, and then check the response.

Katalon Studio: It is a robust and comprehensive automation tool for API, Web, and mobiletesting. Provides easy deployment by including all frameworks, ALM integrations, and plugins in one package. Also, supports both SOAP and REST requests, also supports various types of commands and parameterization functionalities.

Tricentis Tosca: It is a model based test API automation testing tool from Tricentis but also supports API testing. It supports a wide array of protocols including HTTP(s) JMS, SOAP, REST, IBM MQ, NET TCP, etc.

Apart from these popular tools, there are also tools like Apache JMeter, HttpMaster, Parasoft, appigee, Assertible , HP QTP, Karate DSL, and many others. Though these tools make API testing easy.

Originally published at https://www.numpyninja.com on April 30, 2021.