This guide explains how to craft Alertmanager templates to format alert messages, improving clarity and presentation. Alertmanager uses Go’s text/template engine with additional helper functions.

• Alerting rules reference: https://www.dogfei.cn/archives/alertmanager-rule
• Prometheus configuration overview: https://www.dogfei.cn/archives/prometheus-config

Go template essentials used by Alertmanager

Alertmanager renders notifications with Go templates. The core syntax is documented here: https://golang.org/pkg/text/template/

The dot (.) data context

The root data passed to the template is acessible as .. You can traverse nested fields via chaining:

{{ .User.Profile.Age }}

Template-scoped variables

You can bind values to variables and reuse them:

{{ $who := "alex" }}
Hello, {{ $who }}

Defining named templates

{{ define "wechat.message.default" }}
  ...
{{ end }}

Conditionals (if / else if / else)

Falsy values include: false, 0, empty string, nil, empty list/map.

{{ if .Name }}
Hello {{ .Name }}
{{ else if .BackupName }}
Hello {{ .BackupName }}
{{ else }}
Hello, stranger
{{ end }}

Iteration with range

Assume a slice of structs:

type Person struct {
  Name string
  Age  int
}

Template iteration:

{{ range .People }}
- name: {{ .Name }}
  age: {{ .Age }}
{{ end }}

Comparisons

• eq: equal
• ne: not equal
• lt: less than
• le: less than or equal
• gt: greater than
• ge: greater than or equal

{{ if ge .CPU 80 }}
High CPU: {{ .CPU }}%
{{ end }}

Logical operators

• and: all operands true
• or: any operand true
• not: logical negation

{{ if and .Username (gt (len .Password) 8) }}
credentials look valid
{{ else }}
invalid credentials
{{ end }}

{{ if not .Authenticated }}
access denied
{{ end }}

Common helper functions

Useful functions supplied by Alertmanager include:

• title: capitalize words
• toUpper: upper-case letters
• toLower: lower-case letters
• join: concatenate with delimiter
• safeHtml: mark string as safe HTML (disable auto-escaping)
• len: length of strings, lists, maps

Examples:

{{ "alertmanager" | title }}
{{ "WARN" | toLower }}
{{ join "," (list "a" "b" "c") }}
{{ len .Alerts }}

Controlling whitespace

Trim whitespace using - inside delimiters:

{{- /* trim left */ -}}
{{- /* trim both sides */ -}}

Alert data available in templates

Alertmanager passes a full data object to templates. Commonly used fields:

• .Receiver: receiver name
• .Status: overall status (firing or resolved)
• .Alerts: list of alert objects
• .Alerts.Firing: subset of firing alerts
• .Alerts.Resolved: subset of resolved alerts
• .GroupLabels: labels used for grouping
• .CommonLabels: labels common to all alerts
• .CommonAnnotations: annotations common to all alerts
• .ExternalURL: Alertmanager UI address

Example payload shape

{
  "receiver": "webhook",
  "status": "firing",
  "alerts": [
    {
      "status": "firing",
      "labels": {
        "alertname": "NodeMemoryHigh",
        "instance": "10.127.92.100:9100",
        "job": "node",
        "severity": "warning",
        "team": "ops"
      },
      "annotations": {
        "summary": "High memory usage",
        "description": "Memory utilization above 55% (current: 58%)"
      },
      "startsAt": "2020-12-30T07:20:08.775177336Z",
      "endsAt": "0001-01-01T00:00:00Z",
      "generatorURL": "http://prometheus:9090/graph?...",
      "fingerprint": "09f94bd1aa7da54f"
    }
  ],
  "groupLabels": { "alertname": "NodeMemoryHigh" },
  "commonLabels": {
    "alertname": "NodeMemoryHigh",
    "job": "node",
    "severity": "warning",
    "team": "ops"
  },
  "commonAnnotations": { "summary": "High memory usage" },
  "externalURL": "http://alertmanager:9093",
  "version": "4",
  "groupKey": "{}:{alertname=\"NodeMemoryHigh\"}",
  "truncatedAlerts": 0
}

Template example for Enterprise WeChat

Reference (default template): https://raw.githubusercontent.com/prometheus/alertmanager/master/template/default.tmpl

The following template renders separate sections for firing and resolved alerts, with timestamps adjusted to Biejing time (UTC+8). The offset 28800e9 is 28,800 seconds in nanoseconds.

{{ define "wechat.message.default" }}
{{- $tz := 28800e9 -}}

{{- if gt (len .Alerts.Firing) 0 -}}
{{- range $i, $a := .Alerts.Firing -}}
========== Alert Triggered ==========
Type:     {{ $a.Labels.alertname }}
Severity: {{ $a.Labels.severity }}
Summary:  {{ $a.Annotations.summary }}
Details:  {{ $a.Annotations.description }}
Started:  {{ ($a.StartsAt.Add $tz).Format "2006-01-02 15:04:05" }}
{{- if $a.Labels.instance }}
Instance: {{ $a.Labels.instance }}
{{- end }}
================= END ===============
{{ end -}}
{{- end -}}

{{- if gt (len .Alerts.Resolved) 0 -}}
{{- range $j, $r := .Alerts.Resolved -}}
======== Alert Recovered =========
Type:     {{ $r.Labels.alertname }}
Severity: {{ $r.Labels.severity }}
Summary:  {{ $r.Annotations.summary }}
Details:  {{ $r.Annotations.description }}
Started:  {{ ($r.StartsAt.Add $tz).Format "2006-01-02 15:04:05" }}
Ended:    {{ ($r.EndsAt.Add $tz).Format   "2006-01-02 15:04:05" }}
{{- if $r.Labels.instance }}
Instance: {{ $r.Labels.instance }}
{{- end }}
============== END ==============
{{ end -}}
{{- end -}}
{{ end }}

Using the template in Alertmanager

Reference the named template in the receiver configuration. Ensure your template file is loaded via the templates directive in Alertmanager’s configuration.

# Example: alertmanager.yml (snippet)
receivers:
  - name: wechat
    wechat_configs:
      - send_resolved: true
        message: '{{ template "wechat.message.default" . }}'
        to_party: "<party-id>"
        agent_id: "<agent-id>"
        to_user: "<user-id>"
        api_secret: "<corp-secret>"