# Aggregation — Prometheus + Grafana

Source: https://www.skillbyai.com/en/prometheus-grafana/q-agg

> sum, avg, max, topk with by and without.

## Collapse dimensions deliberately

Aggregation operators (`sum`, `avg`, `min`, `max`, `count`, `topk`, `bottomk`, `quantile`) combine series. `by (label)` keeps only the listed labels; `without (label)` removes the listed labels and keeps the rest. Aggregating a rate by `route` gives per-route traffic across all instances; `topk(5, ...)` finds the busiest series. `count by (version) (up)` is a quick inventory of running versions if a version label exists.

## Aggregation examples

PromQL.

```promql
# requests per second per route, across all instances
sum by (route) (rate(http_requests_total{job="orders-api"}[5m]))

# error ratio for the whole service
sum(rate(http_requests_total{job="orders-api", status=~"5.."}[5m]))
  /
sum(rate(http_requests_total{job="orders-api"}[5m]))

# five busiest routes
topk(5, sum by (route) (rate(http_requests_total[5m])))

# CPU busy fraction per host (node_exporter)
1 - avg by (instance) (rate(node_cpu_seconds_total{mode="idle"}[5m]))
```

## Use without to keep useful labels

without (instance, pod) keeps all other labels, so new labels added later are preserved automatically.

**Quiz:** What does sum by (route) (...) do?

- [ ] Removes the route label
- [x] Adds series together, keeping only the route label
- [ ] Sorts by route
- [ ] Filters to one route

*Answer:* Adds series together, keeping only the route label. by keeps the listed labels.