Offline Messages

This plugin persists selected QoS 1/2 messages when no subscribers are online and replays them when a matching subscriber comes online later.

Supported backends:

• MySQL
• Redis

Configuration

The plugin is configured through the standard EMQX plugin configuration API:

PUT /api/v5/plugins/<name-vsn>/config

The schema is defined in priv/config.hocon and includes backend-specific settings for Redis and MySQL.

The plugin only persists messages when:

• the publish QoS is greater than 0
• the topic matches one of the configured message.topic_filters

Build And Test

Build the plugin from the repository root:

make

Run this plugin's Common Test suite:

make plugins/emqx_offline_messages-ct

Database Schema

MySQL

CREATE TABLE IF NOT EXISTS `mqtt_msg` (
  `id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `msgid` varchar(64) DEFAULT NULL,
  `topic` varchar(180) NOT NULL,
  `sender` varchar(64) DEFAULT NULL,
  `qos` tinyint(1) NOT NULL DEFAULT '0',
  `retain` tinyint(1) DEFAULT NULL,
  `payload` blob,
  `arrived` datetime NOT NULL,
  PRIMARY KEY (`id`),
  INDEX topic_index(`topic`)
)
ENGINE=InnoDB DEFAULT CHARSET=utf8MB4;

CREATE TABLE IF NOT EXISTS `mqtt_sub` (
  `clientid` varchar(64) NOT NULL,
  `topic` varchar(180) NOT NULL,
  `qos` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`clientid`, `topic`)
)
ENGINE=InnoDB DEFAULT CHARSET=utf8MB4;

Redis

Redis uses hashes and sorted sets under the mqtt:sub:* and mqtt:msg:* key spaces.

• Subscription state is stored in hashes keyed by client ID.
• Message payloads are stored in hashes keyed by message ID.
• Topic indexes are stored in sorted sets keyed by topic.

Download

Tarballs for each EMQX release:

EMQX Version	Plugin Version	Package
6.1.2	2.0.0	emqx_offline_messages-2.0.0.tar.gz
6.1.3	2.0.0	emqx_offline_messages-2.0.0.tar.gz