Prometheus alert evaluation, are they instant queries?

[Andrew Dedesko]

Hi,

I'm wondering whether prometheus uses instant queries or range queries when evaluating alert expressions?  The context about why I'm asking might help clarify my question.

I'm comparing Grafana Cloud's alerting functionality with prometheus.  From Grafana Cloud we're querying Google Cloud Metrics with PromQL (it's Google's Monarch DB with a PromQL interface).  Grafana Cloud's alerting system takes your PromQL query and performs a range query against Google Cloud Metrics, returning multiple data points over the range you have selected (e.g. 10 minutes ago to now).  Then you need to choose a reducer function to turn the time series into an instant scalar (e.g. min, max, last, mean).

Prometheus alerts don't seem to have an option for specifying a range and also don't have a reducer option.  So this leads me to believe prometheus uses instant queries to evaluate alert expressions.  But I'd like to know for sure.

Thanks for reading!

Here's the Grafana Cloud documentation on alert query ranges and reducers:
https://grafana.com/docs/grafana/latest/alerting/alerting-rules/create-grafana-managed-rule/

[Ben Kochie]

All rule evaluations are instant queries. You do all the "reducer functions" in PromQL itself.

For example, you can use subquery syntax to do something like `avg_over_time()`.

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[Brian Candler]

Even without a subquery, a rule can include a range vector expression and then reduce it to an instant vector, e.g.

expr: avg_over_time(snmp_scrape_duration_seconds[5m]) >= 3

[Ben Kochie]

Yes, that Grafana API seems to be unnecessary complicated compared to just using PromQL.

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[Andrew Dedesko]

That's a good point, Ben and Brian!  And thank you both for clarifying!

[Bryan Boreham]

Grafana can alert off 200 different data sources, most of which do not support PromQL, so it is necessary to offer more features in the client.