Alarm

A node sends alarm messages to inform consumers about alarm state changes.

<node>/alarm/<code>

Examples:

45fe/alarm/tlc.hardware.error   # serious hardware error on node 45fe
45fe/alarm/tlc.detector         # detector error on node 45fe

When an alarm relates to a specific component, the component is identified in the payload, not in the topic path. This means all alarm events for a given code share a single topic, regardless of which component is affected.

Payload

Alarm messages use the same entries array structure as status messages. Each entry records one alarm event with a precise timestamp.

A single-event message has one entry; when multiple alarm events occur within a short period they MAY be batched into a single message with multiple entries.

Event Update

An event update is published when an alarm becomes active or inactive on any component. It contains only the changed alarm(s).

{
  "entries": [
    {
      "ts": "2026-02-24T10:02:30.000Z",
      "component": "dl/7",
      "active": true,
      "values": {"message": "No response from detector"}
    }
  ]
}

When the alarm applies to the node as a whole (no specific component), the component field is omitted:

{
  "entries": [
    {
      "ts": "2026-02-24T10:05:00.000Z",
      "active": true,
      "values": {"message": "Critical hardware failure"}
    }
  ]
}

Batched example — two detector alarms occurring close together:

{
  "entries": [
    {
      "ts": "2026-02-24T10:02:30.000Z",
      "component": "dl/1",
      "active": true,
      "values": {"message": "No response from detector"}
    },
    {
      "ts": "2026-02-24T10:02:31.200Z",
      "component": "dl/7",
      "active": true,
      "values": {"message": "No response from detector"}
    }
  ]
}

Full Update

A full update contains the complete alarm state for the code across all components. It is published:

• When the node first connects
• Periodically (e.g. every 5 minutes) as a heartbeat

Full updates are published with retain = true so new subscribers immediately receive the current alarm state.

{
  "entries": [
    {
      "ts": "2026-02-24T10:02:30.000Z",
      "component": "dl/1",
      "active": true,
      "values": {"message": "No response from detector"}
    },
    {
      "ts": "2026-02-24T10:02:31.200Z",
      "component": "dl/7",
      "active": true,
      "values": {"message": "No response from detector"}
    }
  ],
  "full": true
}

When no alarms are active, the full update has an empty entries array:

{
  "entries": [],
  "full": true
}

Fields

Field	Type	Description
entries	array	One or more alarm event objects
ts	ISO 8601 timestamp	When the alarm state changed
component	string	Component id (omitted when alarm applies to the node as a whole)
active	boolean	true if the alarm is active, false if cleared
values	object	Alarm attributes (e.g. message, severity)
full	boolean	true on full updates. Omitted on event updates.

MQTT Behavior

Property	Value
QoS	1
Retain	true for full updates, false for event updates

Subscription Patterns

45fe/alarm/#            # all alarms from node 45fe
+/alarm/#               # all alarms from all nodes
45fe/alarm/tlc.detector # detector alarms from node 45fe