# Recording Rules — Prometheus + Grafana

Source: https://www.skillbyai.com/en/prometheus-grafana/a-recording

> Precompute expensive queries.

## Store query results as new series

A **recording rule** evaluates a PromQL expression on a schedule and saves the result as a new metric. Dashboards and alerts then read the cheap precomputed series instead of recalculating heavy aggregations over many series. The naming convention is `level:metric:operations`, for example `job:http_requests:rate5m`. Rules live in groups in rule files referenced by prometheus.yml and are validated with `promtool check rules`.

## A rule file with recording rules

Rates and error ratio per job.

```yaml
groups:
  - name: orders-api-recording
    interval: 30s
    rules:
      - record: job:http_requests:rate5m
        expr: sum by (job) (rate(http_requests_total[5m]))
      - record: job:http_requests_errors:rate5m
        expr: sum by (job) (rate(http_requests_total{status=~"5.."}[5m]))
      - record: job:http_requests_error_ratio:rate5m
        expr: job:http_requests_errors:rate5m / job:http_requests:rate5m
```

## Record what dashboards load most

Start with the queries on your most-viewed dashboards and in your alerts.

**Quiz:** What is the main benefit of a recording rule?

- [x] Expensive queries are precomputed, so dashboards and alerts are fast
- [ ] It deletes old data
- [ ] It sends notifications
- [ ] It discovers targets

*Answer:* Expensive queries are precomputed, so dashboards and alerts are fast. Computed once, read many times.