MQTT

Lightweight publish/subscribe messaging for IoT, sensor networks, and real-time telemetry. Supports QoS levels 0–2, topic wildcards (+, #), TLS, and automatic reconnection with re-subscription. Use it for device-to-cloud ingestion, command dispatch, or any scenario where bandwidth-efficient push messaging matters.

Configuration

connector "sensors" {
  type      = "mqtt"
  broker    = "tcp://localhost:1883"
  client_id = "mycel-iot-gateway"
  qos       = 1
}

Option	Type	Default	Description
broker	string	"tcp://localhost:1883"	Broker URL (tcp://, ssl://, ws://)
client_id	string	"mycel"	Client identifier
username	string	—	Authentication username
password	string	—	Authentication password
qos	int	0	Default QoS level (0 = at most once, 1 = at least once, 2 = exactly once)
topic	string	—	Default publish topic
clean_session	bool	true	Whether to start a clean session on connect
keep_alive	duration	"30s"	PINGREQ interval
connect_timeout	duration	"10s"	Connection timeout
auto_reconnect	bool	true	Reconnect automatically on disconnect
max_reconnect_interval	duration	"5m"	Maximum wait between reconnection attempts

TLS

connector "sensors" {
  type      = "mqtt"
  broker    = "ssl://broker.example.com:8883"
  client_id = "mycel-secure"

  tls {
    cert    = "/certs/client.crt"
    key     = "/certs/client.key"
    ca_cert = "/certs/ca.crt"
  }
}

Option	Type	Default	Description
tls.enabled	bool	true when the block is present	Enable TLS
tls.cert	string	—	Client certificate file
tls.key	string	—	Client private key file
tls.ca_cert	string	—	CA certificate file
tls.insecure_skip_verify	bool	false	Skip server certificate verification. Development only

The block is the same on every connector that speaks TLS — see TLS.

Operations

Operation	Direction	Description
<topic>	source	Subscribe to a topic (supports MQTT wildcards +, #)
<topic>	target	Publish a message to a topic

Source (Subscribe)

When used as a flow source, the operation is the MQTT topic filter to subscribe to. MQTT wildcards are supported:

• sensors/+/temperature — matches sensors/room1/temperature, sensors/lab/temperature
• devices/# — matches devices/status, devices/room1/temperature, etc.

Incoming messages provide these fields:

Field	Description
_topic	The actual topic the message was published on
_message_id	Broker-assigned message ID
_qos	QoS level of the received message
_retained	Whether the message was a retained message
*	All JSON fields from the payload are merged at the top level
_raw	Raw string payload (only if payload is not valid JSON)

Target (Publish)

When used as a flow target, the target field is the topic to publish to:

to {
  connector = "sensors"
  target    = "alerts/temperature"
}

Optional params: qos (int), retain (bool).

Example

# Subscribe to sensor readings and store in database
flow "sensor_reading" {
  from {
    connector = "sensors"
    operation = "sensors/+/temperature"
  }
  transform {
    device_id   = "input._topic"
    value       = "input.temperature"
    unit        = "'celsius'"
    received_at = "now()"
  }
  to {
    connector = "db"
    target    = "readings"
  }
}

# Publish alerts via REST
flow "temperature_alert" {
  from {
    connector = "api"
    operation = "POST /alerts"
  }
  to {
    connector = "sensors"
    target    = "alerts/temperature"
  }
}

See the mqtt example for a complete working setup.

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Full configuration reference: See MQTT in the Configuration Reference.