am-executor

prometheus-am-executor

The prometheus-am-executor is a HTTP server that receives alerts from the Prometheus Alertmanager and executes a given command with alert details set as environment variables.

Usage

Usage: ./prometheus-am-executor [options] script [args..]

  -l string
    	HTTP Port to listen on (default ":8080")
  -v	Enable verbose/debug logging

The executor runs the provided script with the following environment variables set:

• AMX_RECEIVER: name of receiver in the AM triggering the alert
• AMX_STATUS: alert status
• AMX_EXTERNAL_URL: URL to reach alertmanager
• AMX_ALERT_LEN: Number of alerts; for iterating through AMX_ALERT_<n>.. vars
• AMX_LABEL_<label>: alert label pairs
• AMX_GLABEL_<label>: label pairs used to group alert
• AMX_ANNOTATION_<key>: alert annotation key/value pairs
• AMX_ALERT_<n>_STATUS: status of alert
• AMX_ALERT_<n>_START: start of alert in seconds since epoch
• AMX_ALERT_<n>_END: end of alert, 0 for firing alerts
• AMX_ALERT_<n>_URL: URL to metric in prometheus
• AMX_ALERT_<n>_LABEL_<label>: alert label pairs
• AMX_ALERT_<n>_ANNOTATION_<key>: alert annotation key/value pairs

Example: Reboot systems with errors

Sometimes a system might exhibit errors that require a hard reboot. This is an example on how to use Prometheus and prometheus-am-executor to reboot a machine a machine based on a alert while making sure enough instances are in service all the time.

Let assume the counter app_errors_unrecoverable_total should trigger a reboot if increased by 1. To make sure enough instances are in service all the time, the reboot should only get triggered if at least 80% of all instances are reachable in the load balancer. A alerting expression would look like this:

ALERT RebootMachine IF
	increase(app_errors_unrecoverable_total[15m]) > 0 AND
	avg by(backend) (haproxy_server_up{backend="app"}) > 0.8

This will trigger an alert RebootMachine if app_errors_unrecoverable_total increased in the last 15 minutes and there are at least 80% of all servers for backend app up.

Now the alert needs to get routed to prometheus-am-executor like in this alertmanager config example.

Finally prometheus-am-executor needs to be pointed to a reboot script:

./prometheus-am-executor examples/reboot

As soon as the counter increases by 1, an alert gets triggered and the alertmanager routes the alert to prometheus-am-executor which executes the reboot script.

Caveats

To make sure a system doesn't get rebooted multiple times, the repeat_interval needs to be longer than interval used for increase(). As long as that's the case, prometheus-am-executor will run the provided script only once.

increase(app_errors_unrecoverable_total[15m]) takes the value of app_errors_unrecoverable_total 15 minutes ago to calculate the increase, it's required that the metric already exists before the counter increase happens. The Prometheus client library sets counters to 0 by default, but only for metrics without dynamic labels. Otherwise the metric only appears the first time it is set. The alert won't get triggered if the metric uses dynamic labels and was incremented the very first time (the increase from 'unknown to 0). Therefor you need to initialize all error counters with 0.

Since the alert gets triggered if the counter increased in the last 15 minutes, the alert resolves after 15 minutes without counter increase, so it's important that the alert gets processed in those 15 minutes or the system won't get rebooted.