This PCA practice question tests your understanding of ensure solution and operations reliability. 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 organization wants to define an SLO for their API hosted on Cloud Endpoints. Which metric should they use as a Service Level Indicator (SLI) for availability?
Number of HTTP 5xx errors
Why wrong: Error count alone doesn't give a ratio of success; need denominator.
Request latency at the 99th percentile
Why wrong: Latency is an SLI for performance, not availability.
Ratio of HTTP 200 responses to total requests
This directly measures the availability of the API.
CPU utilization of backend instances
Why wrong: CPU utilization is internal, not a customer-facing SLI.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Ratio of HTTP 200 responses to total requests
For an availability SLO, the SLI must measure the proportion of successful requests. In Cloud Endpoints, availability is defined as the ratio of successful (HTTP 200) responses to total requests, as this directly reflects whether the API is functioning correctly. Option C is correct because it captures the fraction of requests that completed without error, which is the standard definition of availability in service-level monitoring.
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.
Number of HTTP 5xx errors
Why it's wrong here
Error count alone doesn't give a ratio of success; need denominator.
Request latency at the 99th percentile
Why it's wrong here
Latency is an SLI for performance, not availability.
Ratio of HTTP 200 responses to total requests
Why this is correct
This directly measures the availability of the API.
Related concept
Read the scenario before looking for a memorised answer.
CPU utilization of backend instances
Why it's wrong here
CPU utilization is internal, not a customer-facing SLI.
Common exam traps
Google Cloud often tests the distinction between availability and performance metrics, so the trap here is that candidates confuse latency (a performance SLI) with availability, or they mistakenly think that counting only server-side errors (5xx) is sufficient for an availability SLI, ignoring that availability is a ratio of successful to total requests.
Detailed technical explanation
Under the hood, Cloud Endpoints uses Envoy proxy to intercept all API requests and emits metrics such as `serviceruntime.googleapis.com/api/request_count` and `serviceruntime.googleapis.com/api/response_code`. The availability SLI is computed as the ratio of responses with status code 200 to total requests over a rolling window (e.g., 1 minute). A subtle behavior is that Cloud Endpoints treats HTTP 401 and 403 as successful from an availability perspective because they indicate the API gateway processed the request, whereas 5xx and timeouts count as failures. In a real-world scenario, if a backend is overloaded and returns 503 errors, the availability SLI would drop, triggering an SLO breach even if the API gateway itself is healthy.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Ensure solution and operations reliability — This question tests Ensure solution and operations reliability — Read the scenario before looking for a memorised answer..
The correct answer is: Ratio of HTTP 200 responses to total requests — For an availability SLO, the SLI must measure the proportion of successful requests. In Cloud Endpoints, availability is defined as the ratio of successful (HTTP 200) responses to total requests, as this directly reflects whether the API is functioning correctly. Option C is correct because it captures the fraction of requests that completed without error, which is the standard definition of availability in service-level monitoring.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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