The correct approach is to enable Cloud SQL read replicas. This works because read replicas provide an asynchronous read-only copy of the primary instance, allowing you to scale read traffic without compromising the read-after-write consistency your stateful microservice requires—the primary instance handles all writes with strong consistency, while replicas serve stale reads that eventually become consistent, perfectly matching your tolerance for write latency. On the Google Professional Cloud Developer exam, this scenario tests your understanding of when to use read replicas versus alternatives like connection pooling or caching; a common trap is assuming read replicas guarantee immediate consistency, but they are asynchronous, so they are ideal for workloads that can accept eventual consistency for reads. Remember the memory tip: "Primary for writes, replica for reads—strong on the source, stale on the course."
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a stateful microservice that requires read-after-write consistency but can tolerate some latency for writes. They are currently using a single Cloud SQL instance and want to scale read traffic. Which approach should they take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable Cloud SQL read replicas
Cloud SQL read replicas are the correct choice because they provide an asynchronous read-only copy of the primary instance, which can scale read traffic without compromising the read-after-write consistency required by the stateful microservice. The primary instance handles all writes, ensuring strong consistency for writes, while replicas serve stale reads that eventually become consistent, which aligns with the tolerance for write latency.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 Memorystore to cache reads
Why it's wrong here
Caching may serve stale data, not ideal for read-after-write consistency.
✗
Shard the database manually
Why it's wrong here
Adds significant operational complexity.
✓
Enable Cloud SQL read replicas
Why this is correct
Scales read capacity with eventual consistency, good for the described needs.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use Cloud Bigtable
Why it's wrong here
NoSQL, not suitable for SQL workloads.
✗
Migrate to Cloud Spanner
Why it's wrong here
Provides strong consistency and horizontal scalability but is more expensive and complex.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that caching (Memorystore) is the default solution for scaling reads, but the trap here is that caching does not guarantee read-after-write consistency, whereas read replicas can be configured to serve stale reads while the primary maintains strong consistency for writes.
Detailed technical explanation
How to think about this question
Cloud SQL read replicas use asynchronous replication based on MySQL's binary log or PostgreSQL's streaming replication, meaning the replica may lag behind the primary by milliseconds to seconds. For read-after-write consistency, the application can route critical reads to the primary instance or use session-based read-your-writes consistency by pinning the session to the primary after a write. In practice, this pattern is common for stateful microservices like user session stores or order processing systems where eventual consistency for reads is acceptable.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable Cloud SQL read replicas — Cloud SQL read replicas are the correct choice because they provide an asynchronous read-only copy of the primary instance, which can scale read traffic without compromising the read-after-write consistency required by the stateful microservice. The primary instance handles all writes, ensuring strong consistency for writes, while replicas serve stale reads that eventually become consistent, which aligns with the tolerance for write latency.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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