Each region runs two read/write replicas and a witness replica to maintain availability, Sagar explained, adding that when a zonal outage occurs affecting any one replica in a region, Spanner, due to the dual-region configuration setup, would be able to maintain availability as at least two replicas would be functional.
However, the product manager pointed out that in the event that an entire region faces outage and enterprises are in dual-region configuration setup for Spanner, the databases will lose availability.
“To recover availability, databases must switch to single-region mode, consisting of three replicas in a single region,” Sagar wrote, adding that Google continuously monitors for outages and upon detection automatically fails over and reconfigures the dual-region setup to a single-region setup to ensure availability.