An interesting point to add is that Datomic can also write to Riak. But since riak lacks an atomic CAS, zookeeper is used alongside Riak.

What do you refer to when you say that Datomic inherits the speed of indexing a huge table? The query engine works by fetching blocks of data from storage and placing the blocks in a lazy immutable tree structure that are the indices of Datomic. They are also covering (the indices is where the actual data is stored). So it's not like the query engine runs queries on the huge table stored in mysql or postgres, it's used more like a key/value store. Case in point: you can put memcached between the datomic client (peer) and any of the supported storage engines. So Datomic piggybacks on very little other than the actual capabiliy to safely store data.