Underneath it is postgres, with a few key things rewritten. It has a custom storage layer that isn't too different from a very fancy SAN. Replication is where

Underneath it is postgres, with a few key things rewritten. It has a custom storage layer that isn't too different from a very fancy SAN. Replication is where things get to be very different. All instances use the same underlying store, so replication of storage isn't part of the postgres layer. However, reader nodes need to invalidate cache when writes occur. So they use postgres replication, but the readers skip writing to storage. Lots of rewritten components under the hood to support this different storage paradigm, but the engine itself is still postgres.