2. The blog actually provides the answer:
When the Backup Nodes detect the failure, they will notify the Transaction
coordinator that they committed the transaction successfully. In this
scenario, there is no data loss because the data are backed up and can still
be accessed and used by applications.
In other words, if primary node fails, backups will not wait for a message,
but instead will commit right away and send an ack to the coordinator. Once
coordinator gets all required acs, transaction completes.
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