On-Call Is a System, Not a Schedule#

On-call done wrong burns out engineers and degrades reliability simultaneously. Exhausted responders make worse decisions, and teams that dread on-call avoid owning production systems. Done right, on-call is sustainable, well-compensated, and generates signal that drives real reliability improvements.

Rotation Schedule Types#

Weekly Rotation#

Each engineer is primary on-call for one full week, Monday to Monday. This is the simplest model and works for teams of 5 or more in a single timezone.

Routing Tree#

Alertmanager receives alerts from Prometheus and decides where to send them based on a routing tree. Every alert enters at the root route and travels down the tree until it matches a child route. If no child matches, the root route’s receiver handles it.

# alertmanager.yml
global:
  resolve_timeout: 5m
  slack_api_url: "https://hooks.slack.com/services/T00/B00/xxx"
  pagerduty_url: "https://events.pagerduty.com/v2/enqueue"

route:
  receiver: "default-slack"
  group_by: ["alertname", "namespace"]
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 4h
  routes:
    - match:
        severity: critical
      receiver: "pagerduty-oncall"
      group_wait: 10s
      repeat_interval: 1h
      routes:
        - match:
            team: database
          receiver: "pagerduty-dba"
    - match:
        severity: warning
      receiver: "team-slack"
      repeat_interval: 12h
    - match_re:
        namespace: "staging|dev"
      receiver: "dev-slack"
      repeat_interval: 24h

Timing parameters matter. group_wait is how long Alertmanager waits after receiving the first alert in a new group before sending the notification – this lets it batch related alerts together. group_interval is the minimum time before sending updates about a group that already fired. repeat_interval controls how often an unchanged active alert is re-sent.

Why Runbooks Exist#

An on-call engineer paged at 3 AM has limited cognitive capacity. They may not be familiar with the specific service that is failing. They may have joined the team two weeks ago. A runbook bridges the gap between the alert firing and the correct human response. Without runbooks, incident response depends on tribal knowledge – the engineer who built the service and knows its failure modes. That engineer is on vacation when the incident hits.