The answer is to increase the number of vCPUs on the primary instance. This directly improves Cloud SQL write performance by providing more processing power to handle the higher write throughput during peak hours, which resolves the timeouts without disrupting the existing read replica used for reporting. On the Google Professional Cloud Developer exam, this scenario tests your understanding of vertical scaling for write-heavy workloads versus horizontal scaling with replicas, which only improve read performance. A common trap is assuming you need to add more replicas or switch to a different database engine, but replicas do not help with primary write load. The key insight is that Cloud SQL write performance improvement requires scaling up the primary instance’s compute capacity, not its storage or replica count. Memory tip: “Write needs might, so vCPUs ignite.”
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. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 uses Cloud SQL for MySQL to store customer data. They have enabled automatic backups and a read replica for reporting. The application experiences timeouts during peak hours because the primary instance cannot handle the write load. The team needs to improve write performance without losing the ability to read from replicas. What should they do?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the number of vCPUs on the primary instance.
Option C is correct because increasing the number of vCPUs on the primary Cloud SQL for MySQL instance directly improves its processing capacity to handle higher write throughput. This addresses the root cause of timeouts during peak hours without disrupting the existing read replica architecture, which continues to serve reporting queries. Cloud SQL allows vertical scaling of the primary instance by adjusting machine type, and this change does not affect the ability to read from replicas.
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.
✗
Increase the size of the read replica to handle writes.
Why it's wrong here
Read replicas cannot accept writes.
✗
Promote the read replica to a standalone instance and redirect writes.
Why it's wrong here
Promoting replica doesn't increase write capacity; it becomes a new primary with same limitations.
✓
Increase the number of vCPUs on the primary instance.
Why this is correct
Scaling up the primary instance improves write throughput.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use Cloud Spanner instead of Cloud SQL for better write scalability.
Why it's wrong here
Migration to Spanner is unnecessary and complex; Cloud SQL can be scaled vertically.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume read replicas can be used to offload writes (Option A) or that promoting a replica is a valid scaling strategy (Option B), but Cloud SQL read replicas are strictly read-only and cannot accept write traffic, making these options invalid for improving write performance.
Detailed technical explanation
How to think about this question
Cloud SQL for MySQL uses InnoDB as its default storage engine, which relies on the primary instance's CPU and memory for write-intensive operations such as transaction log writes and buffer pool flushes. Increasing vCPUs directly improves the instance's ability to handle concurrent write connections and reduces query execution time, thereby alleviating timeouts. In a real-world scenario, if the primary instance is already at the maximum vCPU limit for its tier, the team might need to consider sharding or moving to a higher-tier machine type, but vertical scaling is the first recommended step.
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
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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: Increase the number of vCPUs on the primary instance. — Option C is correct because increasing the number of vCPUs on the primary Cloud SQL for MySQL instance directly improves its processing capacity to handle higher write throughput. This addresses the root cause of timeouts during peak hours without disrupting the existing read replica architecture, which continues to serve reporting queries. Cloud SQL allows vertical scaling of the primary instance by adjusting machine type, and this change does not affect the ability to read from replicas.
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.
Are there clue words in this question I should notice?
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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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