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API Testing: Types and Tools

What is API?

API (Application Programming Interface) is a computing interface which enables communication and data exchange between two separate software systems. Software system that executes an API includes several functions/subroutines that another software system can perform. API defines requests that can be made, how to make requests, data formats that can be used, etc. between two software systems.

What is API Testing?

API Testing is a software testing type that validates Application Programming Interfaces (APIs). The purpose of API Testing is to check the functionality, reliability, performance, and security of the programming interfaces. In API Testing, instead of using standard user inputs(keyboard) and outputs, you use software to send calls to the API, get output, and note down the system’s response. API tests are very different from GUI Tests and won’t concentrate on the look and feel of an application. It mainly concentrates on the business logic layer of the software architecture.

Benefits of API Testing


Data is exchanged via XML and JSON formats, so any language can be used for test automation. XML and JSON are typically structured data, making the verification fast and stable. There are also built-in libraries to support comparing data using these data formats.


API testing can be performed in the app prior to GUI testing. Early testing means early feedback and better team productivity. The app's core functionalities can be tested to expose small errors and to evaluate the build's strengths.

Improved test coverage

Most API/web services have specifications, allowing you to create automated tests with high coverage — including functional testing and non-functional testing.

Faster releases

It is common that executing API testing saves up to eight hours compared to UI testing, allowing software development teams to release products faster.

API Testing Types:

1. Validation Testing

Validation testing occurs among the final steps and plays an essential role in the development process. It verifies the aspects of product, behavior, and efficiency. In other words, validation testing can be seen as an assurance of the correct development.

2. Functional testing

Includes testing particular functions in the codebase. These features are the representation of specific scenarios to make sure the API functions are handled well within the planned parameters.

3. UI testing

UI testing is defined as a test of the user interface for the API and other integral parts. UI testing focuses more on the interface which ties into the API rather than the API testing itself. Although UI testing is not a specific test of API in terms of codebase, this technique still provides an overview of the health, usability, and efficiency of the app’s front and back ends.

4. Security testing

This practice ensures the API implementation is secure from external threats. Security testing also includes additional steps such as validation of encryption methodologies, and of the design of the API access control. It also includes user rights management and authorization validation.

5. 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.

6. Runtime and error detection

This testing type is related to the actual running of the API — particularly with the universal results of utilizing the API codebase. This technique focuses on one of the below aspects: monitoring, execution errors, resource leaks, or error detection.

7. Penetration testing

Penetration testing is considered the second test in the auditing process. In this type, users with limited API knowledge will try to assess the threat vector from an outside perspective, which is about functions, resources, processes, or aim to the entire API and its components.

8. Fuzz testing

Fuzz testing is another step in the security audit process. 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."

API Testing Tools:

1. Katalon Studio

Recognized as the Gartner Peer Insights Customers’ Choices for Software Test Automation for three consecutive years, Katalon Studio is the leading test automation solution for API, Web, and Mobile and Windows applications.

Feature highlights:

  • Easy-to-use UI and productivity-centric features for projects of all sizes

  • Supports REST, SOAP requests, and SSL client certificates

  • Enable test import from Swagger (2.0 & 3.0), Postman, WSDL, and WADL

  • Native CI/CD integrations (Jenkins, Azure DevOps, CircleCI, Dockers, etc.)

  • Data-driven testing methods better test coverage and reliability

  • AssertJ support to create fluent assertions in BDD style

  • Support API test data setup using UI testing

  • Built-in reporting platform to centralize reports and activities across tool stacks: version control systems, CI/CD, test automation tools, and ALMs

  • Free API testing courses and tool tutorials on Katalon Academy

2. Postman

Originally a Chrome browser plugin, Postman extended to an on-premise solution for both Mac and Windows.

Feature highlights:

  • Easy-to-use REST client

  • Offer rich interface

  • Available for both automated and exploratory testing

  • Able to run on Mac, Linux, Windows

  • Provide many integrations like support for Swagger & RAML formats

  • Run, test, document, and monitoring features

Users could share the knowledge with the team at ease as they can package up all the requests and expected responses, then send them to their coworkers.

