Guest Blog by Randy SlocumPromo app started by teaching us that continuous feedback helps create valuable code the customer and developers are confident in. We want a fast, reliable, refactored code. We also want code that isolates that codes' failures.But how do you write code like this? With the SOLID principle!S - Single Responsibility. Your applications should do one thing and do it right! Keep functionality simple from everything from applications down to variables.O - Open Closed principle. All artifacts should be open for extension, but closed for modification.L - Liskov's substitution principle: Any part of your code can be substituted by any other types.I - Interface segregation. Keep each interfaces simple with only one function.D - Dependency inversion. Abstractions should not depend on details, details should depend on abstractions. A car should depend on its parts, not the other way around.Exercise these principles, and you can use libraries like Mock and Fixture to help make unit testing fun!