# What Is A Prometheus Rule? 

A **Prometheus rule** defines how Prometheus processes time-series data, primarily for recording precomputed results or triggering alerts. Rules are written in YAML and referenced in Prometheus' configuration under the `rule_files` section.

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### Recording Rules

Recording rules create new time series by precomputing results from PromQL queries. This simplifies complex queries and improves performance for frequent use cases. For example, to calculate the average CPU usage across nodes:

```yaml
groups:
  - name: recording_rules
    rules:
      - record: job:node_cpu:avg
        expr: avg(rate(node_cpu_seconds_total[5m])) by (job)
```

The `record` field specifies the name of the new time series, while `expr` defines the query to compute its value. Recording rules reduce computational overhead and simplify dashboards and alerts.

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### Alerting Rules

Alerting rules evaluate conditions to trigger alerts. When conditions are met, Prometheus generates an alert and sends it to external systems like Alertmanager. For example, to alert when CPU usage exceeds 90% for five minutes:

```yaml
groups:
  - name: alerting_rules
    rules:
      - alert: HighCPUUsage
        expr: avg(rate(node_cpu_seconds_total[5m])) by (instance) > 0.9
        for: 5m
        labels:
          severity: critical
        annotations:
          summary: "High CPU usage on {{ $labels.instance }}"
          description: "CPU usage has exceeded 90% for the last five minutes."
```

The `alert` field names the alert, while `expr` defines the condition to evaluate. The `for` field specifies a waiting period before firing the alert, and labels and annotations provide context for notifications.

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### Configuring Rules

Save rules to a file (e.g., `rules.yml`) and reference it in the Prometheus configuration:

```yaml
rule_files:
  - /path/to/rules.yml
```

Reload Prometheus to apply changes using:  
```bash
kill -HUP $(pgrep prometheus)
```

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### Best Practices

Group related rules logically and precompute complex queries with recording rules to simplify alerting. Use reasonable evaluation intervals to avoid overloading Prometheus and include meaningful labels and annotations for clarity. Test PromQL expressions in the Prometheus UI before deploying them in rules.

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Prometheus rules improve monitoring by enabling faster queries and proactive alerting, making systems easier to manage and maintain.

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