The answer is that a purge request failing to complete successfully is one of three key factors causing Cloud CDN to serve stale content after a purge, alongside a long Cache-Control: max-age header from the origin and the CDN’s asynchronous processing of purge requests. When a purge fails—due to network issues, invalid paths, or timeouts—the cached content remains untouched, so users continue receiving stale data despite the attempted invalidation. On the Google Professional Cloud Architect exam, this concept tests your understanding of CDN cache invalidation mechanics and common failure modes, often appearing as a trap where candidates assume all purges are instantaneous and reliable. A common memory tip is to remember the three culprits as “Max, Miss, and Fail”—the origin’s max-age overriding the purge, a missed or incomplete purge request, and the CDN’s asynchronous failure to apply the invalidation.
Google PCA Ensure solution and operations reliability Practice Question
This PCA practice question tests your understanding of ensure solution and operations reliability. 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 CDN to accelerate content delivery. They notice that some users receive stale content even after purging the cache. Which THREE factors could cause this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The purge request did not complete successfully.
Option B is correct because a purge request that does not complete successfully will leave cached content intact, causing users to receive stale data. Cloud CDN processes purge requests asynchronously, and if the request fails (e.g., due to network issues or invalid paths), the cache is not invalidated. This directly explains why stale content persists despite an attempted purge.
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 content is compressed with gzip.
Why it's wrong here
Compression does not affect cache staleness.
✓
The purge request did not complete successfully.
Why this is correct
Failed purge operations leave stale cache intact.
Related concept
Read the scenario before looking for a memorised answer.
✓
The content was cached at multiple edge locations and not all were purged.
Why this is correct
Purge may not propagate to all edges immediately; some edges may serve stale content.
Related concept
Read the scenario before looking for a memorised answer.
✗
The CDN is configured with signed URLs.
Why it's wrong here
Signed URLs control access, not cache freshness.
✓
The origin server returns a long Cache-Control: max-age header, causing the CDN to ignore the purge.
Why this is correct
Purge invalidates cache, but if the origin sets a long max-age, the CDN may re-cache the old content if the origin still serves it.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a purge is instantaneous and global, leading candidates to overlook that incomplete or failed purge requests can leave stale content at some edge locations.
Detailed technical explanation
How to think about this question
Cloud CDN uses a distributed cache with multiple edge locations, and purge requests are propagated via a global invalidation system that may take time to reach all edges. If a purge fails silently (e.g., due to a malformed URL or rate limiting), the cache remains unchanged. Additionally, the Cache-Control: max-age header does not override a purge; however, if the origin returns a very long max-age, the CDN may re-validate only after that period, effectively ignoring a purge if the origin still serves the same content (Option E is correct for this reason).
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this PCA question in full detail.
Ensure solution and operations reliability — This question tests Ensure solution and operations reliability — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The purge request did not complete successfully. — Option B is correct because a purge request that does not complete successfully will leave cached content intact, causing users to receive stale data. Cloud CDN processes purge requests asynchronously, and if the request fails (e.g., due to network issues or invalid paths), the cache is not invalidated. This directly explains why stale content persists despite an attempted purge.
What should I do if I get this PCA 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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