The one thing to understand first
Azure Database for PostgreSQL — Flexible Server is Microsoft’s Postgres-on-a-disk offering: community PostgreSQL on an Azure VM with Azure managed disk storage, plus a control plane that gives you fine-grained control over maintenance windows, configuration, and high-availability placement. “Flexible” refers to that operational control compared with the older, more locked-down Single Server.
Compute tiers and storage
You pick a compute tier — Burstable for small/intermittent workloads, General Purpose, or Memory Optimized — backed by Azure managed disks with configurable IOPS. As with the rest of family one, a single primary handles writes, and storage durability comes from Azure’s replicated disk underneath.
High availability: zone-redundant and same-zone
Flexible Server’s HA provisions a hot standby kept current by synchronous replication. You can place that standby in a different availability zone (zone-redundant HA, surviving a zone failure) or in the same zone (cheaper, surviving only a node failure). On failure, Azure promotes the standby automatically. As everywhere in family one, the standby is for failover, not reads.