How does it handle the possible inconsistency due to the asynchronous replication?
That would be a big question for me. I can sync databases easily and instantaneously within an availability zone, and nearly instantaneously across availability zones within a region. But once I have to replicate across regions, I add latency to the mix. If replication from US-East to US-West is asynchronous, how would I reconcile the two? I suppose with a catastrophic failure of the primary database in US-East, I could just write off any data that wasn't replicated before the failure, but that doesn't seem like a solid solution. Would it be better to write a transaction layer into the app, so that data isn't considered committed until it has been written and replicated across multiple regions?
There are all kinds of solutions available to you in that scenario. Two-phase commit, on DBs that support it, would probably go a long way towards enabling a transaction layer like you describe.
Usually, though, discussions of DR should start with determining what kinds of RPO and RTO you're willing to pay for and then evaluating which of the available solutions will get you there.
That would be a big question for me. I can sync databases easily and instantaneously within an availability zone, and nearly instantaneously across availability zones within a region. But once I have to replicate across regions, I add latency to the mix. If replication from US-East to US-West is asynchronous, how would I reconcile the two? I suppose with a catastrophic failure of the primary database in US-East, I could just write off any data that wasn't replicated before the failure, but that doesn't seem like a solid solution. Would it be better to write a transaction layer into the app, so that data isn't considered committed until it has been written and replicated across multiple regions?