Glossary · Monitoring

Prometheus

Prometheus is an open-source monitoring system that scrapes numeric metrics over HTTP, stores them as time series and evaluates alerting rules with PromQL.

Updated October 6, 2026

Prometheus is an open-source monitoring and alerting toolkit originally developed at SoundCloud and now a graduated project of the Cloud Native Computing Foundation. It periodically pulls (scrapes) metrics from HTTP endpoints, stores them in its own time-series database and lets you query them with PromQL.

Why it matters

It became a standard in cloud-native environments, especially with Kubernetes. Applications expose a /metrics endpoint in a simple text format, and many exporters exist for databases, hosts and network devices. Alert rules are evaluated by Prometheus and sent to Alertmanager, which handles grouping, silencing and routing to chat, email or paging services. Grafana is commonly used to draw its data.

Example

A scrape target and an alerting rule:

scrape_configs:
  - job_name: web
    static_configs:
      - targets: ["10.0.1.5:9100"]

# alert rule
- alert: HighErrorRate
  expr: sum(rate(http_requests_total{status=~"5.."}[5m])) / sum(rate(http_requests_total[5m])) > 0.05
  for: 5m

Common mistakes

  • High-cardinality labels, which inflate memory use.
  • Expecting long-term storage by default. Local retention is limited and long-term needs a remote store.
  • Monitoring only from inside the cluster, which cannot see outages visible to users.

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