We've already looked at how to setup Slack with the Alertmanager, and saw the default notification. Wouldn't it be nice to customise it?

I'm going to presume we already have a working Slack and Alertmanager setup.

Let's say we have two alerts firing with the labels {alertname="MyAlert",instance="host1"} and {alertname="MyAlert",instance="host2"} that are going to our Slack receiver, and we want to display all the instance labels.

A key idea with the Alertmanager is that a single notification can contain more than one alert. This allows for a great reduction in notification spam, and is what the group_by in the alertmanger.yml controls.

This means we can't simply use .Labels.instance to get to the instance label, as each alert that will be part of the notification has its own instance labels. Instead we need a for loop using the range action of Go templates.

  - name: slack_general
    - api_url: https://hooks.slack.com/services/XXXXXXXXXXXXXXX/XXXXXXXX
      channel: '#general'
      text: 'Instances: {{ range .Alerts }}{{ .Labels.instance }} {{ end }}'

This will produce a notification like:


Annotations can be accessed similarly to Labels. You now know enough to do some basic notification templating!