In the event of a failure, Managed Databases will automatically switch data handling to a standby node to minimize downtime.
The data between the nodes is being replicated using SQL replication.
All database clusters have automated failover, meaning they automatically detect and replace degraded or failing nodes.
With one standby node, if the primary node fails, the service remains available. The standby node is immediately promoted to primary and begins serving requests while a replacement standby node is provisioned in the background.
If both nodes fail simultaneously, the service becomes unavailable until at least one of the nodes is reprovisioned.
To increase the stability further, you could have two standby nodes, that way the cluster would be highly available and very resilient against downtime.
Even if two nodes fail simultaneously, the service remains available while two replacements are provisioned in the background.
The service only becomes unavailable in the unlikely event of all three nodes failing at the same time.