> The other debate is on how parallelism (and concurrency) should be done. Shared-nothing or shared-memory?

Obviously it should be "shared nothing" as it would allow you to horizontally scale your application and code will be much easier to develop and test. But in reality you inevitably need to have a shared state in one way or another. You could try to move that state into some dedicated service, like database, but it's not always performant enough (or even applicable). So you end up with shared memory because of performance.