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Configuring Prometheus Alertmanager for High Availability

Prometheus Alertmanager is a great way to handle your Prometheus alerts. However, a lone instance of Alertmanager can serve as a single point of failure if it goes down. Luckily, you can configure Alertmanager to run in a multi-instance cluster to provide failure resilience. In this hands-on lab, you will make an existing single-instance Alertmanager setup highly available by adding an additional instance.

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Level
Clock icon Intermediate
Duration
Clock icon 1h 0m
Last updated
Clock icon Sep 03, 2025

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Table of Contents

  1. Challenge

    Configure the Two Alertmanager Instances to Form a Cluster

    1. Log in to both the Prometheus Server and Alertmanager 2 server.

    2. On both servers, edit the Alertmanager unit file:

      sudo vi /etc/systemd/system/alertmanager.service

    3. Locate the ExecStart section. On each server, add the other server's private IP address using the cluster.peer flag.

    4. On the Prometheus Server:

      ExecStart=/usr/local/bin/alertmanager 
  --config.file /etc/alertmanager/alertmanager.yml 
  --storage.path /var/lib/alertmanager/ 
  --cluster.peer=10.0.1.102:9094

    5. On the Alertmanager 2 server:

      ExecStart=/usr/local/bin/alertmanager 
  --config.file /etc/alertmanager/alertmanager.yml 
  --storage.path /var/lib/alertmanager/ 
  --cluster.peer=10.0.1.101:9094

    6. On both servers, reload the unit file:

      sudo systemctl daemon-reload

    7. On the Prometheus Server, restart Alertmanager:

      sudo systemctl restart alertmanager

    8. On the Alertmanager 2 server, enable and start Alertmanager:

      sudo systemctl enable alertmanager

      sudo systemctl start alertmanager

    9. Test your cluster setup by creating a silence on one instance and verifying it appears on the other instance. Access both instances in a browser: http://<PUBLIC_IP>:9093

    10. On one instance, click Silences and create a new silence.

    11. Click Silences on the other instance and verify the silence you created appears.

  2. Challenge

    Configure Prometheus to Use Your Multi-Instance Alertmanager Setup

    1. On the Prometheus Server, edit the Prometheus configuration file:

      sudo vi /etc/prometheus/prometheus.yml

    2. Add the new Alertmanager (10.0.1.102:9093) to the list of Alertmanager targets:

      alerting:
  alertmanagers:
  - static_configs:
    - targets:
      - localhost:9093
      - 10.0.1.102:9093

    3. Restart Prometheus to reload the config:

      sudo systemctl restart prometheus

    4. Access the Prometheus server in a browser: http://<PROMETHEUS_SERVER_PUBLIC_IP>:9090

    5. Click Status > Runtime & Build Information.

    6. Verify both of your Alertmanagers appear under the Alertmanagers section.

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