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Open-Source API Testing Tools: Here Are Our Top 5 Picks

API testing, in a nutshell, concerns checking the developed APIs against the desired expectations, in terms of functionality, performance, readability, and security of the application as a whole.

API testing has become a practice within many organizations and teams over the years. Smartbear research concerning 5,000 software professionals in 2017 shows that API testers automated more than 50% of their tests, which only grew till now, crossing 80%.

Before we dive into the open-source API testing tools that are available, let’s make a brief introduction of what API stands for and its functions.

What is an API?

API means Application Programming Interface. It acts as an intermediary software, allowing two applications or software systems to communicate with each other.

What is API
Via Rigor.com

A software that has API running, contains functions that can be executed by another software. In simpler terms, API is a set of functions that allow the creation of an application. This application then can access data or features of another software system, application or another service.

Now, let’s talk about API testing.

What is API Testing?

Since APIs do not have a GUI, API testing is conducted at the business logic layer of the software architecture. Basically, it doesn’t use regular inputs and outputs. In fact, a software is used to send calls to the API in order to receive a response. The response is then validated against the expected outcome.

To perform this testing, there are many open-source tools available. In fact, few new contenders have risen for the occasion, promising great testing experience. Moreover, with AI integrating into various existing technologies and techniques; it was a given that AI testing tools would emerge.

We dealt with the ‘What’- Now let’s discuss the ‘Why’.

Why do we need API testing?

It is imperative that any confusion regarding API testing is dealt with. So why is there a need to perform API testing?

Since shift-left-testing is slowly being adopted by many software development companies– it made test feedback to be produced at a quicker pace. Focusing entirely on UI automation doesn’t do any good as it completely kills off test automation efforts; since UI automation is quite slow.

APIs are a part of modern software development. As many are taking up the microservices approach to software dev, API testing has become even more crucial.

However, API testing would have risen to fame much earlier, if developers and testers weren’t given specific roles to fill; which places API testing in testers’ hands. Unfortunately, since it needs to be performed by the developers from the early stages of the cycle, testers remain clueless with its workings.

Anyhow, these are the top 5 open-source API testing tools that you can make use of.

1. REST-Assured

REST-Assured API Testing Tool
Via Testing Alert

The perfect tool for Java developers for API automation.

REST-Assured is a fluent Java library that one can use to test HTTP-based REST services. This particular tool is built to make the process easier as it integrates with any existing Java-based automation framework.

Apart from this, it also provides a BDD (Behavior-driven development); which is a domain-specific language, making API testing in Java simple. Also, REST-Assured comes with built-in functionalities, allowing developers to avail code chunks without having to write them from scratch.

2. SoapUI

API Testing Tool SoapUI
Via Medium

SoapUI provides seamless automated API testing experience with a simple and user-friendly interface.

So if you and your team deal in complicated API testing scenarios then SoapUI is the right choice. Since its a fully functional test tool, there is hardly any need for producing solutions from scratch. However, in case, you do need to devise a custom functionality or workflow to match your test-cases, then SoapUI allows code to be written using Groovy.

3. Insomnia

API Testing Tool Insomnia
Via Twitter

Insomnia contains simple to use and beautiful interface. Moreover, it has more to offer: creating HTTP requests, organizing tests, viewing response details, reusable values and generate code snippets.

However, these are just ‘some’ of the many features Insomnia has to offer.

4. Fiddler

API Testing Tools
Via Telerik

This one is perfect for the .NET folks. Since Fiddler allows teams that dabble with .NET languages, to develop tests in Fiddler using any of the languages of the respective framework.

What can Fiddler do? It lets you monitor, manipulate and reuse HTTP requests. It also comes with many extensions; one of those is APITest extension. It validates the behavior of web APIs. However, this is lightweight. FiddlerCorr.NET class library is suitable for extensive API testing development.

5. Citrus Framework

API Testing Framework Citrus
Via Citrus Framework

The tests in Citrus use Java or XML. It enables automated integration testing of message-based applications.

Citrus Framework also works with many protocols that include REST, SOAP, HTTP, JMS, TCP/IP, and others.

If we get into the features it supports, it enables the following:

  • Sending and receiving messages
  • Database validation
  • Automatic retries
  • Variable definitions
  • Message timeout simulation
  • Dynamic message contents
  • Error simulation
  • Iterations

Another plus that Citrus offers, is impeccable documentation. Moreover, this API testing framework tool is built to handle any headless protocol. If you are in an enterprise environment, then the flexibility Citrus offers can help you test many different kinds of applications.

These were just a few of the many open-source tools available for API testing. Since there is no one tool that is best, it entirely depends on the organization and its requirements. Whichever tool closely matches would work.