Note: I’m now using an updated SystemD service file at the bottom of this post.

Things go wrong with technology, it’s bound to happen. I needed a simple way to monitor scheduled jobs. The majority of these jobs are spawned systemd services via systemd timers.

While researching for a system I found healthchecks.io which allow you to host your own instance. The next step was to trigger a ping when a systemd service status changed. For this I leveraged OnSuccess/OnFailure hooks with a few custom services.

An example of a service file I’m monitoring:

[Unit]
Description=Example service
OnFailure=[email protected]
OnSuccess=[email protected]

[Service]
ExecStart=/bin/bash -c 'exit 0'
Type=oneshot

[Install]
WantedBy=multi-user.target

We pass the ping UUID for both [email protected] and [email protected] as an argument. These services simply run curl to ping the healthchecks.io instance:

/etc/systemd/system/[email protected]:

[Unit]
Description=Pings healthchecks (%i)

[Service]
ExecStart=/bin/bash -c 'curl -s "https://localhost:8000/ping/%i/fail"'
Type=oneshot

[Install]
WantedBy=multi-user.target


/etc/systemd/system/[email protected]:

[Unit]
Description=Pings healthchecks (%i)

[Service]
ExecStart=/bin/bash -c 'curl -vv "https://localhost:8000/ping/%i"'
Type=oneshot

[Install]
WantedBy=multi-user.target

Combined with the integration’s healthchecks.io offer I’m now notified when something goes wrong. I prefer the simplicity of this setup because it can be integrated with other tooling, for example Vorta/borg which I use for machine backups:

Vorta Post Hook

In future I may look at attaching logs and/or trying to migrate to a single healthcheck service.

Updated 2023-01-18

Based on a colleagues feedback I’m now using this updated SystemD template service file.

[Unit]
Description=Pings healthchecks (%i)

[Service]
ExecStart=/bin/bash -c 'IFS=: read -r UUID ACTION <<< "%i"; if [ "$ACTION" = "start" ]; then LOGS="" && EXIT_CODE="start"; else LOGS=$(journalctl --no-pager -n 50 -u $MONITOR_UNIT) && EXIT_CODE=$MONITOR_EXIT_STATUS; fi && curl -fSs -m 10 --retry 3 --data-raw "$LOGS" "https://localhost:8000/ping/$UUID/$EXIT_CODE"'
Type=oneshot

[Install]
WantedBy=multi-user.target

This allows me to use the same template file, whilst supporting the start and logging options of Healthchecks.io:

[Unit]
Description=Important service
OnFailure=[email protected]:failure.service
OnSuccess=[email protected]:success.service
Wants=[email protected]:start.service

The :failure, :success and :start are important as without them the $MONITOR_* environmental variables are not passed through to the service. See this quote from the manual:

$MONITOR_SERVICE_RESULT, $MONITOR_EXIT_CODE, $MONITOR_EXIT_STATUS, $MONITOR_INVOCATION_ID, $MONITOR_UNIT

Only defined for the service unit type. Those environment variables are passed to all ExecStart= and ExecStartPre= processes which run in services triggered by OnFailure= or OnSuccess= dependencies.

Variables $MONITOR_SERVICE_RESULT, $MONITOR_EXIT_CODE and $MONITOR_EXIT_STATUS take the same values as for ExecStop= and ExecStopPost= processes. Variables $MONITOR_INVOCATION_ID and $MONITOR_UNIT are set to the invocation id and unit name of the service which triggered the dependency.

Note that when multiple services trigger the same unit, those variables will be not be passed. Consider using a template handler unit for that case instead: “[email protected]%n.service” for non-templated units, or “[email protected]%p-%i.service” for templated units.