Google Cloud Functions to only Ack Pub/Sub on success (Problem resolved by GCP)

Question

An early version of Google Cloud Functions had a limitation with regards to retries when errors occurred. They have since provided enhancements that resolve this issue.

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We are using a cloud function triggered by Pub/Sub to ensure delivery of an e-mail. Sometimes the e-mail service takes a long time to respond and our cloud function terminates before we get an error back. Since the message has already been acknowledged our e-mail gets lost.

The cloud function appears to be sending an ACK the Pub/Sub message automatically when we are called. Is there a way to delay the ACK until the successful completion of our code? Alternatively is there a way to catch timeouts and requeue the message for delivery? Something else we could try?

Answer 1

I heard from Google support that they do not currently provide the means to delay the ACK when a cloud function is invoked by Pub/Sub. If you want to use cloud functions with Pub/Sub you need to handle the error case yourself. For example you could have your cloud function requeue a message for the retry with a retry count.

This would seem to make it unnecessarily difficult to guarantee execution with Pub/Sub and cloud functions.

Answer 2

This is a problem because Functions ACKing a message on invoke, even if they crash, prevents the use of the new "dead-letter" feature.

Also, it goes against the docs. see note after this code sample: https://cloud.google.com/functions/docs/calling/pubsub#sample_code