Google PubSub - Counting messages in topic
I've looked over the documentation for Google's PubSub, and also tried looking in Google Cloud Monitoring, but couldn't find any means of figuring out what's the queue size in my topics.
Since I plan on using PubSub for analytics, it's important for me to monitor the queue count, so I could scale up/down the subscriber count.
What am I missing?
Answer
The metric you want to look at is "undelivered messages." You should be able to set up alerts or charts that monitor this metric in Google Cloud Monitoring under the "Pub/Sub Subscription" resource type. The number of messages that have not yet been acknowledged by subscribers, i.e., queue size, is a per-subscription metric as opposed to a per-topic metric. For info on the metric, see pubsub.googleapis.com/subscription/num_undelivered_messages in the GCP Metrics List (and others for all of the Pub/Sub metrics available).