The Literate Programmer: The microservice declaration of independence

In order for "microservice architecture" to be a useful way of characterising systems, it needs an accepted and well-understood definition. Mark Baker laments the lack of a precise description, because he feels its absence compromises the value of the term:

Really unfortunate that the Microservices style isn't defined in terms of architectural constraints #ambiguity #herewegoagain

In their article, Martin and James have this to say about the style's definition:

We cannot say there is a formal definition of the microservices architectural style, but we can attempt to describe what we see as common characteristics for architectures that fit the label.

I'm still optimistic that a clearer picture is yet to emerge, though it might not be as unambiguous as the REST architectural style, which is defined in terms of six constraints.

My own opinion is that microservice architectures can be understood through a single abstract architectural constraint which can be interpreted along many different degrees of freedom.

X can be varied independently of the rest of the system.

What might X be? There are many qualities that might be locally varied in an architecture to beneficial effect. Here are some ways we could introduce novelty into a service without wishing to disturb the greater system:

• Select implementation and storage technology
• Test
• Deploy to production
• Recover from failure
• Monitor
• Horizontally scale
• Replace

There are other aspects of systems that are harder to formalise, but which the human beings that work on our software systems might wish to do with without being troubled by the fearsome complexity of the whole architecture:

• Understand
• Describe
• Work on with a close-knit team
• Share two pizzas in celebration of a new feature

I don't wish to imply that this is an exhaustive list. To me, the commonality lies in the concept of independent variation, not the kinds of variation themselves.

The microservice declaration of independence