Proposal

Scaling by unacknowledged messages and oldest message are good, reactive scaling metrics. If a queue is behind, both of these are good for responding to how much is currently in the queue. They are, however, reactive and likely can lead to constant scaling up and down to keep up with consistent message volumes.

It would be nice if there was a trigger based on the rate of incoming messages (pubsub.googleapis.com/topic/message_sizes) to keep up with current rate, and use a reactive metric like unacknowledged messages to scale up when things get behind.

Scaler Source

Google Pub/Sub incoming messages

Scaling Mechanics

Scale based on metric

Authentication Source

GCP

Anything else?

No response