Add blog post about Alertmanager templating

content/blog/2016-03-03-custom-alertmanager-templates.md

+---
+title: Custom Alertmanager Templates
+created_at: 2016-03-03
+kind: article
+author_name: Fabian Reinartz
+---
+
+The Alertmanager handles alerts sent by Prometheus servers and sends
+notifications about them to different receivers based on their labels.
+
+A receiver can be one of many different integrations such as PagerDuty, Slack,
+email, or a custom integration via the generic webhook interface (for example [JIRA](https://github.com/fabxc/jiralerts)).
+
+## Templates
+
+The messages sent to receivers are constructed via templates.
+Alertmanager comes with default templates but also allows defining custom
+ones.
+
+In this blog post, we will walk through a simple customization of Slack
+notifications.
+
+We use this simple Alertmanager configuartion that sends all alerts to Slack:
+
+```yaml
+global:
+  slack_api_url: '<slack_webhook_url>'
+
+route:
+  receiver: 'slack-notifications'
+  # All alerts in a notification have the same value for these labels.
+  group_by: [alertname, datacenter, app]
+  
+receivers:
+- name: 'slack-notifications'
+  slack_configs:
+  - channel: '#alerts'
+```
+
+By default, a Slack message sent by Alertmanager looks like this:
+
+![](/assets/blog/2016-03-03/slack_alert_before.png)
+
+It shows us that there are 3 firing alerts, followed by the label values of
+the alert grouping (alertname, datacenter, app) and further label values the
+alerts have in common (critical).
+
+## Customize 
+
+If you have alerts, you should also have documentation on how to handle them –
+a runbook. A good approach to that is having a wiki that has a section for
+each app you are running with a page for each alert.
+
+Suppose we have such a wiki running at `https://internal.myorg.net/wiki/alerts`.
+Now we want links to these runbooks shown in our Slack messages.
+
+In our template, we need access to the "alertname" and the "app" label. Since
+these are labels we group alerts by, they are available in the `GroupLabels`
+map of our templating data.
+
+We can directly add custom templating to our Alertmanager's [Slack configuration](/docs/alerting/configuration/#slack-receiver-slack_config)
+that is used for the `text` section of our Slack message.
+The [templating language](https://godoc.org/text/template) is the one provided
+by the Go programming language.
+
+```yaml
+global:
+  slack_api_url: '<slack_webhook_url>'
+  
+route:
+- receiver: 'slack-notifications'
+  group_by: [alertname, datacenter, app]
+  
+receivers:
+- name: 'slack-notifications'
+  slack_configs:
+  - channel: '#alerts'
+    # Template for the text field in Slack messages.
+    text: 'https://internal.myorg.net/wiki/alerts/{{ .GroupLabels.app }}/{{ .GroupLabels.alertname }}'
+```
+
+We reload our Alertmanager by sending a `SIGHUP` or restart it to load the
+changed configuration. Done.
+
+Our Slack notifications now look like this:
+
+![](/assets/blog/2016-03-03/slack_alert_after.png)
+
+### Template files
+
+Alternatively, we can also provide a file containing named templates, which
+are then loaded by Alertmanager. This is especially helpful for more complex
+templates that span many lines.
+
+We create a file `/etc/alertmanager/templates/myorg.tmpl` and create a
+template in it named "slack.myorg.text":
+
+```
+{{ define "slack.myorg.text" }}https://internal.myorg.net/wiki/alerts/{{ .GroupLabels.app }}/{{ .GroupLabels.alertname }}{{ end}}
+```
+
+Our configuration now loads the template with the given name for the "text"
+field and we provide a path to our custom template file:
+
+```yaml
+global:
+  slack_api_url: '<slack_webhook_url>'
+  
+route:
+- receiver: 'slack-notifications'
+  group_by: [alertname, datacenter, app]
+
+receivers:
+- name: 'slack-notifications'
+  slack_configs:
+  - channel: '#alerts'
+    text: '{{ template "slack.myorg.text" . }}{{ end }}'
+
+templates:
+- '/etc/alertmanager/templates/myorg.tmpl'
+```
+
+We reload our Alertmanager by sending a `SIGHUP` or restart it to load the
+changed configuration and the new template file. Done.