Database / Google Spanner Database Interview questions
What is the difference between Spanner and Cloud SQL?
| Cloud Spanner | Cloud SQL |
| Horizontally scalable, distributed across zones/regions automatically. | Runs on a single managed instance (with read replicas), vertically scaled. |
| Strong external consistency via TrueTime and Paxos. | Standard MySQL/PostgreSQL/SQL Server consistency model. |
| Custom dialect (GoogleSQL) or PostgreSQL-compatible. | Real MySQL, PostgreSQL, or SQL Server engines. |
| Priced by node/processing unit capacity, not storage tiers alone. | Priced by instance size (vCPU/RAM) and storage. |
Cloud SQL is the right default for typical application databases that fit on one machine and need full engine compatibility, including extensions and stored procedures. Spanner is worth the added complexity when a single instance's write throughput or storage ceiling becomes the actual bottleneck, or when the application needs strong consistency across multiple regions rather than just read replicas.
Which service scales horizontally across zones automatically?
When is Cloud SQL usually the better default choice?
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