Configure Alerting Rules

Set up Prometheus alerting rules and Alertmanager for reliable, actionable incident notifications.

See Extended Examples for complete configuration files and templates.

When to Use

• Implementing proactive monitoring with automated incident detection
• Routing alerts to appropriate teams based on severity and service ownership
• Reducing alert fatigue through intelligent grouping and deduplication
• Integrating monitoring with on-call systems (PagerDuty, Opsgenie)
• Establishing escalation policies for critical production issues
• Migrating from legacy monitoring systems to Prometheus-based alerting
• Creating actionable alerts that guide responders to resolution

Inputs

• Required: Prometheus metrics to alert on (error rates, latency, saturation)
• Required: On-call rotation and escalation policies
• Optional: Existing alert definitions to migrate
• Optional: Notification channels (Slack, email, PagerDuty)
• Optional: Runbook documentation for common alerts

Procedure

Step 1: Deploy Alertmanager

Install and configure Alertmanager to receive alerts from Prometheus.

Docker Compose deployment (basic structure):

version: '3.8'
services:
  alertmanager:
    image: prom/alertmanager:v0.26.0
    ports:
      - "9093:9093"
    volumes:
      - ./alertmanager.yml:/etc/alertmanager/alertmanager.yml
    # ... (see EXAMPLES.md for complete configuration)

Basic Alertmanager configuration (alertmanager.yml excerpt):

global:
  resolve_timeout: 5m
  slack_api_url: 'https://hooks.slack.com/services/YOUR/SLACK/WEBHOOK'

route:
  receiver: 'default-receiver'
  group_by: ['alertname', 'cluster', 'service']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 4h

  routes:
    - match:
        severity: critical
      receiver: pagerduty-critical

# ... (see EXAMPLES.md for complete routing, inhibition rules, and receivers)

Configure Prometheus to use Alertmanager (prometheus.yml):

alerting:
  alertmanagers:
    - static_configs:
        - targets:
            - alertmanager:9093
      timeout: 10s
      api_version: v2

Got: Alertmanager UI accessible at http://localhost:9093, Prometheus "Status > Alertmanagers" shows UP status.

If fail:

• Check Alertmanager logs: docker logs alertmanager
• Verify Prometheus can reach Alertmanager: curl http://alertmanager:9093/api/v2/status
• Test webhook URLs: curl -X POST <SLACK_WEBHOOK_URL> -d '{"text":"test"}'
• Validate YAML syntax: amtool check-config alertmanager.yml

Step 2: Define Alerting Rules in Prometheus

Create alerting rules that fire when conditions are met.

Create alerting rules file (/etc/prometheus/rules/alerts.yml excerpt):

groups:
  - name: instance_alerts
    interval: 30s
    rules:
      - alert: InstanceDown
        expr: up == 0
        for: 5m
        labels:
          severity: critical
          team: infrastructure
        annotations:
          summary: "Instance {{ $labels.instance }} is down"
          description: "{{ $labels.instance }} has been down for >5min."
          runbook_url: "https://wiki.example.com/runbooks/instance-down"

      - alert: HighCPUUsage
        expr: 100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100) > 80
        for: 10m
        labels:
          severity: warning
        annotations:
          summary: "High CPU usage on {{ $labels.instance }}"
          # ... (see EXAMPLES.md for complete alerts)

Alert design best practices:

• for duration: Prevents flapping alerts. Use 5-10 minutes for most alerts.
• Descriptive annotations: Include current value, affected resource, and runbook link.
• Severity levels: critical (pages on-call), warning (investigate), info (FYI)
• Team labels: Enable routing to correct team/channel
• Runbook links: Every alert should have a runbook URL

Load rules into Prometheus:

# prometheus.yml
rule_files:
  - "rules/*.yml"

Validate and reload:

promtool check rules /etc/prometheus/rules/alerts.yml
curl -X POST http://localhost:9090/-/reload

Got: Alerts visible in Prometheus "Alerts" page, alerts fire when thresholds exceeded, Alertmanager receives fired alerts.

If fail:

• Check Prometheus logs for rule evaluation errors
• Verify rule syntax with promtool check rules
• Test alert queries independently in Prometheus UI
• Inspect alert state transitions: Inactive → Pending → Firing

Step 3: Create Notification Templates

Design readable, actionable notification messages.

Create template file (/etc/alertmanager/templates/default.tmpl excerpt):

{{ define "slack.default.title" }}
[{{ .Status | toUpper }}] {{ .GroupLabels.alertname }}
{{ end }}

{{ define "slack.default.text" }}
{{ range .Alerts }}
*Alert:* {{ .Labels.alertname }}
*Severity:* {{ .Labels.severity }}
*Summary:* {{ .Annotations.summary }}
{{ if .Annotations.runbook_url }}*Runbook:* {{ .Annotations.runbook_url }}{{ end }}
{{ end }}
{{ end }}

# ... (see EXAMPLES.md for complete email and PagerDuty templates)

Use templates in receivers:

receivers:
  - name: 'slack-custom'
    slack_configs:
      - channel: '#alerts'
        title: '{{ template "slack.default.title" . }}'
        text: '{{ template "slack.default.text" . }}'

Got: Notifications formatted consistently, include all relevant context, actionable with runbook links.

