Fix GCP Pub/Sub Ack Deadline Errors

When working with GCP Pub/Sub, you may encounter a configuration error that prevents your data pipeline or messaging system from working. This guide explains the most common mistake with ack deadline and shows the exact fix.

A Common Mistake

Setting the ack deadline too short for a subscription, causing messages to be redelivered before the subscriber finishes processing them.

The incorrect command:

gcloud pubsub subscriptions create my-sub --topic=my-topic --ack-deadline=10

Error output:

Created subscription with 10s ack deadline.
Message processing takes 30 seconds (includes API calls, data transformation, database writes).
At 10s: message not acked. Pub/Sub redelivers the message.
At 20s: message processed twice (original + redelivered).
At 30s: duplicate write to database.

The Correct Approach

The right way to configure ack deadline in GCP Pub/Sub:

gcloud pubsub subscriptions create my-sub --topic=my-topic --ack-deadline=60

Successful result:

Created subscription with 60s ack deadline.
Message processing takes 30 seconds.
At 30s: message acked successfully.
No redeliveries. No duplicate processing. The message is processed exactly once.

How to Prevent This

Set ack deadline to 2-3x your p99 processing time. Min: 10s, max: 600s. Modify ack deadline after creation with gcloud pubsub subscriptions update. Subscribers can modify ack deadline dynamically (modifyAckDeadline). Monitor undelivered messages with subscription/num_undelivered_messages metric.

FAQ

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