MqttTrigger

The MQTT Trigger operation subscribes to a configured full topic and starts an execution path when a matching MQTT message is received. It exposes MQTT metadata outputs including QoS, retained flag, response-topic related fields, content type, and computed message-expiry seconds.

Trigger

This operation is a trigger. It starts a new execution path when an MQTT message matching the subscribed topic is received. The > output pulses true during the trigger callback and returns to false afterward.

Inputs

This operation has no inputs.

Outputs

Port	Type	Default	Description
>	Any	false	Untyped trigger pulse. true during callback, then false.
TimeStamp(ns)	Integer	Nil	Arrival timestamp in nanoseconds (system_clock epoch).
Topic	String	Nil	Full received topic.
Payload	Binary	Nil	Raw message payload.
QoS	Integer	Nil	Incoming QoS level.
MessageExpiry(s)	Integer	Nil	Remaining lifetime in seconds. Computed from expiry metadata or Nil if unavailable.
ResponseTopic	String	Nil	MQTT 5 response topic, if present.
CorrelationData	Binary	Nil	MQTT 5 correlation data, if present.
UserProperties	Map	Nil	MQTT 5 user properties.
ContentType	String	Nil	MQTT 5 content type.

Properties

Name	Type	Default	Description
Topic	TextLine	""	Full topic to subscribe to. Must start with <slot>/. Hint: Add full topic as <slot>/…

This Operation can never err: it has no Input to check, and a message that arrives is a message, so its Node carries no On Error policy.

State on Reload

State is preserved across restarts for the following outputs:

• Topic
• Payload
• TimeStamp(ns)

Metadata outputs (QoS, MessageExpiry, ResponseTopic, CorrelationData, UserProperties, ContentType) are not persisted. Saved state is discarded if the configured Topic changed.

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How It Works

1. Startup — The configured topic is validated (must start with <slot>/) and the operation subscribes to it.

2. Message receive — When a matching message arrives, the operation sets all output values: TimeStamp(ns), Topic, Payload, QoS, and any MQTT 5 metadata fields.

3. MessageExpiry computation — If an absolute expiresAtMs exists, the remaining seconds are computed as ceil((expiresAtMs - nowMs) / 1000) floored at 0. Otherwise, messageExpiryInterval is used if present, or the output is Nil.

4. Trigger pulse — The > output pulses to true for the callback execution, starting the downstream execution path, then returns to false.

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Reset Behavior

When the operation is reset:

• Topic, Payload, TimeStamp(ns), QoS, ResponseTopic, CorrelationData, UserProperties, ContentType, MessageExpiry(s) are cleared to Nil.
• Retained is set to false.

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Notes

• Topic validation requires the <slot>/... prefix when mqttbroker.requireSlotPrefix is enabled; otherwise any structurally valid topic is accepted. Isolation between slots does not depend on the prefix.
• Topic matching and ACL behavior are enforced by the MQTT stack/plugin path.

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Configuration

One host setting bounds MQTT Trigger. It is read at start-up from the host’s configuration file; see the MQTT Broker configuration reference.

Key	Type	Default	Effect
mqttbroker.maxTriggerPerTask	Integer	-1	How many MQTT Trigger nodes one Task may contain. Negative means unlimited.

The cap travels to the editor in the operation’s schema, so the editor refuses to add a Trigger beyond it rather than letting you build a Task the server will reject.

The broker-wide settings a Trigger runs under — the granted QoS level, whether retained messages are stored, the packet-size ceiling, and the per-device rate limits — are on the same page.