Google ACE Practice Question: Ensuring Successful Operation of a Cloud Solution
You have a Pub/Sub subscription that is accumulating a backlog of messages. Which Cloud Monitoring metric should you alert on to detect this condition?
⚠ Common exam trap
ACE often tests the difference between rate metrics (sent, ack) and backlog age metrics; candidates may mistakenly choose ack_message_count thinking it reflects backlog, but it does not.
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
✓
pubsub.googleapis.com/subscription/oldest_unacked_message_age
The metric oldest_unacked_message_age measures the age of the oldest unacknowledged message in a subscription. A growing backlog directly causes this age to increase, making it the most reliable indicator of a subscription falling behind. Alerting on this metric allows you to detect when messages are not being processed in a timely manner.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
pubsub.googleapis.com/subscription/oldest_unacked_message_age
Why this is correct
This metric tracks the maximum age of the oldest message that has not yet been acknowledged by any subscriber for the subscription. It directly reflects backlog depth and consumer lag: when a subscription is accumulating a backlog, this value grows steadily because messages sit unacked for longer periods. It is the ideal signal for alerting on message processing delays because it captures the time dimension of the backlog, not just its size.
- ✗
pubsub.googleapis.com/subscription/sent_messages_count
Why it's wrong here
The sent_messages_count metric records the total number of messages delivered, i.e., sent from the subscription to its subscribers. It is a throughput measure of delivery attempts, not a measure of messages waiting in the subscription. A subscription can have a serious backlog while this metric shows a high value, because subscribers may receive messages but fail to process them quickly, so this metric does not expose accumulation.
- ✗
pubsub.googleapis.com/subscription/unacked_messages_by_region
Why it's wrong here
This metric shows the number of unacknowledged messages grouped by the region of the subscribers, giving a geographic breakdown of concurrent unacked messages. It lacks the time dimension, so it cannot indicate how long those messages have been waiting; a subscription could have a constant unacked count while the oldest message age climbs, which means the backlog is not fresh but elderly. Counting unacked messages without their age does not reveal accumulation trends, so it is insufficient for the stated need.
- ✗
pubsub.googleapis.com/subscription/ack_message_count
Why it's wrong here
The ack_message_count metric counts the number of acknowledgment messages sent by subscribers to confirm successful processing. It measures the subscriber's processing throughput, not the number of messages still awaiting acknowledgement. A rising ack count actually indicates that messages are being processed and removed from the subscription, so it would not flag a growing backlog; in fact, it could be high while a backlog still grows if the inflow of messages outpaces the ack rate.
Go deeper
Related to this question
Learn chapter
Cloud Monitoring and Logging
Key term
Alerting policy
An alerting policy is a set of rules that defines when to send notifications about a system condition that needs attention.
Key term
Cloud Monitoring
Cloud monitoring is the process of observing, measuring, and managing an organization's cloud infrastructure and applications to ensure performance, availability, and security.
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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 ACE 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 ACE exam.