This is Part 2 of a guest post by Kris Beevers, founder and CEO, NSONE, a purveyor of a next-gen intelligent DNS and traffic management platform. Here's Part 1.

Integration and functional testing is crucial

Unit testing is hammered home in every modern software development class.  It’s good practice. Whether you’re doing test-driven development or just banging out code, without unit tests you can’t be sure a piece of code will do what it’s supposed to unless you test it carefully, and ensure those tests keep passing as your code evolves.

In a distributed application, your systems will break even if you have the world’s best unit testing coverage. Unit testing is not enough.

You need to test the interactions between your subsystems. What if a particular piece of configuration data changes – how does that impact Subsystem A’s communication with Subsystem B? What if you changed a message format – do all the subsystems generating and handling those messages continue to talk with each other? Does a particular kind of request that depends on results from four different backend subsystems still result in a correct response after your latest code changes?

Unit tests don’t answer these questions, but integration tests do. Invest time and energy in your integration testing suite, and put a process in place for integration testing at all stages of your development and deployment process. Ideally, run integration tests on your production systems, all the time.

There is no such thing as a service interrupting maintenance