Grouping categorizes alerts of similar nature into a single notification. This is especially useful during larger outages when many systems fail at once and hundreds to thousands of alerts may be firing simultaneously.

Example: Dozens or hundreds of instances of a service are running in your cluster when a network partition occurs. Half of your service instances can no longer reach the database. Alerting rules in Prometheus were configured to send an alert for each service instance if it cannot communicate with the database. As a result hundreds of alerts are sent to Alertmanager.

As a user, one only wants to get a single page while still being able to see exactly which service instances were affected. Thus one can configure Alertmanager to group alerts by their cluster and alertname so it sends a single compact notification.

Grouping of alerts, timing for the grouped notifications, and the receivers of those notifications are configured by a routing tree in the configuration file.