You need to set up monitoring for a Vertex AI model that serves predictions in real-time. The model is expected to have a latency SLA of under 100ms. Which metric should you configure an alert on to ensure the SLA is met?
A latency SLA is a tail-latency commitment, so p99 captures the slowest 1% of prediction requests that breach the 100ms threshold. Alerting on p99 detects SLA violations that average latency would hide behind fast responses.
Why this answer
P99 latency measures the worst-case latency experienced by 99% of requests, which is the standard metric for enforcing a strict SLA like under 100ms. Monitoring p99 ensures that even the slowest 1% of requests do not violate the threshold, providing a robust guarantee for real-time predictions.
Exam trap
Google Cloud often tests the misconception that median (p50) latency is sufficient for SLAs, but the trap is that SLAs require tail-latency guarantees (p99 or p999) to catch performance outliers that violate the threshold.
How to eliminate wrong answers
Option A is wrong because p50 latency (median) ignores the tail latency, meaning half of the requests could exceed 100ms without triggering an alert, failing the SLA. Option B is wrong because prediction drift score measures changes in model input/output distributions over time, not latency, and is irrelevant for SLA compliance. Option D is wrong because the number of prediction requests per second (throughput) does not measure individual request latency; high throughput can occur even if latency spikes above 100ms.