Why is my Google Pub/Sub topic not receiving automatic messages from the source while manual publish works?

Below are some steps and points to help diagnose why your source stopped publishing messages and to ensure everything is working as expected.

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1. Verify the Source

Since manual publishing works, the most likely culprit is that the source isn’t triggering as expected:

• Check Logs:
  Look at logs for your source (e.g., Cloud Function or Cloud Scheduler). Verify if there are any errors or if the function isn’t being invoked at all. For Cloud Functions, check Cloud Logging; for Cloud Scheduler, examine its job history and look for execution errors or misconfigurations.

• Health & Status:
  If your source is an external service, verify connectivity and any potential authentication/authorization issues that might prevent it from publishing. If it integrates with a third-party system, confirm it’s still sending events as expected.

• Configuration Changes:
  Review recent changes to configurations or code. Sometimes a small update might disable or alter the triggering behavior of the source.

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2. Check Service Account and Permissions

Even though your service account appears to have Owner privileges, it is a good idea to:

• Double-check IAM Settings:
  Ensure there haven’t been any recent policy changes that could interfere with the role. Although not likely if you have Owner, sometimes organization-level policies or VPC Service Controls might affect operations.

• Audit Logs:
  Check the Cloud Audit Logs for denied operations or errors when invoking the publish step. This can trigger clues if there are any permission-related issues not immediately obvious.

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3. Audit Pub/Sub Quota and Limits

Even though your quota usage is zero, ensure:

• Quota Resets:
  Confirm that no quota limits are being hit indirectly. For example, if there is a misconfiguration that prevents the asynchronous or repeated triggers from occurring.

• Undocumented/Edge Cases:
  There are no widely reported undocumented limitations in Pub/Sub that would cause the behavior you describe. However, it’s a good idea to check the Pub/Sub status dashboard or Google Cloud Status page in case of any ongoing service disruptions or outages.

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4. Reproduce the Flow End-to-End

• Test the Source in Isolation:
  Manually trigger the source (if possible) and see if it attempts to publish to the topic. For example, if using a Cloud Function, invoke it with test parameters to see if the code path that publishes the message is being executed without error.

• Instrument Additional Logging:
  If possible, add logging around the point where the source publishes to Pub/Sub. That extra logging could help confirm that the code is reaching the publish call and what the response is from the Pub/Sub API.

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5. Check Event Trigger Configurations

• Trigger Conditions:
  Look for any conditionals or error handling in your source that might be skipping the publish operation. For instance, if there are preconditions or environment variables that might have changed.

• Network and Connectivity:
  Ensure the source can reach the Pub/Sub API endpoint. Network issues (e.g., misconfigured VPC Service Controls or firewall rules) might prevent communication despite service account permissions being correct.

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Summary

1. Investigate source logs (Cloud Function, Cloud Scheduler, or external service) to ensure that the trigger is firing and that no errors occur during invocation.
2. Examine audit logs for any permission or connectivity errors.
3. Review recent changes to the source's configuration or deployment that might affect message publishing.
4. Test the source directly using forced execution and additional logging to isolate the issue.
5. Confirm network and environment settings, especially if your source needs to reach an external API endpoint.

By following these steps, you should be able to narrow down the issue. If the source still does not publish messages after these checks, consider reaching out to Google Cloud Support for further investigation and insights into any potential platform issues.