Read-Only Replicas and Zero-Downtime Moving with Google Cloud Spanner

Published on march 27, 2023

A new regional and multi-regional capability has been added to Google Cloud Spanner. Several improvements have been made to the distributed SQL database, including the addition of configurable read-only replicas and the introduction of a “zero-downtime” instance move service.

Spanner supports regional and multi-regional configurations, with regional configurations providing 99.99% availability and multi-regional configurations providing 99.999% availability and protection against regional outages.

There are three types of replicas offered by Cloud Spanner: read-write replicas, read-only replicas, and witness replicas. A read-only replica provides low-latency stale reads to nearby clients and increases the read scalability of a node. Read-only replicas do not contribute to the write latency since they do not participate in the vote to commit writes.

Spanner’s zero-downtime instance move service supports regional, multi-regional, and custom deployments with configurable read-only replicas. It supports moving production instances from any configuration and region to another without downtime. Donsky discusses the previous challenge

Customers can move an instance to a different location on Google Cloud by combining the new service with the option to customize configurations with additional read replicas. It may take from a few hours to a few days for the operation to be completed, but Cloud Spanner maintains high availability and strong consistency throughout the process, thus maintaining the Service Level Agreement.

Both the source and destination instance configurations are subject to hourly compute and storage charges during the change. Currently, the zero-downtime move service requires opening a support ticket with Google and is subject to certain limitations: instances cannot be moved between projects or accounts, Spanner free trial instances are not supported, and an instance must contain at least one node (1000 processing units).

In a separate announcement, Spanner fine-grained access control is now generally available, enabling database administrators to define database roles, grant privileges to the roles, and create IAM policies to grant permissions to IAM principals. It was announced earlier this year that Spanner would support regional endpoints, which would enable data to be stored and processed within the same region to comply with regulatory requirements. However, the feature has been retracted and will be included in a future release.