Prometheus HA Replicas and Alertmanager Deduplication: Label-Set Identity Across Replicas

Integration Boundary

Prometheus HA replicas each expose firing and resolved alerts to a shared Alertmanager via the Prometheus Alerting API at /api/v1/alerts. Alertmanager deduplicates incoming alerts by comparing label sets, treating alerts with identical labels as a single group for notification purposes.

Unresolved Behavior

The deduplication logic is version-sensitive and depends on whether replicas share identical external_labels. When replicas carry divergent external_labels, the same alerting rule produces distinct alert identities in Alertmanager, potentially triggering duplicate notifications. Annotations are excluded from the grouping key, yet alerts differing only in annotations may still appear as separate entries depending on the Alertmanager version and receiver configuration. Prometheus does not query Alertmanager for active silences or inhibitions, so it continues sending alerts that Alertmanager may suppress.

Verification Gap

Documented behavior does not fully specify how Alertmanager reconciles alerts when replicas emit identical label sets but divergent annotation payloads, nor does it clarify whether fingerprint comparison at /api/v2/alerts reliably confirms HA merging across all supported versions.

• Does Alertmanager merge alerts from HA replicas that share identical label sets but carry different annotation values, and does this behavior vary by version?
• How can operators verify that external_labels configuration across replicas guarantees single-notification semantics without deploying a test rule?