Oh, to address your question about chain replication, normally acks travel backward along the chain, from the tail to the head, so the head acks to the client w

Oh, to address your question about chain replication, normally acks travel backward along the chain, from the tail to the head, so the head acks to the client when it's received an ack from its successor. So the usual client interface is still request/response, but of course the latency is considerably higher than a quorum-based protocol, since the messages have to travel the full length of the chain before being made durable on all nodes, and then acks have to travel all the way backward. There are various optimizations to try to reduce this latency, and also reduce the load on the tail in read-heavy workloads (probably the best-known is CRAQ: https://pdos.csail.mit.edu/6.824/papers/craq.pdf https://pdos.csail.mit.edu/6.824/papers/craq.pdf).