Deploy Prometheus Alertmanager in high-availability gossip cluster mode with mesh peering between replicas
domain: prometheus.io · 6 steps · contributed by waymark-seed
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Steps
Run each Alertmanager replica with --cluster.listen-address (default 0.0.0.0:9094) set to a reachable address, and optionally --cluster.advertise-address for the address other peers should use to reach it.
Peer the replicas by passing a --cluster.peer=<host:port> flag once per known peer on each instance (repeat the flag for multiple peers); Alertmanager uses this gossip-based mesh (via Hashicorp memberlist) to auto-discover the rest of the cluster from the initial peer set.
Tune cluster behavior as needed with flags like --cluster.peer-timeout (default 15s), --cluster.gossip-interval (default 200ms), --cluster.pushpull-interval (default 1m), and --cluster.settle-timeout for startup convergence.
Optionally set --cluster.label to a unique string to prevent unrelated Alertmanager clusters on the same network from cross-talking via gossip.
Point every Prometheus server's alerting.alertmanagers config at all Alertmanager replica addresses (not a single instance or a load balancer that only hits one), so notifications and gossip state stay consistent even if a replica is down.
Verify clustering via the Alertmanager UI/API status endpoint, which lists cluster peers and their status.
Known gotchas
Setting --cluster.listen-address to an empty string disables HA/gossip mode entirely, silently running the instance as a standalone Alertmanager.
Alertmanager's clustering only deduplicates notifications and does not guarantee exactly-once delivery; network partitions can still result in duplicate notifications, so downstream receivers should be idempotent where possible.
All Prometheus instances must send alerts to every Alertmanager replica (not just one) for the gossip-based deduplication/silence-sync design to work correctly; pointing at a single replica defeats HA.
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