If fail:

• Test template rendering: amtool template test --config.file=alertmanager.yml
• Check template syntax errors in Alertmanager logs
• Use {{ . | json }} to debug template data structure

Step 4: Configure Routing and Grouping

Optimize alert delivery with intelligent routing rules.

Advanced routing configuration (excerpt):

route:
  receiver: 'default-receiver'
  group_by: ['alertname', 'cluster', 'service']
  group_wait: 30s

  routes:
    - match:
        team: platform
      receiver: 'team-platform'
      routes:
        - match:
            severity: critical
          receiver: 'pagerduty-platform'
          group_wait: 10s
          repeat_interval: 15m
          continue: true   # Also send to Slack

# ... (see EXAMPLES.md for complete routing with time intervals)

Grouping strategies:

# Group by alertname: All HighCPU alerts bundled together
group_by: ['alertname']

# Group by alertname AND cluster: Separate notifications per cluster
group_by: ['alertname', 'cluster']

Got: Alerts routed to correct teams, grouped logically, timing appropriate for severity.

If fail:

• Test routing: amtool config routes test --config.file=alertmanager.yml --alertname=HighCPU --label=severity=critical
• Check routing tree: amtool config routes show --config.file=alertmanager.yml
• Verify continue: true if alert should match multiple routes

Step 5: Implement Inhibition and Silencing

Reduce alert noise with inhibition rules and temporary silences.

Inhibition rules (suppress dependent alerts):

inhibit_rules:
  # Cluster down suppresses all node alerts in that cluster
  - source_match:
      alertname: 'ClusterDown'
      severity: 'critical'
    target_match_re:
      alertname: '(InstanceDown|HighCPU|HighMemory)'
    equal: ['cluster']

  # Service down suppresses latency and error alerts
  - source_match:
      alertname: 'ServiceDown'
    target_match_re:
      alertname: '(HighLatency|HighErrorRate)'
    equal: ['service', 'namespace']

# ... (see EXAMPLES.md for more inhibition patterns)

Create silences programmatically:

# Silence during maintenance
amtool silence add \
  instance=app-server-1 \
  --author="ops-team" \
  --comment="Scheduled maintenance" \
  --duration=2h

# List and manage silences
amtool silence query
amtool silence expire <SILENCE_ID>

Got: Inhibition reduces cascade alerts automatically, silences prevent notifications during planned maintenance.

If fail:

• Test inhibition logic with live alerts
• Check Alertmanager UI "Silences" tab
• Verify silence matchers are exact (labels must match perfectly)

Step 6: Integrate with External Systems

Connect Alertmanager to PagerDuty, Opsgenie, Jira, etc.

PagerDuty integration (excerpt):

receivers:
  - name: 'pagerduty'
    pagerduty_configs:
      - routing_key: 'YOUR_INTEGRATION_KEY'
        severity: '{{ .CommonLabels.severity }}'
        description: '{{ range .Alerts.Firing }}{{ .Annotations.summary }}{{ end }}'
        details:
          firing: '{{ .Alerts.Firing | len }}'
          alertname: '{{ .GroupLabels.alertname }}'
        # ... (see EXAMPLES.md for complete integration examples)

Webhook for custom integrations:

receivers:
  - name: 'webhook-custom'
    webhook_configs:
      - url: 'https://your-webhook-endpoint.com/alerts'
        send_resolved: true

Got: Alerts create incidents in PagerDuty, appear in team communication channels, trigger on-call escalations.

If fail:

• Verify API keys/tokens are valid
• Check network connectivity to external services
• Test webhook endpoints independently with curl
• Enable debug mode: --log.level=debug

Validation

• Alertmanager receives alerts from Prometheus successfully
• Alerts routed to correct teams based on labels and severity
• Notifications delivered to Slack, email, or PagerDuty
• Alert grouping reduces notification volume appropriately
• Inhibition rules suppress dependent alerts correctly
• Silences prevent notifications during maintenance windows
• Notification templates include runbook links and context
• Repeat interval prevents alert fatigue for long-running issues
• Resolved notifications sent when alerts clear
• External integrations (PagerDuty, Opsgenie) create incidents

Pitfalls

• Alert fatigue: Too many low-priority alerts cause responders to ignore critical ones. Set strict thresholds, use inhibition.
• Missing for duration: Alerts without for fire on transient spikes. Always use 5-10 minute windows.
• Overly broad grouping: Grouping by ['...'] sends individual notifications. Use specific label grouping.
• No runbook links: Alerts without runbooks leave responders guessing. Every alert needs a runbook URL.
• Incorrect severity: Mislabeling warnings as critical desensitizes team. Reserve critical for emergencies.
• Forgotten silences: Silences without expiration can hide real issues. Always set end times.
• Single route: All alerts to one channel loses context. Use team-specific routing.
• No inhibition: Cascade alerts during outages create noise. Implement inhibition rules.

Related Skills

• setup-prometheus-monitoring - Define metrics and recording rules that feed alerting rules
• define-slo-sli-sla - Generate SLO burn rate alerts for error budget management
• write-incident-runbook - Create runbooks linked from alert annotations
• build-grafana-dashboards - Visualize alert firing history and silence patterns