PDE Maintaining and Automating Data Workloads Practice Question
A data engineer needs to alert when Pub/Sub subscription has messages older than 1 hour. Which Cloud Monitoring metric and filter should they use?
⚠ Common exam trap
PDE often tests the distinction between message count and message age metrics, tempting candidates to choose num_undelivered_messages when the requirement is about age.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Metric: subscription/oldest_unacked_message_age; filter: subscription_id
The metric subscription/oldest_unacked_message_age directly reports the age of the oldest unacknowledged message in a subscription, which is exactly what's needed to alert when messages are older than 1 hour. The filter subscription_id scopes the metric to the specific subscription. This metric is designed for monitoring message backlog age and is the correct choice for this requirement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Metric: topic/send_message_operation_count; filter: topic_id
Why it's wrong here
topic/send_message_operation_count measures publish operations on the topic, not delivery latency at the subscription, so it cannot detect messages older than one hour. It is tempting because it monitors Pub/Sub activity, and it is the correct metric for alerting on publish failures or send operation rates.
- ✗
Metric: subscription/ack_message_count; filter: subscription_id
Why it's wrong here
ack_message_count counts acknowledgements over a period, revealing throughput rather than how long the oldest unacknowledged message has waited. It is tempting because low acknowledgement rates suggest stuck messages, and it is the correct metric for alerting on acknowledgement rate or consumer throughput degradation.
- ✗
Metric: subscription/num_undelivered_messages; filter: subscription_id
Why it's wrong here
num_undelivered_messages counts backlog size, not message age, so it cannot detect messages older than one hour. It is tempting because a large backlog often correlates with staleness, and it is the correct metric for alerting on queue depth or volume thresholds rather than age.
- ✓
Metric: subscription/oldest_unacked_message_age; filter: subscription_id
Why this is correct
The `subscription/oldest_unacked_message_age` metric reports the age of the oldest unacknowledged message, directly satisfying the one-hour staleness threshold. Filtering by `subscription_id` scopes the alert to the specific Pub/Sub subscription, so Cloud Monitoring evaluates only that subscription's backlog age rather than aggregating across the project.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Google Cloud exam blueprint
This PDE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PDE exam.