Api Load Testing Open Source

API testing (Application Programming Interface Testing) is software testing that focuses on determining if the built APIs fulfill expectations for the application’s functionality, dependability, performance, and security.

According to Google Trends, interest in API/Web services testing has gradually increased over the previous few years. Having the appropriate API automation testing methodology, tool, and solution is more critical than ever. API testing is an essential part of any effective CI/DevOps process.

Open source load testing tools aren’t always ideal, but they’re better than no load testing at all. A modest investment in one can pay long-term dividends. Regular load testing means more reliable performance and fewer slowdowns in production.

That said, there are a few limitations to these solutions:

  • Virtual users originate on your servers, which means you’re testing under ideal conditions compared to third-party load testing environments. In addition, you must build and maintain the underlying infrastructure.
  • Most open source load testing tools are protocol-based rather than browser-based, which means that there’s no JavaScript execution or HTML/CSS rendering taking place. Browser-based tools usually aren’t cost effective for in-house tests.
  • There’s no support if you have trouble implementing or maintaining the solution, which means you could end up spending engineers’ hours. There’s also no guarantee the project will continue to be supported over the long term by the community.

It’s important to keep these factors in mind when opting to use open source load testing tools. While they tend to have a lower upfront cost, they may not have the right feature sets for your use cases and may have a higher total cost of ownership thanks to regular maintenance, time spent, and amount of resources required. This can quickly add up and what was once “free” is quickly not “free” anymore.

This article will give a breakdown of the best API testing tools including both open-source and commercial solutions that testing teams may use to meet their requirements.

1. Katalon Studio

Katalon Studio is a free test automation solution for APIs, web applications, desktop apps, and mobile applications. It is quickly becoming a prominent tool for API/Web services testing, promoting itself as a complete end-to-end automation solution for developers and testers.

Katalon Studio supports SOAP and REST requests, as well as a variety of commands and parameterization features. Furthermore, Katalon Studio’s ability to combine UI and API/Web services for different platforms (Windows, Mac OS, and Linux) has been cited as a distinct feature among top API tools.

Feature highlights:

  • Support both SOAP and Rest
  • All-in-one shop for API, WebUI, Desktop App, Mobile testing, and the combined capabilities.
  • Support data-driven approach.
  • Support automated and exploratory testing
  • Support CI/CD integration.
  • Support AssertJ, one of the most potent assertion libraries, to create fluent assertion with BDD style
  • Suitable for both non-techies and pros through Manual and Groovy Scripting modes.
  • Can be integrated with Katalon TestOps — test orchestration platform.

Katalon Studio has been named one of the 2020 Gartner Peer Insights Customers’ Choices for Software Test Automation, with over 600 favorable evaluations, confirming its position at the forefront of the field.

Pricing: Free — $69/license/month

2. SoapUI

SoapUI is a headless functional testing tool dedicated to API testing, allowing users to test REST and SOAP APIs and Web Services easily.

Free package: Using the free package of SOAPUI, users can get the full source code and build their preferred features.

  • Create test quickly and easily with Drag and drop, Point-and-click
  • Reusability of Scripts: load tests and security scans can be reused for functional test cases in just several steps

Pro package:

  • Powerful data-driven testing: Data loaded from files, and databases, and Excel so that they can simulate how consumers interact with the APIs
  • Support native CI/CD integrations, asynchronous testing

The latest version SoapUI 5.6, released in July 2020, has updated third-party libraries (org.apache.HTTP components, commons-logging, commons-codec, and JUnit) and removed unused third-party libraries (Jackson, KeenIO). To enhance testing security, SoapUI now disables the Load and Save project scripts by default.

Pricing: Free – $709/year

3. Postman

Being originally a Chrome browser plugin, Postman now extends their solution with the native version for both Mac and Windows.

Postman is a good choice for API testing for those who don’t want to deal with coding in an integrated development environment using the same language as the developers.

  • Easy-to-use REST client
  • Rich interface which makes it easy to use
  • Can be used for both automated and exploratory testing
  • Can be run on Mac, Windows, Linux & Chrome Apps
  • Has a bunch of integrations like support for Swagger & RAML formats
  • Has Run, Test, Document and Monitoring Features
  • Doesn’t require learning a new language
  • Enable users to easily share the knowledge with the team as they can package up all the requests and expected responses, then send to their colleagues.

Starting from version 7.3 in 2020, Postman helps users better organize the collections and API elements (mock server, monitors, tests, and documentation) generated from API schemas by using the new advanced preferences. The latest version 8.7.0 for Mac, Windows, and Linux App has been released in June 2021 with “Warnings” in API validation, with the addition of Security Warnings enabled for all APIs on Postman.

Pricing: Free – $12/user/month

4. Tricentis Tosca

Tricentis Tosca is a continuous testing platform for Agile and DevOps. Benefits of Tricentis Tosca include:

  • Supports many array of protocols: HTTP(s) JMS, AMQP, Rabbit MQ, TIBCO EMS, SOAP, REST, IBM MQ,NET TCP
  • Integrates into the Agile and DevOps Cycle
  • Maximize reuse and maintainability with model-based test automation
  • API tests can be used across mobile, cross-browser, packaged apps, etc…
  • Achieve sustainable automation with new technology
  • Reduce the time of regression testing
  • Interactive testing provides test managers with the ability to execute manual testing and collect results without having to configure Tosca environments.

With the latest version — Tricentis Tosca 14.2 LTS, you can expand Tosca Data Integrity to access data sources that aren’t natively supported. To conduct row-by-row comparisons or load data into the cache database, you may create a bespoke solution and connect it into Tosca Data Integrity.

Pricing: Contact Sales

5. Apigee

Apigee is a cross-cloud API testing tool, allowing users to measure and test API performance, supports and build API using other editors like Swagger. Apigee is recognized as one of the leaders in the Gartner Magic Quadrant 2019 for Full Lifecycle API Management for the fourth consecutive time.

  • It is multi-step and powered by Javascript
  • Allows the design monitor, deploy, and scale APIs
  • Identify performance issues by tracking API traffic, error rates, and response times
  • Easily create API proxies from the Open API Specification and deploy them in the cloud
  • Cloud, on-premise, or hybrid deployment model on a single code base
  • PCI, HIPAA, SOC2, and PII for apps and APIs
  • Apigee is purpose-built for digital business, and the data-rich mobile-driven APIs and apps that power it.

Starting from version 4.19.01 in February 2019, Apigee gives users even more flexibility to manage their APIs with features like Open API 3.0 support, TLS security, self-healing with apigee-monit, virtual host management improvements, and more software support. The latest (March 2021) released with minor bug fixes and security improvements.

Pricing: Contact Sales


There is no one perfect tool for everyone: Every organization has different requirements. Virtually all of the API test tools listed above work well and are great options. The key is to read through the descriptions carefully and decide which best fits your team’s needs. 

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