FluxMQ
Clients

MQTT

Connect using MQTT 3.1.1 or 5.0 over TCP

MQTT

Last Updated: 2026-02-06

FluxMQ supports MQTT 3.1.1 and MQTT 5.0 over TCP.

Quick Start

mosquitto_sub -h localhost -p 1883 -t "sensors/#" -v

In another terminal:

mosquitto_pub -h localhost -p 1883 -t "sensors/temp" -m "22.5" -q 1

MQTT v5 Consumer Groups (Queue Topics)

Use MQTT v5 SUBSCRIBE user property consumer-group for queue consumers:

mosquitto_sub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -i worker-1 -q 1 -t '$queue/orders/#' -v \
  -D subscribe user-property consumer-group workers

Start another worker in the same group:

mosquitto_sub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -i worker-2 -q 1 -t '$queue/orders/#' -v \
  -D subscribe user-property consumer-group workers

Publish messages to the queue:

mosquitto_pub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -t '$queue/orders' -m '{"id":"order-1"}' -q 1

Queue ACK/NACK/REJECT with mosquitto_pub

mosquitto_sub does not automatically send queue acknowledgments.
To ack (or nack/reject), publish to queue ack topics with MQTT v5 user properties:

# Ack
mosquitto_pub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -t '$queue/orders/$ack' -m '' \
  -D publish user-property message-id 'orders:42' \
  -D publish user-property group-id 'workers'

# Nack (retry)
mosquitto_pub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -t '$queue/orders/$nack' -m '' \
  -D publish user-property message-id 'orders:42' \
  -D publish user-property group-id 'workers'

# Reject (no retry)
mosquitto_pub -V mqttv5 -h localhost -p 1883 -u usr -P pwd \
  -t '$queue/orders/$reject' -m '' \
  -D publish user-property message-id 'orders:42' \
  -D publish user-property group-id 'workers' \
  -D publish user-property reason 'invalid payload'

message-id format is typically <queue>:<offset> (for example orders:42).

Special Topics

TopicPurpose
$queue/<queue>Publish to a durable queue root
$queue/<queue>/<routing-key>Publish or consume queue messages with routing keys
$queue/<queue>/#Subscribe to all messages in a queue
$queue/<queue>/$ackAcknowledge a queue message
$queue/<queue>/$nackNegative-acknowledge (retry)
$queue/<queue>/$rejectReject (no retry)
$share/<group>/<filter>MQTT shared subscription filter

Notes

  • Use single quotes around $queue/... topics in shell commands to avoid $ expansion.
  • Queue consumer groups require MQTT v5 (-V mqttv5) if you want explicit consumer-group assignment.
  • Reusing the same consumer group with different queue filters creates distinct internal groups per filter (group@pattern in logs).
  • TLS and mTLS listeners are configured via server.tcp.tls and server.tcp.mtls.
  • Shared subscriptions are supported.

CLI Tip

To inspect SUBSCRIBE/PUBLISH packets and reason codes during troubleshooting, add -d:

mosquitto_sub -d -V mqttv5 -h localhost -p 1883 -t '$queue/orders/#' \
  -D subscribe user-property consumer-group workers

Learn More

Security

TLS/mTLS listeners, inter-broker TLS, and rate limiting

HTTP

Publish messages over the HTTP bridge

On this page

MQTTQuick StartMQTT v5 Consumer Groups (Queue Topics)Queue ACK/NACK/REJECT with mosquitto_pubSpecial TopicsNotesCLI TipLearn More