3. Apigee

Apigee is a cross-cloud API testing tool, enabling users to measure and test API performance, support and build API. Apigee also provides PCI, HIPAA, SOC2, and PII for apps. This tool has been named one of the leaders in the 2019 Gartner Magic Quadrant for Full Lifecycle API Management four times in a row.

Feature highlights:

  • Allows design to monitor, implement and extend API

  • It is multi-step and powered by Javascript

  • Define performance issues by tracking API traffic, error rate, and response time

  • Easily create an API proxy based on open API specifications and deploy it in the cloud

This tool is designed for digital businesses and the data-rich mobile-driven APIs and apps that power it. Since version 4.19.01 in 2019, Apigee gives users more flexibility to manage their APIs with features (e.g., Open API 3.0 support, TLS security, Self-healing with apigee-monit, Virtual host management improvements). Now, the latest version is enhanced with minor bug fixes and security improvements.

4. JMeter

Initially, JMeter was created for performance testing, however, it is commonly used for functional API testing.

Feature highlights:

  • Cache and offline replay of test results

  • Automatically work with CSV files, thus enabling the team to create unique parameter values for the API tests at speed

  • Able to include the API tests in CI pipeline thanks to JMeter and Jenkins integrations

  • Available for both static and dynamic resources performance testing

The latest release JMeter 5.4.1 is packed with versatile features and enhancements such as new themes, bug fixes, visual representation of disabled elements, revamped Groovy library, and updated JMeter templates for functional testing. This tool is most suitable for Web application performance and load testing.

5. Rest-assured

This open-source Java Domain-specific language enables testing REST services more simply. It can be used to validate and verify the response of these requests.

Feature highlights:

  • Offer a bunch of baked-in functionalities to help users proceed with the codeless practice

  • Integrates seamlessly with Serenity automation framework, thus users can combine the UI and REST tests in one framework that extracts excellent reports

  • Support BDD Given/When/Then syntax

  • Supports POST, GET, PUT, DELETE, OPTIONS, PATCH and HEAD requests

  • Users do not need to be an HTTP expert to utilize this tool

Version 4.0.0 required at least Java 8, offered support for Apache Johnzon, and fixed many issues with the initial OSGi support. The latest version 4.4.0 is released with various improvements and bug fixes.

6. Assertible

Assertible is an API testing tool that focuses on automation and reliability.

Feature highlights:

  • Support automating API tests throughout CI/CD pipeline

  • Assist in running API tests after deployments and integrates with familiar tools (e.g., GitHub, Slack, and Zapier)

  • Support validating HTTP responses with turn-key assertions

  • Help testers update their tests when their specifications change by The Sync feature. Hence it’s needless to manually update tests after adding new parameters or changing the response of the API

In October 2019, Assertible added the Encrypted variables feature, which provides a new way to store passwords, tokens, and secret data fields. This offering is to improve API testing security practices.

7. Soap UI

This headless functional testing tool is dedicated to API testing. It allows users to test REST and SOAP APIs and Web Services with no hassles.

For the Free package, users can:

  • Acquire the full source code and build their preferred features at hand

  • Create test effortlessly with Drag and drop, Point-and-click

  • Reuse load tests and security scans for functional test cases in just several steps with Reusability of Scripts feature

For the Pro package, users can:

  • Powerful data-driven testing – which means users can simulate how consumers interact with the APIs thanks to data loaded from files, databases, and Excel

  • Support native CI/CD integrations and asynchronous testing

The latest version SoapUI 5.6 has updated third-party libraries (org.apache.HTTP components, commons-logging, commons-codec, and JUnit) and removed unused third-party libraries such as Jackson, KeenIO.

8. Karate DSL

This new API testing tool assists users in creating scenarios for API-based BDD tests simply without writing definition steps. Indeed, KarateDSL creates those definitions itself, thus users can quickly kickstart the API testing.

