QA Testing

Getting Started with API Testing for Web and Mobile Applications

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APIs (or Application Programming Interface) are an essential component of software applications. They save developers valuable time by allowing them to integrate features and capabilities from other applications that would otherwise have to have been written from scratch.

For many applications, they depend on having robust, dependable access to an APIs endpoints in order for it to function properly, while also being mindful of things like security and the overall API performance.

Developers who plan to extend aspects of their application as part of an API or who want to validate the reliability of their existing API should consider implementing thorough API Testing.

What is API Testing?

API Testing is a method of software testing which aims to identify defects within an API in order to prevent errors in applications that make use of it. There are many types of API Testing and it is through these methods that testers are able to validate the security, reliability, and performance of an API.

API Testing functions differently than traditional manual testing - instead of interacting with the GUI of a traditional application, a tester will use software to send calls to the API and, as the system provides outputs, they will record the response. From there, they can compare the results to the expectations of a test case and determine whether or not to file an issue.

Illustration of a web browser window with the words 'API' placed inside of it and surrounded by a gear icon.

Why is API Testing Important?

Whether it’s shopping online, searching for a place to stay, or streaming your favorite movie, APIs are very much ingrained in the experience of using the web or applications. We depend on resilient APIs to be able to reliably serve us necessary information from application servers. When an API is either error prone or there is a noticeable delay between the client and the server, this can be a frustrating user experience.

By introducing API Testing early in the development process (as part of a shift-left approach) teams can identify critical issues within their API, make improvements for a more stable experience, and then monitor the performance of the API after it has been deployed to production.

Types of API Testing

Unit Testing

Focused on creating units of isolated application code and writing tests for each. They are often created directly by developers and are useful for validating that a section of an application’s code behaves as intended.

Functional Testing

Similar to unit testing except that a tester will examine a larger portion of the API and determine whether or not it can correctly perform an expected task within predefined parameters. This also includes checking that the values in the API are reflected (and vice versa.)

Load Testing

Also a form of performance testing; this API testing type seeks to stress test the API by simulating peak user volume so that it can be evaluated how the API will perform under load.

Security Testing

Checking that the API is not vulnerable to external threats and that certain security requirements are being met for authentication, encryption, and API access.

Integration Testing

In this testing type, a tester will validate that when mapping one API to another, the mapped values are correct and the two APIs are able to communicate effectively between each other.

Side view of a computer monitor displaying colorful application code.

Methods of API Testing

Manual vs Automated

When it comes to API Testing, there are two popular approaches: manual and automated testing. While each has their advantages and disadvantages, which you choose will ultimately depend on the project’s needs.

Manual Testing involves using real, human testers that are manually executing pre-scripted tests. This approach is ideal when the test scenarios that are being covered are complex or unpredictable since a human can validate and analyze the results but also runs the risk of being subject to human error.

Automated testing instead takes advantage of software tools to execute predefined test cases. This approach is ideal for regression testing and scenarios where it requires a larger scale of input. While test automation can often be quicker and more efficient for routine testing tasks than a human tester would, it also requires a significant investment in tools and technology infrastructure to execute.

Over the shoulder view of a developer working on application code on their computer

The best solution is to combine both manual and automated testing. For example, using manual testers for scenarios which would be too complex to automate and test automation for scenarios which would be more efficient to execute programmatically.

Popular Software Tools for Automated Testing

There are a variety of software tools available for API test automation. Here are a few options you might consider:


Bio from their website: “Postman is an API platform for building and using APIs. Postman simplifies each step of the API lifecycle and streamlines collaboration so you can create better APIs—faster.”

Pricing: Free plan -> $49 per user, per month for Enterprise plan when purchasing annually.

Apache JMeter

Bio from their website: “The Apache JMeter™ application is open source software, a 100% pure Java application designed to load test functional behavior and measure performance.”

Pricing: Open Source (Free)


Bio from their website: “...SoapUI is the only open source tool that covers the entire testing spectrum (functional, security, load, mocking). With its easy-to-use graphical interface, SoapUI is the entry point for API testing so you can validate REST, SOAP, and GraphQL-based web services with ease.”

Pricing: Open Source (Free)

Types of Bugs API Testing Will Help Detect

As API Testing is conducted, you should expect to come across some of these common issues:

Broken or Missing Functionality

These issue types come from when an API fails to meet its functional requirements or the implementation is missing features that were originally planned. Addressing these issues is essential to meeting user’s functional expectations.

Compatibility Issues

If an API is having issues integrating with a platform, framework, or language, it’s likely due to a compatibility issue. This can lead to errors and unpredictable behavior that affect a user’s experience.

Improper Error Handling

This type of issue occurs when an API receives an invalid request but does not provide an adequate response that can be used to help diagnose and troubleshoot effectively. An API should always handle errors using clear and consistent messaging.

Security Vulnerabilities

An API with a security vulnerability runs the risk of exposing sensitive information about your users or allowing access to unauthorized API functions. Cyberattacks are becoming more common each year and your API could be a potential target.


With potentially hundreds of thousands of users making API requests at any given time, it is important to perform performance testing to validate that your API is able to respond promptly, even when under heavy load.

You also want to make sure that your API is always readily available for users. By identifying bottlenecks and optimizing performance, excessive downtime or outages can be avoided and user satisfaction will increase.

Abstract illustration of a tester working on a laptop device that has a checkmark displayed on its screen. Surrounding them is a web of API testing related iconography.


As we’ve explored in this post, API testing is an essential component of the development process for any website or application which makes use of an API in order to deliver some type of functionality.

Whether you choose to approach this using manual testing, automated testing or a combination of the both, having a testing partner with the experience and tools to execute effective API testing will save you and your users valuable time and frustration.

Please visit our API Testing services page to learn more about what PLUS QA can offer for businesses looking to API test their applications and get in touch with us for more information.