You are building a backend service for an ecommerce platform that will persist transaction data from mobile and web clients. After the platform is launched, you expect a large volume of global…

Cloud Spanner is a globally distributed, strongly consistent, and horizontally scalable relational database service that supports SQL queries. Its multi-region configuration ensures high availability and resilience, making it ideal for high-volume, global transaction data like those in an e-commerce platform that requires strong consistency and SQL analytical capabilities.

Firestore is a NoSQL document database, not a relational database, and while it offers scalability and global presence, its primary querying mechanism is not SQL, and aggregation queries have limitations compared to a full SQL engine, making it less suitable for complex SQL analytics.

Cloud SQL for PostgreSQL, while a relational database supporting SQL, is typically scaled vertically or with read replicas for reads, but writes are limited by a single primary instance, making it less suitable for extreme global write scalability compared to Cloud Spanner. Multi-region support in Cloud SQL refers to disaster recovery, not active-active multi-region writes.

BigQuery is an analytical data warehouse designed for large-scale data analysis and is excellent for SQL queries over massive datasets, but it is not optimized for transactional workloads or storing individual real-time events that require atomic updates, as it is primarily an OLAP rather than an OLTP system.