You're replicating to a new cluster, and each cluster needs a writer instance. It is recommended that customers apply the read_only parameter to the replica, but by default they are able to write to this instance. This allows for architectures where the replica is read & writable.
'Create Cross-Region Read Replica' sets up binary log replication, utilizing MySQL's built-in functionality to replicate changes to another database. Binlog replication comes with its own challenges, including higher potential for replicas to fall behind the writer for heavy workloads. It is native to MySQL though, and has been around a long time. 'Add Region' sets up Aurora Global Database, which uses Aurora's storage-based replication. Aurora GlobalDB is going to give customers lower (typically less than 1 second) and more consistent replica lag. There are a few technical limitations to consider if using GlobalDB, but this also allows for customers to have an easier way to failover and use advanced features like Write Forwarding. In addition to a second cluster being setup, customers are also charged for Replicated Write I/Os to the secondary region(s).
No, we don't auto-failover to another region. This requires manual effort from the customer.
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