[wb14123]

This kind of replication can be configured in vanilla Postgres with something like ANY 3 (s1, s2, s3, s4) in synchronous_standby_names? Doc: https://www.postgresql.org/docs/current/runtime-config-repli...

[ctapobep]

I don't think it's possible with ANY set up. All you get is that some replicas are more outdated than others. But they won't return 2 conflicting states when ReplicaA says tx1 wrote (but not tx2), while ReplicaB says tx2 wrote (but not tx1). Which is what Long Fork and Parallel Snapshot are about.

So Amazon Multi-cluster seems to replicate changes out of order?

[mattashii]

Kinda. I think it's "just" PostgreSQL behaviour that's to blame here: On replicas, transaction commit visibility order is determined by the order of WAL records; on the primary it's based on when the backend that wrote the transaction notices that its transaction is sufficiently persisted.

See also my comment https://news.ycombinator.com/item?id=43843790 elsewhere in this thread