Both Cloud SQL and Cloud Spanner are fully managed relational databases on Google Cloud, and both support ACID transactions and SQL queries. The decision between them comes down to scale, geography, and cost. Cloud SQL vs Cloud Spanner is a comparison the Associate Cloud Engineer exam expects you to navigate, and the two services represent very different points on the spectrum of relational database options available on GCP.
Cloud SQL is a cost-effective, fully managed relational database service. It supports MySQL, PostgreSQL, and SQL Server. If you have an existing MySQL or PostgreSQL workload, Cloud SQL is the natural destination for a lift-and-shift migration. Google manages the infrastructure, backups, patches, and replication. You interact with it the same way you would with a self-managed MySQL or PostgreSQL instance.
Cloud SQL is well suited for small to medium datasets, single-region deployments, and transactional workloads. Think e-commerce order management, application user databases, inventory systems, or CRM data. These workloads share a pattern: many small operations happening continuously, with strong consistency required across all of them. Cloud SQL handles this well because it is built with ACID compliance, meaning concurrent transactions maintain data integrity.
Cloud SQL is not designed for global scale. Read replicas are confined to the same region as the primary instance. Failover replicas provide high availability within a region or between zones, but they do not give you a globally distributed active-active database. If your application needs to serve users across multiple continents with consistent read and write performance from all of them, Cloud SQL will start to show its limits.
Cloud SQL provides two mechanisms for availability beyond the primary instance. Read replicas are copies of the primary that handle read traffic, reducing load on the primary and improving read scalability. These replicate asynchronously, so there may be a brief lag between writes to the primary and visibility on the replicas. Read replicas stay in the same region as the primary.
Failover replicas serve a different purpose: disaster recovery. A failover replica replicates synchronously from the primary, which means it stays current. If the primary fails, the failover replica can take over. Failover replicas should be set up in a different zone or region from the primary to protect against zonal or regional outages.
Cloud SQL also supports point-in-time recovery through binary logging in MySQL and write-ahead logging in PostgreSQL. These mechanisms allow you to restore a database to any specific past state, which is valuable for recovering from accidental data changes or corruption.
Cloud SQL supports three backup approaches. Automatic backups run daily within a configured 4-hour window and are retained for up to 7 days by default. If you need longer retention, you need to export backups to Cloud Storage. Manual backups can be created at any time and retained as long as needed. Scheduled exports to Cloud Storage support long-term retention for compliance purposes. The exam has tested the distinction between automatic backups with their 7-day default retention and exports for longer-term storage needs.
Cloud Spanner is a globally distributed relational database. It is designed for applications that need global availability, global scalability, and global consistency at the same time. Spanner's key claim is that it addresses the CAP theorem's traditional trade-offs: it provides strong consistency and high availability even across geographically distributed deployments, something most distributed systems have to choose between.
Spanner is the right choice when you genuinely need a relational database that operates across multiple regions with consistent reads and writes from anywhere in the world. Global financial transaction systems, large-scale gaming applications, and SaaS products with a worldwide user base where regional data inconsistency would be unacceptable are examples of workloads where Spanner's characteristics matter.
The trade-off is cost. Spanner is considerably more expensive than Cloud SQL. It is priced per node with multi-region configurations running significantly higher than equivalent Cloud SQL instances. For workloads that fit comfortably within Cloud SQL's capabilities, paying for Spanner's global consistency is unnecessary overhead.
The decision usually comes down to a few questions. Does your application need to serve reads and writes globally with strong consistency from multiple regions? If yes, Spanner. Is this a lift-and-shift of an existing MySQL or PostgreSQL workload? Cloud SQL. Is the dataset small to medium and single-region? Cloud SQL. Is cost a meaningful constraint and global distribution not required? Cloud SQL.
For the Associate Cloud Engineer exam, the scenario will usually include a clear signal. Global consistency across regions points to Spanner. Single-region transactional workloads, MySQL or PostgreSQL compatibility, or cost sensitivity all point to Cloud SQL. The exam rarely asks you to choose between them without providing enough context to make the right call.
One Cloud SQL feature worth knowing separately is the Cloud SQL Auth Proxy. The Proxy provides a secure connection between applications and Cloud SQL instances using IAM-based authentication. It handles connection encryption and hides the Cloud SQL instance's IP address, making it the recommended connection method for applications running in GCP. Service accounts with the cloudsql.instances.connect IAM permission are used to authenticate through the Proxy.
The GCP Study Hub Associate Cloud Engineer course covers Cloud SQL and Cloud Spanner in depth, including high availability configuration, backup strategies, and how each database service is tested on the ACE exam.
