Practices vs Principles

It struck me when reading Scaling the Practice of Architecture that people often use the term “principle” in a sloppy way:

There is a great deal I could write here about bad architectural principles but I’ll stick to the key aspects. Firstly, they are not practices. Practices are how you go about something, such as following TDD, or Trunk Based Delivery, or Pair Programming. This is not to say that practices are bad […] they’re just not architectural principles.

I’ve probable been using the term in a wrong way more than once. Principles don’t tell you exactly how to do something. They are just criterions to evaluate decisions. All things being equal, take the decision that fulfills the principle the most. Examples of well-known design principles are for instance

  • Single-responsibility principle
  • Keep it simple, stupid
  • Composition over inheritance

A practice, on the other hand, is a way of doing something. Examples of practices are:

  • Pair Programming
  • ​​​​​​​Shift left with CI/CD
  • Limit Work in Progress (WIP)

A lot of documents confuse the two. For instance, the SAFe Lean-Agile principle are actually mostly practices.

It could look like principles are for software design and practices are for software delivery. But you can have principles for software delivery, too. For instance, “maximize autonomy” could be a delivery principle. It doesn’t tell you how. It just tell you that if you have two options to design the organization, you should go the the one that maximizes autonomy. On the other hand, a software design practice could be to “model visually”.

Another confusion in this area come with another term similar to principles and practices: values. A value is a judgment of what we consider important. Usually they define behaviors and are then adjective (but “profit” could be a value yet isn’t an adjective). “Autonomy”, could be for instance a value. A value embodies implicitly the principle of favoring this value over others. For instance, if you value “autonomy”, you will automatically follow the principle “maximize autonomy”. If you adhere to a value, the corresponding principle comes for free.

Finally, there are “conventions” and “guideline”. Conventions tell you how to do things exactly and are mandatory. You can check if you adhere to a convention or not. This is unlike principles or practices, which have room for interpretation. A guideline is like a convention, but optional. Examples of convention or guidelines are:

  • Interfaces are versioned
  • Sanitize all inputs
  • Limit WIP to 3

Using a full example of value/principle/practice/guideline with in one area, we could have

  • value: resilience
  • principle: tolerate failures
  • practice: chaos testing
  • guideline: use tolerant reader

Granted, no matter how we try to distinguish the terms from one other, there will be some overlap in some cases. Natural language is messy. But I think it’s worth using the terms in the most appropriate ways if possible. It helps create a mental model that works. If you mix practices, principles, value and guidelines together, people might not notice immediately, but it creates a cognitive friction that makes it harder to actually apply underlying ideas.

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