Set alert conditions

The alert condition defines when your alert fires. You’ll set a threshold that, when crossed, triggers the alert. For multi-dimensional queries, each series is evaluated independently—so one rule can generate alerts for multiple hosts or services.

To set your alert conditions, complete the following steps:

1. In the Define query and alert condition section, scroll to the Expressions area.

   Grafana automatically adds a Reduce expression and a Threshold expression to process your query results.

2. Configure the Reduce expression:

   • Function: Select how to reduce the time series to a single value.

     For example, select Last to use the most recent value, or Mean to use the average over the evaluation window.

   • Input: Should reference your query (typically A).

3. Configure the Threshold expression:

   • Input: Should reference your Reduce expression (typically B).

   • Condition: Select the comparison operator.

     Expand table

     If alerting on	Condition	Example
     High CPU	IS ABOVE	Fire when CPU > 80%
     Low disk space	IS BELOW	Fire when free space < 10GB
     Error rate	IS ABOVE	Fire when errors/min > 10
     Log count	IS ABOVE	Fire when error logs > 50 in 5m

   • Value: Enter your threshold value.

     Expand table

     Alert type	Example value	Notes
     CPU percentage	0.8 or 80	Depends on query output format
     Error rate	10	Errors per minute
     Log count	50	Errors in evaluation window

4. Click Preview to test your condition against current data.

   The preview shows which series would currently fire (if any) based on your threshold. Use this to validate your threshold is appropriate.

5. Adjust the threshold if needed based on the preview results.

   Tip: A good threshold gives you time to respond before the situation becomes critical. For metrics, if your normal CPU is 40-60%, alerting at 80% gives you warning. For logs, if your app normally logs 1-2 errors per minute, alerting at 10 catches real problems.

In the next milestone, you’ll configure how often Grafana evaluates this condition and how long it must persist before firing.