The answer is a connection draining timeout that is too short. This is correct because connection draining allows existing in-flight requests to complete before an instance is removed from the load balancer’s pool; if the timeout is set too low, active requests are abruptly terminated, resulting in 502 errors even when health checks pass successfully. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how Cloud Load Balancer manages instance lifecycle and graceful shutdowns—a common trap is to blame health checks or SSL configuration, but those would cause consistent failures or SSL-specific errors, not intermittent timeouts. The key distinction is that health checks confirm the instance is alive, while connection draining handles the graceful completion of existing traffic. Memory tip: think of “draining” as a slow sink—if you pull the plug too fast (short timeout), water (requests) spills over, causing 502 errors.
PCD Deploying applications Practice Question
This PCD practice question tests your understanding of deploying 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.
An application running on Compute Engine instances behind a Cloud Load Balancer experiences intermittent 502 errors. The health checks pass but sometimes requests time out. What is the most likely cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The connection draining timeout is too short.
Option D is correct because if connection draining timeout is too short, in-flight requests are terminated causing 502 errors even if health check passes. Option A could cause but not specifically 502. Option B would cause SSL errors. Option C would cause consistent failure.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 load balancer is misconfigured with wrong backend type.
Why it's wrong here
Misconfiguration would likely cause consistent failures.
✗
The backend instances are running out of memory.
Why it's wrong here
Memory exhaustion may cause other errors, not specifically 502.
✗
The SSL certificate has expired.
Why it's wrong here
SSL certificate issues result in TLS errors, not 502.
✓
The connection draining timeout is too short.
Why this is correct
Short connection draining timeout causes in-flight requests to be terminated, leading to 502 errors.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCD NAT questions on configuration and troubleshooting.
Deploying applications — This question tests Deploying applications — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The connection draining timeout is too short. — Option D is correct because if connection draining timeout is too short, in-flight requests are terminated causing 502 errors even if health check passes. Option A could cause but not specifically 502. Option B would cause SSL errors. Option C would cause consistent failure.
What should I do if I get this PCD question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCD NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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