[falcolas]

Since it sounds like you have the information on what it is based upon (if only the principles which were used to address distributed ACID consistency), it would be good to get this information dissiminated - it's hard to trust that it will "just work" when we have so many examples of distributed ACID not working well.

[awgupta]

You can think of Aurora as a single-instance database where the lower quarter is pushed down into a multi-tenant scale-out storage system. Transactions, locking, LSN generation, etc all happen at the database node. We push log records down to the storage tier and Aurora storage takes responsibility for generation of data blocks from logs.

So, the ACI components of ACID are all done at the database tier using (largely) traditional techniques. Durability is where we're using distributed systems techniques around quorums, membership management, leases, etc, with the important caveat that we have a head node generating LSNs, providing a monotonic logical clock, and avoiding those headaches.

Our physical read replicas receive redo log records, update cached entries and have readonly access to the underlying storage tier. The underlying storage is log-structured with nondestructive writes, so we can access data blocks in the past of what is current at the write master node - that's required if the replica needs to read a slightly older version of a data block for consistency reasons.

Make sense?