I have a Google Cloud Function subscribed to a topic. Our GCP Pub/Sub publishes a message to the topic when cloud scheduler invoke GCP Pub/Sub each 5 minutes. The problem is that the cloud functions gets sometimes invoked twice 90s after invoking first one.
The acknowledgement deadline on the subscription is 600 seconds. So, I can't figure it out why GCF is invoked twice in 90s by GCP Pub/Sub. Does invoking twice 90s after related to something?
Your duplicate could either be on the publish side or on the subscribe side. If the duplicate messages have different message IDs, then your duplicates are generated on the publish side. This could be caused by retries on the publish side in response to retryable errors. If the messages have the same message ID, then the duplication is on the subscribe side within Pub/Sub.
Cloud Pub/Sub offers at-least-once delivery semantics. That means it is possible for duplicates to occur, even if you acknowledge the message and even if the acknowledgement deadline has not passed. If you want stronger guarantees around delivery, you can use Pub/Sub's exactly once feature, which is currently in public preview. However, this will require you to set up your Cloud Function with an HTTP trigger and to create a push subscription in Pub/Sub that points to the address of the function because there is no way to set the exactly once setting on a subscription created by Cloud Functions.
