Developing and testing interdependent applications is difficult. Maybe you’re working on a mobile application that needs to talk to a legacy API. Or a microservice that relies on two other services that are still in development.

The problem is the same: how do you develop and test against external dependencies which you cannot control?

You could use mocking libraries as substitutes for external dependencies. But mocks are intrusive, and do not allow you to test all the way to the architectural boundary of your application.

Stubbed services are better, but they often involve too much configuration or may not be transparent to your application.

Then there is the problem of managing test data. Often, to write proper tests, you need fine-grained control over the data in your mocks or stubs. Managing test data across large projects with multiple teams introduces bottlenecks that impact delivery times.

Integration testing “over the wire” is problematic too. When stubs or mocks are swapped out for real services (in a continuous integration environment for example) new variables are introduced. Network latency and random outages can cause integration tests to fail unexpectedly.

Hoverfly was designed to provide you with the means to create your own “dependency sandbox”: a simulated development and test environment that you control.

Hoverfly grew out of an effort to build “the smallest service virtualization tool possible”.