Cloud Spanner Strong Consistency for Global Writes with CDN for Reads

A team is designing a globally distributed application on Google Cloud that requires strong consistency for writes but can tolerate eventual consistency for reads. The application expects millions of concurrent users. Which two strategies should they implement? (Choose two.)

Quick Answer

The correct answer is to use Cloud Spanner for global application strong consistency writes with Cloud CDN for reads. This works because Cloud Spanner leverages TrueTime and Paxos-based synchronous replication to guarantee strong consistency across regions for every write, while Cloud CDN serves stale-but-acceptable read data from edge caches, perfectly matching the requirement for eventual consistency on reads. On the Google Professional Cloud Developer exam, this scenario tests your understanding that Spanner is the only globally distributed database offering external consistency, and the common trap is assuming you need Spanner for reads too—remember, CDN handles read-heavy traffic efficiently. A useful memory tip: “Spanner for the pen, CDN for the lens”—writes must be strongly consistent, but reads can tolerate a slightly outdated view.

⚠ Common exam trap

The PCD exam often tests the misconception that a single database can handle both strong consistency and high scalability for reads and writes, leading candidates to choose Firestore or Bigtable without considering the specific consistency requirements for writes versus reads.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Use Cloud Spanner for write operations requiring strong consistency.

Cloud Spanner provides strongly consistent writes globally through synchronous replication using the TrueTime API and Paxos-based consensus. This ensures that all write operations are immediately consistent across regions, meeting the requirement for strong consistency on writes.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
Use Cloud Spanner for write operations requiring strong consistency.
Why this is correct
Cloud Spanner provides externally consistent reads and writes across global regions using TrueTime and atomic clocks, satisfying the strong consistency requirement for write operations in the stem. Its synchronous replication ensures that all writes are immediately visible to subsequent reads, directly addressing the need for write consistency despite eventual read tolerance.
✗
Use Firestore in multi-region mode for all operations.
Why it's wrong here
Firestore multi-region provides strong consistency but not optimized for global writes with low latency.
✓
Use global HTTP(S) Load Balancer with Cloud CDN for read-heavy traffic.
Why this is correct
Cloud CDN caches content globally, providing low-latency reads with eventual consistency.
✗
Deploy Cloud SQL with cross-region replication for read scalability.
Why it's wrong here
Cross-region replication is asynchronous, leading to eventual consistency.
✗
Use Cloud Bigtable for reading data with strong consistency.
Why it's wrong here
Bigtable does not provide strong consistency for reads.

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Same concept, more angles

1 more way this is tested on PCD

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A multi-region application uses Cloud Spanner. The team needs to ensure that a write is immediately visible to all subsequent reads, even those performed in different regions. Which consistency mode should they use?

hard

A.Eventual consistency
B.Global consistency
C.Bounded staleness
✓ D.Strong consistency

Why D: Strong consistency (D) ensures that once a write is acknowledged, any subsequent read, regardless of region, will reflect that write. Cloud Spanner uses the TrueTime API and Paxos-based replication to provide external consistency (a form of strong consistency) across regions, making it the correct choice for immediate global visibility.

Last reviewed: Jun 11, 2026

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