Notifying to Slack and Mattermost

Hemdal includes built-in notifiers for popular collaboration and alerting platforms, including Slack and Mattermost.

Notifier Configuration Fields

Each notifier attached to an alert supports the following fields:

• module: The notifier implementation module (e.g. Hemdal.Notifier.Slack or Hemdal.Notifier.Mattermost, default: Hemdal.Notifier.Slack).
• token: For Slack, the webhook URL (e.g. https://hooks.slack.com/services/YOUR_SLACK_WEBHOOK or /services/YOUR_SLACK_WEBHOOK). For Mattermost, the authorization token or webhook path.
• username: The username displayed as the sender (default: "Hemdal").
• log_level: Level of notifications to deliver:
  • "debug": Delivers all notifications on every state transition and recurring check.
  • "warn": Delivers notifications whenever the status changes (e.g. :ok -> :warn, :warn -> :error, :error -> :ok).
  • "error": Delivers notifications only when transitioning into or out of the :error (broken) state.
• metadata: A map of additional configuration options:
  • "channel": Destination channel override (e.g. "#alerts" or "@username").
  • "icon": Custom emoji icon for the bot (e.g. ":rotating_light:" or ":warning:").
  • "base_url": (For Mattermost) The base URL of the Mattermost server (e.g. "https://mattermost.example.com/api/v4").

Slack Notification Configuration Example

Using Elixir Environment Backend

config :hemdal, Hemdal.Config, [
  [
    id: "aea48656-be08-4576-a2d0-2723458faefd",
    name: "Production Database Check",
    host: [
      id: "2a8572d4-ceb3-4200-8b29-dd1f21b50e54",
      module: Hemdal.Host.Local,
      name: "db-primary"
    ],
    command: [
      name: "check_pg_ready",
      type: "line",
      command: "pg_isready -h localhost && echo '[\"OK\", \"PostgreSQL is accepting connections\"]' || echo '[\"FAIL\", \"PostgreSQL is down\"]'"
    ],
    notifiers: [
      [
        module: Hemdal.Notifier.Slack,
        token: "https://hooks.slack.com/services/YOUR_SLACK_WEBHOOK",
        username: "Hemdal Monitor",
        log_level: "warn",
        metadata: %{
          "channel" => "#infrastructure-alerts",
          "icon" => ":fire:"
        }
      ]
    ]
  ]
]

Using JSON Backend

In alerts.json:

[
  {
    "id": "aea48656-be08-4576-a2d0-2723458faefd",
    "name": "Production Database Check",
    "host_id": "2a8572d4-ceb3-4200-8b29-dd1f21b50e54",
    "command": {
      "name": "check_pg_ready",
      "type": "line",
      "command": "pg_isready -h localhost && echo '[\"OK\", \"PostgreSQL is accepting connections\"]' || echo '[\"FAIL\", \"PostgreSQL is down\"]'"
    },
    "notifiers": [
      {
        "module": "Hemdal.Notifier.Slack",
        "token": "https://hooks.slack.com/services/YOUR_SLACK_WEBHOOK",
        "username": "Hemdal Monitor",
        "log_level": "warn",
        "metadata": {
          "channel": "#infrastructure-alerts",
          "icon": ":fire:"
        }
      }
    ]
  }
]

Mattermost Notification Configuration Example

[
  module: Hemdal.Notifier.Mattermost,
  token: "your-mattermost-token-here",
  username: "Hemdal Bot",
  log_level: "warn",
  metadata: %{
    "base_url" => "https://mattermost.yourdomain.com/api/v4",
    "channel" => "monitoring-alerts"
  }
]

Message Formatting and Colors

Notifications sent to Slack and Mattermost include styled rich message attachments:

The attachment includes detailed fields:

• Host Name: Name of the target host
• Command Name: Name of the executed command
• Description: Output message or error details
• Status: Current status (OK, WARN, FAIL, OFF)
• Prev. Status: Previous state before transition
• Duration: Duration of failure in seconds
• Incoming Status: Raw status from command execution