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
Deleting the subscription loses the backlog and is not a solution. Increasing retention duration does not reduce backlog.
- ✗
Reduce the acknowledgment deadline to force faster processing.
Why it's wrong here
Reducing the deadline may cause messages to be redelivered prematurely, increasing duplicates and potentially slowing down processing due to repeated messages.
- ✗
Change the subscription type from push to pull.
Why it's wrong here
The type of subscription is not likely the root cause; both push and pull can handle high throughput if subscribers are adequate.
- ✓
Increase the number of subscribers or the throughput capacity of the existing subscribers.
Why this is correct
Adding more subscribers (e.g., scaling out Dataflow workers) increases the rate of message processing, reducing the backlog.
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