$SYS - System Topic

The EMQX Broker periodically publishes its running status, message statistics, client online and offline events to the system topic starting with $SYS/.

The $SYS topic path begins with $SYS/brokers/{node}/. {node} is the name of the node where the event/message is generated, for example:

$SYS/brokers/emqx@127.0.0.1/version
$SYS/brokers/emqx@127.0.0.1/uptime

$SYS system message publish interval is configured in etc/emqx.conf:

broker.sys_interval = 1m

TIP

By default, only MQTT clients on localhost is allowed to subscribe to the $SYS topic. Please refer to built-in ACL to modify the ACL rules for publish and subscription.

Most of the data of the $SYS topic in EMQX Broker can be obtained through other method with lower Couplings. The device online and offline status can be obtained through Webhook), and the node and cluster status can be obtained through HTTP API - Statistics Metrics.

Cluster status information

Client Online and Offline Events

$SYS topic prefix: $SYS/brokers/${node}/clients/

The Payload of the ‘connected’ event message can be parsed into JSON format:

{
    "username": "foo",
    "ts": 1625572213873,
    "sockport": 1883,
    "proto_ver": 4,
    "proto_name": "MQTT",
    "keepalive": 60,
    "ipaddress": "127.0.0.1",
    "expiry_interval": 0,
    "connected_at": 1625572213873,
    "connack": 0,
    "clientid": "emqtt-8348fe27a87976ad4db3",
    "clean_start": true
}

The Payload of the ‘disconnected’ event message can be parsed into JSON format:

{
    "username": "foo",
    "ts": 1625572213873,
    "sockport": 1883,
    "reason": "tcp_closed",
    "proto_ver": 4,
    "proto_name": "MQTT",
    "ipaddress": "127.0.0.1",
    "disconnected_at": 1625572213873,
    "clientid": "emqtt-8348fe27a87976ad4db3"
}

Statistics

System topic prefix : $SYS/brokers/${node}/stats/

Client statistics

Subscription statistics

Topic statistics

Routes statistics

The topics.count and topics.max are numerically equal to routes.count and routes.max.

Throughput (bytes/packets/message) statistics

System Topic Prefix : $SYS/brokers/${node}/metrics/

sent and received bytes statistics

sent and received MQTT packets statistics

MQTT sent and received messages statistics

Alarms - system alarms

System Topic Prefix: $SYS/brokers/${node}/alarms/

Sysmon - system monitoring

System Topic Prefix: $SYS/brokers/${node}/sysmon/