This PCA practice question tests your understanding of manage and provision cloud infrastructure. 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 global e-commerce site uses an external HTTPS load balancer with a backend service pointing to a managed instance group. Some users report 503 errors during peak traffic. The backend instances are healthy and not overloaded. 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.
The CDN cache is not warming up properly
Why wrong: CDN issues cause cache misses, not 503.
The backend service's health check interval is too short
Why wrong: Health check issues cause 502, not 503.
The SSL certificate is expired
Why wrong: Expired certificate causes SSL errors, not 503.
The load balancer's max rate per backend is configured too low
The load balancer enforces a rate limit at the backend level; exceeding it produces 503.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The load balancer's max rate per backend is configured too low
A 503 error from an external HTTPS load balancer with healthy backends typically indicates that the load balancer is throttling requests. The 'max rate per backend' setting limits the number of requests per second that the load balancer forwards to each backend instance. When this limit is exceeded, the load balancer returns 503 errors even though the instances themselves are not overloaded, which matches the scenario of peak traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The CDN cache is not warming up properly
Why it's wrong here
CDN issues cause cache misses, not 503.
The backend service's health check interval is too short
Why it's wrong here
Health check issues cause 502, not 503.
The SSL certificate is expired
Why it's wrong here
Expired certificate causes SSL errors, not 503.
The load balancer's max rate per backend is configured too low
Why this is correct
The load balancer enforces a rate limit at the backend level; exceeding it produces 503.
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.
Common exam traps
Google Cloud often tests the misconception that 503 errors always indicate backend overload or health check failures, when in fact the load balancer's rate limiting configuration can cause 503s with perfectly healthy instances.
Detailed technical explanation
In Google Cloud Load Balancing, the 'max rate per backend' is a per-instance rate limit (requests per second) enforced at the load balancer level. When the rate is exceeded, the load balancer queues or drops requests and returns HTTP 503, even if the backend has capacity. This is distinct from backend instance CPU/memory overload; it is a configuration-based throttle. Real-world tuning often requires monitoring load balancer logs for 'rateLimited' status codes and adjusting the limit based on peak traffic patterns.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Manage and provision cloud infrastructure — This question tests Manage and provision cloud infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: The load balancer's max rate per backend is configured too low — A 503 error from an external HTTPS load balancer with healthy backends typically indicates that the load balancer is throttling requests. The 'max rate per backend' setting limits the number of requests per second that the load balancer forwards to each backend instance. When this limit is exceeded, the load balancer returns 503 errors even though the instances themselves are not overloaded, which matches the scenario of peak traffic.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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