It is my understanding that for automatic failover to occur in an Always On AG environment, the replicas just need to be configured with "automatic failover," "synchronous commit" and the environment must have proper quorum (i.e. odd number of voting members).
However, according to the "SQL Server 2019 Always On" textbook by Peter A. Carter, for automatic failover to work, the secondary cannot be active and must be passive. Passive meaning the Readable Secondary setting is set to 'No' so it is unavailable for direct connections or read access, and obviously no backups can be done on them either.
Here is the exact statement from the textbook:
"Although we can change the value of Readable Secondary through the GUI while at the same time configuring a replica for automatic failover without error, this is simply a quirk of the wizard. In fact, the replica is not accessible since active secondaries are not supported when configured for automatic failover."
Here's a screenshot of the "quirk"
Note: According to the textbox, the Readonly Secondary should be set to 'No' if you want automatic failover to work. Thinking back, I don't recall a successful automatic failover occurring with this set up and I believe when an issue occurred, the AG was in a RESOLVING state. Note, we have third server for proper quorum.
I've searched through Microsoft's documentation to confirm whether or not the author's statement is true but have yet to find anything that explicitly states that the secondary replica must be passive for automatic failover to work. This Microsoft article (Failover and Failover Modes (Always On Availability Groups)) states the conditions required for automatic failover but does not mention needing a passive secondary replica.
So is the author correct and Microsoft has failed to advertise this OR is the author incorrect and automatic failover does work when the secondary replica is active/readable? My objective is to have a highly available environment with automatic failover but do we need to sacrifice offloading read operations in order to accomplish this? Adding another server is not an option.