The answer is that the timeouts are most likely caused by an external service rate limit being hit as the Cloud Function scales. When you increase the max instances from 10 to 100, you amplify concurrent invocations, each of which calls the same external API or database. If that external service enforces a rate limit, the sudden flood of requests from the scaled-out functions will be throttled or rejected, causing each invocation to hang until it times out. This is a classic scaling anti-pattern on the Google Professional Cloud Developer exam: horizontal scaling can expose or worsen a bottleneck rather than fix it. The trap is assuming more instances always improve performance, but the real constraint is the external dependency’s capacity. A quick memory tip: “More instances, more requests—if the external service can’t keep up, timeouts pile up.”
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.
Exhibit
Refer to the exhibit.
Cloud Logging error log:
```json
{
"severity": "ERROR",
"message": "Execution of function myfunc timed out after 54000ms",
"function": "myfunc",
"execution_id": "abc123"
}
```
A Cloud Function (background function, event-driven) consistently logs this timeout error. The function processes messages from Pub/Sub. After increasing the max instances from 10 to 100, the error rate increases. What is the most likely cause of the timeouts?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
Cloud Logging error log:
```json
{
"severity": "ERROR",
"message": "Execution of function myfunc timed out after 54000ms",
"function": "myfunc",
"execution_id": "abc123"
}
```
A
The function depends on an external service that is rate-limited; scaling up causes more calls and timeouts
With more instances, more concurrent calls to the external service may exceed its rate limit, causing timeouts.
B
Increase memory allocation to speed up processing
Why wrong: Memory may help but not address the dependency issue; increased errors with scale suggests external limit.
C
Use a larger instance type (Cloud Functions does not have instance types)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The function depends on an external service that is rate-limited; scaling up causes more calls and timeouts
Option A is correct because increasing the max instances from 10 to 100 amplifies the number of concurrent function invocations. If the function depends on an external service (e.g., a third-party API or database) that enforces rate limits, the higher concurrency causes more requests to be throttled or rejected, leading to increased timeouts. This is a classic scaling anti-pattern where horizontal scaling exacerbates a bottleneck instead of relieving it.
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.
✓
The function depends on an external service that is rate-limited; scaling up causes more calls and timeouts
Why this is correct
With more instances, more concurrent calls to the external service may exceed its rate limit, causing timeouts.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Increase memory allocation to speed up processing
Why it's wrong here
Memory may help but not address the dependency issue; increased errors with scale suggests external limit.
✗
Use a larger instance type (Cloud Functions does not have instance types)
Why it's wrong here
Cloud Functions uses resource-based scaling (CPU/memory), not instance types.
✗
Migrate the function to Cloud Run for longer timeouts
Why it's wrong here
Cloud Run also has timeouts and does not solve the scaling dependency issue.
✗
The function timeout is set too low; increase it to 9 minutes
Why it's wrong here
Increasing timeout may mask the problem but not fix the root cause; also, error rate increasing with instances suggests another issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that scaling up instances always improves performance, when in reality it can worsen timeouts if the bottleneck is an external dependency with fixed capacity or rate limits.
Detailed technical explanation
How to think about this question
Under the hood, Cloud Functions uses a load balancer and autoscaler that creates new instances based on the number of concurrent requests. When max instances is raised, the autoscaler can spawn more instances simultaneously, each making independent calls to the external service. If the external service enforces a global rate limit (e.g., 100 requests per second), the aggregated request rate from 100 instances can easily exceed that limit, causing HTTP 429 or connection timeouts. This is a common failure mode in event-driven architectures where downstream capacity is not elastic.
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: The function depends on an external service that is rate-limited; scaling up causes more calls and timeouts — Option A is correct because increasing the max instances from 10 to 100 amplifies the number of concurrent function invocations. If the function depends on an external service (e.g., a third-party API or database) that enforces rate limits, the higher concurrency causes more requests to be throttled or rejected, leading to increased timeouts. This is a classic scaling anti-pattern where horizontal scaling exacerbates a bottleneck instead of relieving it.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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