If you try and do max_over_time(rate(my_counter_total[5m])[1h]) or predict_linear(rate(my_counter_total[5m])[1d], 3600) in Prometheus it won't work. How can you combine these functions?


There are two general types of functions in PromQL that take timeseries as input, those that take a vector and return a vector (e.g. abs, ceil, hour, label_replace), and those that take a range vector and return a vector (e.g. rate, deriv, predict_linear, *_over_time).

There are no functions that take a range vector and return a range vector, nor is there a way to do any form of subquery (prior to Prometheus 2.7.0). Even with support for subqueries, you wouldn't want to use them regularly as they'd be expensive. So what to do instead?


The answer is to use a recording rule for the inner function, and then you can use the outer function on the time series it creates.

You can add the path of a file to read rules from to your prometheus.yml:

  - my_rules.rules


And put your rules in that file:

instance:my_counter:rate5m = rate(my_counter_total{job="myjob"}[5m])

# You can use this new metric now in rules, alerts or graphs.

instance:my_counter:predict_linear1d_1h_rate5m = 
    predict_linear(instance:my_counter:rate5m{job="myjob"})[1d], 3600)

One thing to keep in mind is the time ranges that the inner function operates on. A rate over ten minutes is going to be much smoother than a rate over one minute.

Finally even though you now know this technique don't forget, to always rate then sum!


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