Cloud Sql Cross Region Failover Promotion Topology

What the Solution Is

In Cloud SQL for PostgreSQL, cross-region replication operates asynchronously. When an unplanned regional disaster incapacitates the primary instance in us-central1, disaster recovery requires manually or programmatically promoting a designated cross-region read replica to assume the read-write primary role.

How It Meets All Requirements

• Minimizing Data Inconsistency and Loss: Connecting to db-west via a PostgreSQL client to check pg_catalog.pg_last_wal_receive_lsn() and pg_catalog.pg_last_wal_replay_lsn() verifies that the replica has fully applied all write-ahead log (WAL) transactions received from the primary before the outage, preventing stale or partially applied state upon promotion.
• Application Connectivity and Routing: Performing a replica failover automatically updates the Cloud SQL DNS write endpoint to route incoming application write traffic directly to the newly promoted primary in us-west1 without necessitating application redeployment.
• High Availability & Topology Restoration: Because cross-region replicas do not inherit HA automatically if not pre-configured, High Availability must be enabled on the promoted instance. Furthermore, Cloud SQL does not automatically re-parent sibling read replicas; the former db-east replica of the failed primary must be deleted and recreated pointing to the new us-west1 primary using a distinct instance name.

Key Advantages

• Guarantees Transactional Completeness: Assesses WAL replay progress to avoid promoting an instance with unprocessed buffered WAL files.
• Transparent Application Redirection: Utilizes DNS write endpoint abstraction to re-route client connection pools seamlessly.
• Restores Fault Tolerance: Re-establishes regional HA and cross-region analytical offloading.

Why This Is Optimal

This procedure correctly addresses data consistency verification, dynamic application routing via DNS endpoints, and the architectural requirement to rebuild orphaned sibling read replicas from the new primary.