[mjb]

I don't think you can simplify things that far. Shared-nothing is certainly a good architectural goal, and there are many types of distributed systems where it is both highly desirable and achievable. On the other hand, there are many distributed systems (and parallel systems) where coordination is strictly necessary.

Think about implementing something like the TPC-C load on a distributed database. Many parts of that load can be done sharded, with a shared-nothing model. Other parts need some level of coordination, and some need true serializability (which requires significant coordination). The pieces that can be built on shared-nothing primitives should be built on shared-nothing primitives. However, it's not useful to pretend that the pieces that require coordination simply don't exist.

Similarly, immutability is a good goal. An append-only log of immutable data items is often a very good mental model, and frequently a good implementation model, for distributed databases. It isn't universally applicable, though, and may put constraints on the possible supportable operations that add complexity to other parts of the system.