Right function for a metric that grows steadily

I have a counter cart_transactions_messages_processed_total. Trying to set an alert, if the counter increases continually during/for an hour.

From the documentation, I learnt that, if the counter value drops the counter resets altogether, if so, how to take this into consideration while creating a rule ?

Will this be a right approach - increase(cart_transactions_messages_processed_total[1h]) ?

increase(cart_transactions_messages_processed_total[1h]) returns increase of your counter over an hour.

Based on your question, I assume you want to fire alert, if counter was increased every minute of some hour. If this is correct, you'll need something like this:

  - alert: ConstantIncrease
    expr: increase(cart_transactions_messages_processed_total[1m]) > 0
    for: 1h
    labels:
      severity: page
    annotations:
      summary: Counter is constantly increasing

Here, expression increase(cart_transactions_messages_processed_total[1m]) > 0 executed every evaluation_interval, and if it returns result for one hour, alert will be fired.

No This is not Right Approach

The "increase" function calculates the difference btw first & last value within the specified time range,which would not capture the steady increase over time.

Instead, you should use the "rate" function

rate(cart_transactions_messages_processed_total[1h])