PDE Maintaining and Automating Data Workloads Practice Question
A company uses Cloud Pub/Sub for a real-time data pipeline. The subscription has a backlog of millions of messages that are not being processed quickly enough. In Cloud Monitoring, you observe that the 'subscription/num_undelivered_messages' metric is high and growing, while 'subscription/oldest_unacked_message_age' is also increasing. Which action is MOST likely to reduce the backlog?
⚠ Common exam trap
A common mistake is to think that reducing the acknowledgment deadline or changing subscription type will speed up processing, when in reality these actions can increase redeliveries or do not address the root cause of insufficient subscriber capacity.
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
✓
Increase the number of subscribers or the throughput capacity of the existing subscribers.
The backlog indicates that subscribers cannot keep up with the message flow. Increasing the number of subscribers or scaling their throughput capacity directly addresses the processing bottleneck, allowing messages to be pulled and acknowledged faster. Cloud Pub/Sub scales horizontally, so adding more pull subscribers or increasing the resources of existing ones (e.g., more worker threads, higher CPU/memory) reduces the backlog.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Delete the subscription and recreate it with a larger message retention duration.
Why it's wrong here
Recreating the subscription resets its backlog and discards undelivered messages; retention duration governs how long unacked messages persist, not how fast they are consumed. It is tempting because a fresh subscription appears to clear the backlog. Increasing subscriber throughput or concurrency addresses the growing undelivered count and message age.
- ✗
Reduce the acknowledgment deadline to force faster processing.
Why it's wrong here
Shortening the acknowledgment deadline causes Pub/Sub to redeliver unacked messages sooner, multiplying duplicate work and worsening the backlog. It is tempting because a shorter deadline does surface stuck messages faster, and it is the right lever when a single consumer hangs indefinitely rather than when throughput is simply too low.
- ✗
Change the subscription type from push to pull.
Why it's wrong here
Switching push to pull changes only the delivery mechanism, not consumer throughput, so the backlog and oldest unacked message age keep growing. It is tempting because pull suits high-volume batch consumers, and it would be correct when the existing push endpoint is rate-limited or returning errors that throttle delivery.
- ✓
Increase the number of subscribers or the throughput capacity of the existing subscribers.
Why this is correct
Scaling subscriber count or per-subscriber throughput directly raises the rate at which messages are pulled and acknowledged, so the undelivered-message count and oldest-unacked age both fall. The backlog stems from consumption capacity lagging behind publish rate, not from retention or delivery configuration, so adding consumer capacity addresses the actual constraint.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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