These are essentially blank zones in which we run user programs.
So if all three datacenters in a three-DC deployment become partitioned from one another, requests may fail rather than serve potentially incorrect data. Manta must survive failure of any service, physical server, rack, or even an entire datacenter, assuming it's been deployed appropriately.
Asynchronous replication means that the asynchronous peer may be slightly behind the other two.
The idea with configuration replication in this way is that if the primary crashes, we take several steps to recover: This allows us to quickly restore read-write service in the event of a postgres crash or an OS crash on the system hosting the primary.
This part is particularly complicated, so pay attention!
The metadata tier is replicated for availability and sharded for scalability. Sharding means dividing the entire namespace into one or more in order to scale horizontally.