Define the guarantee precisely
Pub/Sub exactly-once delivery addresses a subscription delivery contract under documented conditions. It is not a promise that a business action can never happen twice. Two separate publishes of the same business event can have different message identifiers, and an application can repeat an external effect before it knows whether a previous attempt completed.
The guarantee also has a regional boundary for subscribers. A fleet spread across regions without a deliberate connection strategy can fall outside the intended guarantee. Use the supported pull subscription and client-library features, including acknowledgment result support, and verify the actual endpoint configuration. A checkbox alone does not establish the whole delivery path.
Prepare the subscription settings, subscriber locations, endpoint configuration, client library versions, and a test event ledger. Identify how the application records successful processing and handles acknowledgment errors. Keep a stable business identifier even when adopting this feature; transport message identity and business identity serve different purposes.
Verify the supported setup
- Inspect
gcloud pubsub subscriptions describe SUBSCRIPTION --project=PROJECT_ID. Confirm exactly-once delivery is enabled on the intended subscription and that the consumption mode supports it. Check whether ordering is also enabled because the combination adds coordination requirements. - Map every subscriber deployment and its connection endpoint. For workloads outside Google Cloud or spread across regions, use the endpoint strategy documented for keeping subscribers in one region. Review current regional versus locational endpoint semantics rather than assuming every location-looking hostname has identical guarantees.
- Use a supported client library and the acknowledgment method that exposes success or failure. Treat expired or invalid acknowledgment identifiers as a state to handle, not an exception to suppress. Record processing and acknowledgment outcomes without logging sensitive message bodies.
- Test region-consistent delivery and controlled acknowledgment failure in staging. Verify how the application behaves if it finishes work but does not obtain a successful acknowledgment response. Preserve business-level repeat detection for uncertain effects and independently duplicated publishes.
Illustrative scenario
A hypothetical entitlement processor has active subscribers in two regions using the default global service endpoint. Its owners enable exactly-once delivery and expect all repeats to disappear. The review finds both a regional connection issue and a producer that occasionally publishes the same business event twice. The team corrects the subscriber strategy and retains a business event ledger. Those actions address two distinct sources of repeated work.
| Observation | Boundary to check | Correct interpretation |
|---|---|---|
| Fleet spans regions | Subscriber endpoint and location | Regional guarantee needs deliberate configuration |
| Same business event has different message IDs | Publisher retry and event identity | Delivery feature does not merge separate publishes |
| Ack call reports an error | Ack deadline and supported library | Delivery completion is not yet established |
| Latency rises with ordering enabled | Coordination and per-key progress | Measure the combined feature's behavior |
Verify the useful outcome
Publish uniquely identified test events and reconcile publisher records, subscriber processing, and successful acknowledgment results. Test a repeated publish with the same business identifier to demonstrate the application's separate protection. Confirm that monitoring can distinguish a legitimate redelivery after unsuccessful acknowledgment from an unexpected repeat after acknowledged delivery. Include subscriber restarts in the test without creating production side effects.
Operating procedure
Monitor acknowledgment failures, expired deadlines, subscriber region drift, and business duplicate detection. Review endpoint configuration when introducing disaster recovery or active-active deployments. Recheck client support after major library changes. Document exactly which repeated-work cases the feature addresses and which remain an application responsibility so operational expectations stay testable.
Explore Clouvenix cloud architecture for implementation and operational handover.
Technical reference
exactly-once delivery, endpoint behavior, and subscriber troubleshooting.