the top level is a shared-nothing architecture.
Each node of the system could actually be a shared-memory system
with a few processors. Alternatively, each node could be a shared-disk system,
and each of the systems sharing a set of disks could be a shared-memory system.
Thus, a system could be built as a hierarchy, with shared-memory architecture
with a few processors at the base, and a shared-nothing architecture at the top,
with possibly a shared-disk architecture in the middle.