Mult-AZ RDS deployments consistency and availability

Replication with the multi-AZ standby is synchronous as described in the following documentation.

https://aws.amazon.com/rds/faqs/?nc1=h_ls

No, a Multi-AZ standby cannot serve read requests. Multi-AZ deployments are designed to provide enhanced database availability and durability, rather than read scaling benefits. As such, the feature uses synchronous replication between primary and standby. Our implementation makes sure the primary and the standby are constantly in sync, but precludes using the standby for read or write operations. If you are interested in a read scaling solution, please see the FAQs on Read Replicas.

There's a good explanation here - https://aws.amazon.com/rds/features/multi-az/.

1. Note there's direct communication between the primary's EBS volume and standby's EBS volume so data can stay in sync when an instance is down. The purpose of this multi-az setup is to maintain availability when either one of the two instances is down.

2. With 2 standbys replication is async so there can be replica lag as with normal read replicas, but you can expect it to be less in general due to the in-memory caching layer with this setup.