Google ACE Practice Question: Ensuring Successful Operation of a Cloud Solution

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  The subscription's backlog size

  Why this is correct

  The subscription's backlog size is the primary indicator of delivery problems: it counts messages that have been published but not yet acknowledged. When troubleshooting a subscription that is not delivering, an ever-growing backlog means messages are arriving faster than the subscriber can process them, or the subscriber has stopped pulling entirely. Large backlog also correlates with slow processing and can help you decide whether to scale out subscribers or inspect subscriber logs.

• ✗

  The topic's retention duration

  Why it's wrong here

  The topic's retention duration controls how long Pub/Sub retains messages on the topic itself, independent of subscription delivery. Even if the retention window is very short, messages that exist when a subscriber pulls are still delivered at the same rate; retention only affects message durability after the window expires. Thus, changing topic retention cannot speed up or slow down message delivery, and it is not a valid cause when a subscription is stuck.

• ✓

  The subscriber's processing latency

  Why this is correct

  The subscriber's processing latency directly impacts how fast messages are drained from the subscription. If each message takes a long time to handle—due to heavy computation, external API calls, or slow I/O—the effective throughput drops, and the backlog grows even if the subscriber is healthy. High processing latency also increases the chance that the acknowledgment deadline expires, leading to redeliveries that further compound the slowdown.

• ✗

  The message ordering key

  Why it's wrong here

  The message ordering key ensures that messages sharing the same key are delivered to subscribers in the order they were published, but it does not govern delivery speed. Ordering requires the Pub/Sub service to maintain per-key sequencing, which can reduce parallelism, but an ordering key alone will not prevent a subscription from delivering messages. If delivery is slow, the cause is elsewhere, such as processing time or ack deadlines.

• ✓

  The acknowledgment deadline

  Why this is correct

  The acknowledgment deadline is the maximum time a subscriber has to ack a message before Pub/Sub redelivers it. A deadline that is too short for the subscriber's actual processing time causes premature redelivery, duplicate processing, and wasted effort, which can stall the subscription and inflate the backlog. Diagnosing this requires comparing the deadline to the observed subscriber latency; if redelivery counts are high, the deadline should be increased or processing time reduced.

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