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MQTT Subscribe

This block subscribes to an MQTT topic and triggers when messages are received. Use it to receive data from MQTT brokers and other devices.

Overview​

The MQTT Subscribe block listens for messages on specified MQTT topics, enabling your device to receive commands, data, and notifications from cloud services and other devices.

Configuration​

  • Topic: MQTT topic to subscribe to (supports wildcards)
  • QoS: Quality of Service level (0, 1, or 2)
  • Output: Variable to store received message

Topic Wildcards​

MQTT supports wildcards in subscriptions:

Single-level Wildcard (+)​

sensors/+/temperature

Matches:

  • sensors/room1/temperature
  • sensors/room2/temperature
  • Does NOT match: sensors/room1/other/temperature

Multi-level Wildcard (#)​

sensors/#

Matches:

  • sensors/temperature
  • sensors/room1/temperature
  • sensors/building/floor2/room3/temperature

Use # only at end of topic pattern.

Quality of Service​

Subscribe with appropriate QoS:

  • QoS 0: At most once (may miss messages)
  • QoS 1: At least once (may get duplicates)
  • QoS 2: Exactly once (highest reliability)

Actual QoS is minimum of publish and subscribe QoS levels.

Message Processing​

When message received:

  1. MQTT Subscribe block triggers
  2. Message payload available in output variable
  3. Process message content
  4. Extract data and take action

Message Format​

Handle different message formats:

Plain Text​

Parse simple text messages

JSON​

{"command": "turn_on", "device": "light1"}

Parse JSON to extract values

Binary​

Process raw binary data

Dynamic Subscriptions​

Subscribe to topics dynamically:

  • Include device ID in pattern
  • Location-based subscriptions
  • Role-based topic filtering

Example: devices/${deviceId}/commands/#

Use Cases​

Common MQTT subscription scenarios:

  • Receive commands from cloud
  • Listen for sensor data from other devices
  • Subscribe to alerts and notifications
  • Receive configuration updates
  • Monitor status of multiple devices

Multiple Subscriptions​

Create multiple subscribe blocks for:

  • Different topic patterns
  • Separate message handlers
  • Priority-based processing
  • Organized application logic

Message Filtering​

Filter messages after receiving:

  • Check sender device ID
  • Validate message format
  • Filter by priority
  • Discard invalid messages

Use Comparison blocks for filtering.

Subscription Persistence​

MQTT subscriptions:

  • Automatically resubscribe after reconnection
  • Persist across application restarts
  • Restored on broker reconnection

Use On MQTT Connected to verify subscription status.

Retained Messages​

When subscribing to topic with retained message:

  • Immediately receive last retained message
  • Useful for initial state synchronization
  • Get latest value without waiting

Error Handling​

Handle subscription errors:

  • Invalid topic pattern: Check topic syntax
  • Authorization failed: Verify topic permissions
  • Connection lost: Wait for automatic reconnection
  • Buffer overflow: Reduce subscription scope

Security​

Secure MQTT subscriptions:

  • Use TLS/SSL for encryption
  • Implement topic-based ACLs
  • Validate message source
  • Sanitize received data

Performance​

Optimize subscription performance:

  • Use specific topics instead of broad wildcards
  • Limit number of subscriptions
  • Process messages efficiently
  • Avoid blocking operations in handlers

Command Pattern​

Implement command/response pattern:

  1. Subscribe to devices/${deviceId}/commands
  2. Receive command message
  3. Process command
  4. Publish response to devices/${deviceId}/responses

Integration Examples​

Subscribe to messages from:

  • Cloud platforms (AWS, Azure, Google Cloud)
  • Home automation systems
  • Industrial SCADA systems
  • Mobile applications
  • Other IoT devices

Troubleshooting​

Common subscription issues:

  • No messages received: Check topic name and wildcards
  • Missing messages: Increase QoS level
  • Duplicate messages: Normal with QoS 1, handle idempotently
  • Authorization error: Check broker topic permissions

See Also​