When it comes to high performing scalable distributed systems, coordination is a killer. It’s the dominant term in the Universal Scalability Law. When we can avoid or reduce the need for coordination things tend to get simpler and faster. See for example Coordination avoidance in database systems, and more recently the amazing performance of Anna which gives a two-orders-of-magnitude speed-up through coordination elimination. So we should avoid coordination whenever we can.

So far so good, but when exactly can we avoid coordination? Becoming precise in the answer to that question is what the CALM theorem is all about. You’re probably familiar with Brooks’ distinction between essential complexity and accidental complexity in his ‘No silver bullet’ essay. Here we get to tease apart the distinction between essential coordination, a guarantee that cannot be provided without coordinating, and accidental coordination, coordination that could have been avoided with a more careful design.

Also: https://blog.acolyer.org/2019/03/08/a-generalised-solution-to-distributed-consensus/