$SYS - System Topic

The EMQ X 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

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

Most of the data of the $ SYS topic in EMQ X 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

Topic Description
$SYS/brokers cluster node list
$SYS/brokers/${node}/version EMQ X Broker version
$SYS/brokers/${node}/uptime EMQ X Broker startup time
$SYS/brokers/${node}/datetime EMQ X Broker time
$SYS/brokers/${node}/sysdescr EMQ X Broker description

Client Online and Offline Events

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

Topic Description
${clientid}/connected Online event. This message is published when a client goes online
${clientid}/disconnected Offline event. This message is published when a client is offline

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

{
    "username":"undefined",
    "ts":1582687922392,
    "sockport":1883,
    "proto_ver":5,
    "proto_name":"MQTT",
    "keepalive":300,
    "ipaddress":"127.0.0.1",
    "expiry_interval":0,
    "connected_at":1582687922,
    "connack":0,
    "clientid":"emqtt-8348fe27a87976ad4db3",
    "clean_start":true
}

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

{
    "username":"undefined",
    "ts":1582688032203,
    "reason":"tcp_closed",
    "disconnected_at":1582688032,
    "clientid":"emqtt-8348fe27a87976ad4db3"
}

Statistics

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

Client statistics

Topic Description
connections/count Total number of current clients
connections/max Maximum number of clients

Subscription statistics

Topic Description
suboptions/count number of current subscription options
suboptions/max total number of maximum subscription options
subscribers/count number of current subscribers
subscribers/max maximum number of subscriptions
subscriptions/count total number of current subscription
subscriptions/max maximum number of subscriptions
subscriptions/shared/count total number of current shared subscriptions
subscriptions/shared/max maximum number of shared subscriptions

Topic statistics

Topic Description
topics/count total number of current topics
topics/max maximum number of topics

Routes statistics

Topic Description
routes/count total number of current Routes
routes/max maximum number of Routes

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

Topic Description
bytes/received Accumulated received bytes
bytes/sent Accumulated sent bytes

sent and received MQTT packets statistics

Topic Description
packets/received Accumulative received MQTT packets
packets/sent Accumulative sent MQTT packets
packets/connect/received Accumulative received packets of CONNECT
packets/connack/sent Accumulative sent packets of CONNACK
packets/publish/received Accumulative received packets of PUBLISH
packets/publish/sent Accumulative sent packets of PUBLISH
packets/publish/error Accumulative handling packets of PUBLISH error
packets/publish/auth_error Accumulative denied packets of PUBLISH
packets/publish/dropped Accumulative dropped packets of PUBLISH
packets/puback/received Accumulative received packets of PUBACK
packets/puback/sent Accumulative sent packets of PUBACK
packets/puback/inuse Accumulative dropped packets of PUBACK
packets/puback/missed Accumulative missed packets of PUBACK
packets/pubrec/received Accumulative received packets of PUBREC
packets/pubrec/sent Accumulative sent packets of PUBREC
packets/pubrec/inuse Accumulative dropped packets of PUBREC
packets/pubrec/missed Accumulative missed packets of PUBREC
packets/pubrel/received Accumulative received packets of PUBREL
packets/pubrel/sent Accumulative sent packets of PUBREL
packets/pubrel/missed Accumulative missed packets of PUBREL
packets/pubcomp/received Accumulative received packets of PUBCOMP
packets/pubcomp/sent Accumulative sent packets of PUBCOMP
packets/pubcomp/inuse Accumulative dropped packets of PUBCOMP
packets/pubcomp/missed Accumulative missed packets of PUBCOMP
packets/subscribe/received Accumulative received packets of SUBSCRIBE
packets/subscribe/error Accumulative handling packets of SUBSCRIBE error
packets/subscribe/auth_error Accumulative denied packets of SUBSCRIBE
packets/suback/sent Accumulative sent packets of SUBACK
packets/unsubscribe/received Accumulative received packets of UNSUBSCRIBE
packets/unsuback/sent Accumulative sent packets of UNSUBACK
packets/pingreq/received Accumulative received packets of PINGREQ
packets/pingresp/sent Accumulative sent packets of PINGRESP
packets/disconnect/received Accumulative received packets of DISCONNECT
packets/disconnect/sent Accumulative sent packets of DISCONNECT
packets/auth/received Accumulative received packets of AUTH
packets/auth/sent Accumulative sent packets of AUTH

MQTT sent and received messages statistics

Topic Description
messages/received Accumulative received messages
messages/sent Accumulative sent messages
messages/qos0/received Accumulative received messages of QoS0
messages/qos0/sent Accumulative sent messages of QoS0
messages/qos1/received Accumulative received messages QoS1
messages/qos1/sent Accumulative sent messages QoS1
messages/qos2/received Accumulative received messages of QoS2
messages/qos2/sent Accumulative sent messages of QoS2
messages/publish Accumulative PUBLISH messages
messages/dropped Total number of dropped messages
messages/dropped/expired Total number of dropped messages (Expired)
messages/dropped/no_subscribers Total number of dropped messages (No subscriber)
messages/forward Total number of messages forwarded by the node
messages/retained Accumulative retained messages
messages/delayed Accumulative delayed messages
messages/delivered Accumulative delivered messages
messages/acked Accumulative acked messages

Alarms - system alarms

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

Topic Description
alert newly generated alarm
clear cleared alarm

Sysmon - system monitoring

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

Topic Description
long_gc GC Overtime alarm
long_schedule Alarm for Excessive Scheduling Time
large_heap ALarm for Heap Memory Occupancy
busy_port Alarm for Port busy
busy_dist_port Alarm for Dist Port busy

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