GCP Professional Data Engineer Practice Question

A financial-services startup is migrating its globally distributed ledger system to Google Cloud. Key requirements are:

Up to 15 000 strongly consistent transactions per second across three continents.
Single-row and multi-row transactions must be fully ACID.
The service must continue operating through a full regional outage with an application RTO under one minute and an RPO of zero.
Operations teams want Google to handle patching, replication, and automatic scaling.

Which Google Cloud database deployment best satisfies these reliability and consistency requirements while minimizing operational overhead?

Use Cloud SQL for PostgreSQL with high availability enabled and cross-region read replicas.
Store the ledger in Cloud Bigtable with multi-cluster routing across the required continents.
Deploy Cloud Spanner in a multi-region configuration (for example, nam3) to store the ledger.
Migrate the data to Firestore in Datastore mode hosted in a multi-region location.

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Answer Description

A multi-region Cloud Spanner instance is uniquely designed for planet-scale relational workloads. It uses synchronous, quorum-based replication across geographically separated regions, delivering external strong consistency and full ACID semantics for both single-row and multi-row transactions. Multi-region configurations carry a 99.999 % availability SLA and provide zero-RPO, sub-minute RTO failover during a regional outage, all managed automatically by Google (including patching, replication, and horizontal scaling).

Cloud SQL with cross-region read replicas relies on asynchronous replication, which exposes a non-zero RPO and can require minutes for failover. Cloud Bigtable with multi-cluster routing offers high availability and global reads but is not an ACID relational database and provides eventual consistency across clusters. Firestore in Datastore mode supports ACID transactions across up to 25 entity groups, yet it lacks a relational schema and may not deliver the required cross-continental strong consistency or transaction throughput at the specified scale. Thus, only a multi-region Cloud Spanner deployment meets all stated requirements with minimal operational effort.

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