Q. NEO replaces what ? FileStorage ? ZEO ?
Both FileStorage and ZEO.
Q. Why NEO, and not RelStorage + *SQL replication ?
This layout lacks load balancing: data is fetched/stored in a single relational database, which can cause a bottleneck as data transfer grows.
With NEO, objects are scattered over multiple storage (currently, multiple relational databases), balancing the load over multiple servers and getting around that bottleneck.
Q. NEO is a different protocol than ZEO ?
ZEO implement a RPC to expose remotely the FileStorage interface whereas NEO as its own application level protocol to load/store/replicate objects and administrate/monitor/synchronise the cluster.