API mocking vs service virtualization: Key differences and use cases

There’s an endless list of tools and processes that dev teams can use to test their apps. Mock APIs and service virtualization are two common categories of tools. You’ve probably heard about them before, and while they both serve a similar purpose—to enable testing—there are some key differences between their use cases that you should be familiar with.

The service simulation spectrum

To appreciate the differences between simulation processes such as API mocking and service virtualization, it’s helpful to think of them as a spectrum - with different techniques offering different levels of control, self-service, and flexibility. In some cases, simplicity will come at the expense of testing potential, and limit testing to smaller, isolated parts of a larger application.

Let’s look at each option available for app developers: 

• Service virtualization is best used when there’s a need for full functional or integration testing. This normally means building a virtual environment that mirrors the test subject system 1-1. The aim of service virtualization is to test a system holistically and see how its various components interact with one another. 

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• Hosted API mocking involves the use of external services or platforms for creating and managing mock objects, commonly via mock APIs. These services provide interfaces for defining and executing mock experiments in the cloud, such as those focused on network latency and failure scenarios. Hosted mocking, just like service virtualization, can be used for integration testing depending on the provider. 

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• In-code mocking creates mock objects directly within an application’s codebase and is usually performed by using mocking libraries. It’s a highly customizable process that gives developers the freedom to define mock behaviors and expectations within the test code,  making it a good fit for unit testing where specific interactions and behaviors need to be tested in isolation.. However, doing so changes the code base by definition -  which means that by definition you aren't testing the code that will go live in production.

How are service virtualization and API mocking similar?

API mocking falls somewhere between hosted mocking and service virtualization on the service simulation hierarchy. This is because, for most modern use cases, API mocking combines all the benefits of both hosted mocking and service virtualization without the need to set up complex and expensive testing environments. 

Service virtualization and API mocking, for example, both require servers that simulate certain behaviors in response to network traffic. With service virtualization, most dev teams would need to manually set up and configure this server environment. This is both time-consuming and expensive due to infrastructure costs and license fees. In contrast, hosted API mocking cuts out most of these manual processes and costs while also providing a solid environment that teams can use to focus on their development and mocking workflows. 

How are they different?

While the two are similar, it’s important to note the differences. 

It was previously thought that service virtualization was best for scenarios where testing required a realistic simulation of the entire system, and that API mocking was best for unit tests where the goal was to isolate and test specific functions.

Nowadays, we’re living in a world driven by APIs. The majority of apps you come across are now dependent on them, and this is changing the way dev teams approach testing. It’s now possible for teams to run full integration tests through API mocking because that’s the technology their applications are built on. While service virtualization will always have its place, it may be relegated as the testing methodology reserved exclusively for older technologies that rely on virtual machines and desktop GUIs. 

That’s where the key difference between the two technologies lies: API mocking is agile, easier to work with, and focused on modern workflows. It enables testing on the fly and is often cheaper due to the absence of license fees and billing arrangements tied to usage. In a nutshell, you can think of API mocking as the modern way of doing service virtualization in a world that’s increasingly dependent on APIs.

When to use API mocking over service virtualization

Modern, hosted API mocking is a powerful tool for running everything from focused testing of specific functions and components to full integration testing for applications that are based on APIs. 

In other words, API mocking can fully replace traditional service virtualization processes where the circumstances allow it. Mocking should also be the go-to choice for dev teams that need to verify outputs against predefined expectations, such as failure scenarios and timeouts.

As we mentioned earlier, however, service virtualization still has its place. It plays a particularly important role when testing involves legacy systems that don’t use modern web APIs or where desktop-based workflows form part of the wider testing process. Most dev teams today will find that API mocking serves their testing needs better than old-school service virtualization without all the associated legwork and costs. 

Use WireMock Cloud for your API mocking

WireMock Cloud includes all the features you need to mock the APIs that you depend on. 

Built on the open-source WireMock library—the leading enterprise-grade mocking framework—WireMock Cloud is offered as a hosted solution for a wide range of workflows including API prototyping, mock-driven development, scenario testing, and more. All in the cloud and ready to go. 

If you want to learn more, you can try it out and begin mocking instantly by signing up for our free forever account.

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