"Side database" is a good way to put it IMHO. Another way to look at is how databases like Timesten is used for in-memory, short term, frequently needed data sets in telecom networks. The blockchain will act like the in-memory database with all the "consensus" data.I believe this is not as complicated as a sidechain.

The usage of RocksDB is excellent choice as its a proven database within google as BigTable and then developed and actively used by Facebook, Google etc.

@blocktrades : If you have time, your insights will be great in this context.