Feature highlights:

  • Build on top of Cucumber-JVM

  • Run a test and generate reports likewise to any standard Java project

  • Supports configuration switching/staging, multithreaded parallel execution

9. Rest Console

This tool is a REST/HTTP Client for Google Chrome that enables users to visualize and construct custom HTTP requests to test with any RESTful API service.

Feature highlights:

  • Construct POST or PUT body via raw input

  • Support to modify custom headers through intuitive UI

  • Assist users in creating query parameters at ease

  • Able to be used for several authentications like Plain, Basic, OAuth

  • Supports customizable interface

The most recent release, Rest Console v4.0.2 has been upgraded with several features to enhance user experience and bug fixes (e.g., oAuth improvements, Collapsible sections, Clickable Links in Response, UI enhancements).

10. API Fortress

Testers and developers can create and automate functional tests since API Fortress is a continuous platform for API testing.

Feature highlights:

  • Easy-to-use intuitive UI for any skill level

  • Plug-and-play stimulates your continuous API Testing

  • Support Web Services, Test REST, SOAP, GraphQL, and Microservices

  • It’s a web-based collaborative tool for teams that helps testers work within a browser and requires no downloads.

The latest version in September 2020 offers many new features which allow a test to continue after a fatal error, improve multipart for on-prem and load testing, and K/V store for mocking services. API Fortress is free to use and easy to customize to the specific requirements of your project.

11. Pyresttest

It is a Python-based REST API testing platform that supports tests in JSON or YAML config files, no coding knowledge is required.

Feature highlights:

  • Generate and validate mechanisms to build complete test scenarios

  • Allow easy deployment on-server for smoke tests/health checks with minimal dependencies (pycurl, pyyaml, optionally future)

  • Returns exit codes on failure to slot into automated configuration orchestration tools

12. Hoppscotch

Hoppscotch is a tool in the API tool category of the technology stack. It is a free and fast API request generator with a minimalistic UI design, allowing you to send requests and get/copy responses in real-time. Therefore, you can create requests faster, saving valuable development time. In August 2021, Hoppscotch was updated to version v2.2.0 with some minor bugs being fixed. As an open-source tool, you can feel free to customize it to your project requirements.

13. Taurus

Taurus is a free and open-source framework for simple and direct API testing because it is easy to learn. Taurus helps you hide the complexity of running performance tests without learning any programming languages; instead, all you need to understand is the basic YAML structure of Taurus.

Taurus v1.15.4 updates various features, improves user experience, and fixes bugs.

14. Citrus Framework

The tests provide a Java fluent API to specify the test logic and are fully automated. Citrus can act on both sides as a client or a consumer during the test when replacing the actual request/response message over the wire.

Citrus 3.1.0 is the latest stable version. It is available in the central Maven repository so that you can add Citrus as a Maven dependency to your project. It’s a free, open-source tool with custom scripting functionalities that you can utilize your programming skills to fit project requirements.

15. Airborne

Airborne is an API automation testing framework with a Ruby-based RSpec-driven framework. Since Airborne is a programming framework, it has no user interface apart from the text file to create code. Besides that, to use Airborne testers need to remember a few critical methods in the toolset and some Ruby and RSpec fundamentals.

API Testing Tool Selection

This is the evaluation criteria I used in making my API automation testing tools selection:

  1. User Interface (UI): Is the layout clean and inviting? Does it have well-designed navigation elements that flow naturally from one related task or menu to the other? A hallmark of a good user interface is the ability to highlight important tasks in their immediate context without being obtrusive.

  2. Usability: API testing teams need to hit the ground running. They shouldn’t spend an inordinate amount of time figuring out how an automation tool works. So, I evaluated the level of difficulty required to learn, master, and use an automation tool.

  3. Integrations: DevOp and QA teams utilize myriad tools, so it’s crucial the API testing solution they choose can communicate and integrate easily with other tools they’re already using. I looked at how the API testing tool seamlessly integrates with other CI or deployment pipeline elements.

  4. Value for $: Here, I looked at how competitive a tool’s pricing rates are compared to others. An extraordinary testing tool should provide value for money to the customer over and beyond its price tag.

Resources:, Wikipedia

The Tech Platform



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