MQTT Delayed Publishing

What is MQTT Delayed Publishing?

Delayed publishing is an extended feature of MQTT supported by RobustMQ MQTT. When a client publishes a message using the special topic prefix $delayed/{DelayInterval}, it triggers the delayed publishing functionality, allowing messages to be published at a user-configured time interval.

The core feature of delayed publishing is time control: messages are not immediately published to the target topic, but are published after a specified delay time, implementing scheduled message sending functionality.

When to use MQTT Delayed Publishing?

Delayed publishing is suitable for the following scenarios:

• Scheduled tasks: Tasks that need to be executed at specific times, such as scheduled reminders and scheduled reports
• Retry mechanisms: Delayed retries after failures to avoid system pressure from immediate retries
• Batch processing: Delaying multiple messages to the same time point for batch processing
• Traffic control: Controlling message sending rates through delayed publishing to avoid system overload
• Business logic: Operations that need to wait for a certain time before execution, such as cooldown periods and waiting periods

Syntax Rules for Delayed Publishing

The specific format for delayed publishing topics is as follows:

$delayed/{DelayInterval}/{TopicName}

Parameter Description:

• $delayed: Messages using $delayed as the topic prefix will be considered as messages that need delayed publishing
• {DelayInterval}: Specifies the time interval for delayed publishing of the MQTT message, in seconds, with a maximum allowed interval of 4294967 seconds
• {TopicName}: The target topic name for the MQTT message

Important Notes:

• If {DelayInterval} cannot be parsed as an integer, the server will discard the message
• Delay time is in seconds, supporting a maximum delay time of 4294967 seconds (approximately 49.7 days)
• Delayed messages are stored on the server until the delay time is reached

Characteristics of Delayed Publishing

• Precise time control: Can precisely control the publishing time of messages
• Persistent storage: Delayed messages are persistently stored, ensuring normal execution after server restart
• High reliability: Supports QoS levels to ensure reliable message delivery
• Flexible configuration: Supports delayed publishing to any topic

Using Delayed Publishing via MQTTX

Using MQTTX CLI

1. Publish delayed message

   mqttx pub -t "$delayed/10/sensor/temperature" -m "25.5°C" -h '117.72.92.117' -p 1883

   This message will be published to the sensor/temperature topic after 10 seconds.

2. Subscribe to target topic

   mqttx sub -t "sensor/temperature" -h '117.72.92.117' -p 1883 -v

   After waiting 10 seconds, subscribers will receive the delayed published message.

3. Publish messages with different delay times

   # Publish after 15 seconds
   mqttx pub -t "$delayed/15/device/status" -m "online" -h '117.72.92.117' -p 1883
   
   # Publish after 60 seconds
   mqttx pub -t "$delayed/60/system/backup" -m "backup completed" -h '117.72.92.117' -p 1883
   
   # Publish after 3600 seconds (1 hour)
   mqttx pub -t "$delayed/3600/report/daily" -m "daily report ready" -h '117.72.92.117' -p 1883

Practical Application Examples

Scheduled Reminder System

# Set reminder for 30 seconds later
mqttx pub -t "$delayed/30/reminder/meeting" -m '{"title":"Meeting Reminder","time":"14:00","location":"Conference Room A"}' -h '117.72.92.117' -p 1883

# Set reminder for 1 hour later
mqttx pub -t "$delayed/3600/reminder/break" -m '{"title":"Break Reminder","message":"Time for a break"}' -h '117.72.92.117' -p 1883

# Subscribe to reminder topics
mqttx sub -t "reminder/+" -h '117.72.92.117' -p 1883 -v

Retry Mechanism

# Retry after 5 seconds delay on failure
mqttx pub -t "$delayed/5/retry/upload" -m '{"file_id":"123","retry_count":1}' -h '117.72.92.117' -p 1883

# Retry after 30 seconds delay on failure
mqttx pub -t "$delayed/30/retry/sync" -m '{"data_id":"456","retry_count":2}' -h '117.72.92.117' -p 1883

# Subscribe to retry topics
mqttx sub -t "retry/+" -h '117.72.92.117' -p 1883 -v

Batch Processing

# Delay multiple messages to the same time point (10 minutes later)
mqttx pub -t "$delayed/600/batch/process" -m '{"batch_id":"001","type":"data_analysis"}' -h '117.72.92.117' -p 1883
mqttx pub -t "$delayed/600/batch/process" -m '{"batch_id":"002","type":"data_analysis"}' -h '117.72.92.117' -p 1883
mqttx pub -t "$delayed/600/batch/process" -m '{"batch_id":"003","type":"data_analysis"}' -h '117.72.92.117' -p 1883

# Subscribe to batch processing topic
mqttx sub -t "batch/process" -h '117.72.92.117' -p 1883 -v

Traffic Control

# Control message sending rate, send one message every 2 seconds
mqttx pub -t "$delayed/2/rate/limited" -m "message 1" -h '117.72.92.117' -p 1883
mqttx pub -t "$delayed/4/rate/limited" -m "message 2" -h '117.72.92.117' -p 1883
mqttx pub -t "$delayed/6/rate/limited" -m "message 3" -h '117.72.92.117' -p 1883

# Subscribe to rate-limited topic
mqttx sub -t "rate/limited" -h '117.72.92.117' -p 1883 -v

Business Logic Delay

# Device cooldown period, can restart after 30 minutes
mqttx pub -t "$delayed/1800/device/cooldown" -m '{"device_id":"D001","action":"restart_allowed"}' -h '117.72.92.117' -p 1883

# User operation waiting period, can perform sensitive operations after 5 minutes
mqttx pub -t "$delayed/300/user/security" -m '{"user_id":"U001","operation":"sensitive_allowed"}' -h '117.72.92.117' -p 1883

# Subscribe to business logic topics
mqttx sub -t "device/+" -h '117.72.92.117' -p 1883 -v
mqttx sub -t "user/+" -h '117.72.92.117' -p 1883 -v

Delay Time Examples

Delay Time	Example Topic	Description
15 seconds	$delayed/15/sensor/data	Publish to sensor/data after 15 seconds
60 seconds	$delayed/60/system/status	Publish to system/status after 1 minute
3600 seconds	$delayed/3600/report/hourly	Publish to report/hourly after 1 hour
86400 seconds	$delayed/86400/report/daily	Publish to report/daily after 1 day

Important Notes

1. Time format: Delay time must be an integer in seconds
2. Maximum delay: Maximum supported delay time is 4294967 seconds (approximately 49.7 days)
3. Topic format: Must use the $delayed/{DelayInterval}/{TopicName} format
4. Message persistence: Delayed messages are persistently stored and can execute normally after server restart
5. QoS support: Delayed publishing supports MQTT QoS levels
6. Error handling: Messages will be discarded if the delay time format is incorrect