Cloud SQL Cross-Region Replica Lag and Failover

What Cross-Region Read Replicas Are

In Google Cloud SQL, a cross-region read replica is a separate database instance provisioned in a different geographic region from the primary instance. Data modifications on the primary instance are replicated asynchronously across Google's global network to the replica instance.

How it meets the requirements:

• Geographical distribution and read latency: Creating a read replica in europe-west1 allows European analysts to direct analytical and reporting queries to a local database instance, significantly reducing network latency compared to querying the primary database across the Atlantic.
• Disaster recovery and RTO: A cross-region read replica can be promoted to a standalone primary database instance if the original primary region experiences an outage, providing a fast Recovery Time Objective (RTO) for regional disaster recovery.
• Offloading primary workload: Offloading heavy read traffic and complex reporting queries to the replica reduces CPU and memory contention on the primary transactional database.

Key operational considerations:

• Asynchronous replication lag: Cross-region replication operates asynchronously, meaning there can be a slight delay before writes on the primary appear on the replica.
• Replication lag monitoring: When the primary handles heavy write loads or large batch operations, replication lag can increase, resulting in the replica serving temporarily stale data.
• Instance sizing: The replica must be adequately provisioned with sufficient compute, memory, and storage IOPS to process incoming replication changes alongside analytical read workloads.

Why this is optimal:

Cross-region replication directly solves both the latency requirement for geographically dispersed users and the regional disaster recovery requirement, making it the standard Google Cloud architecture for multi-region database availability.