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客户端可以通过订阅$SYS树来查询broker的信息。标记为static的topic只在客户端订阅时发送一次，其他的topic会每隔sys_interval 秒更新一次， 如果sys_interval 为0，则不会发送更新。

注意，如果使用命令行与$SYS topic交互，shell会把$解析为环境变量，所以需要将topic用单引号括起来例如’ $SYS/...’或者使用转义字符，例如 \$SYS/..

下面列出了所有相关的topic

• $SYS/broker/bytes/received

Broker从启动之后至今接收到的字节总数

The total number of bytes received since the broker started.

• $SYS/broker/bytes/sent

Broker从启动之后至今发送的字节总数

The total number of bytes sent since the broker started.

• $SYS/broker/clients/connected,

$SYS/broker/clients/active (不再使用)

当前连接的客户端的总数

The number of currently connected clients.

• $SYS/broker/clients/expired

The number of disconnected persistent clients that have been expired and removed through the persistent_client_expiration option.

• $SYS/broker/clients/disconnected,

$SYS/broker/clients/inactive (deprecated)

The total number of persistent clients (with clean session disabled) that are registered at the broker but are currently disconnected.

• $SYS/broker/clients/maximum

The maximum number of clients that have been connected to the broker at the same time.

• $SYS/broker/clients/total

The total number of active and inactive clients currently connected and registered on the broker.

• $SYS/broker/connection/#

When bridges are configured to/from the broker, common practice is to provide a status topic that indicates the state of the connection. This is provided within $SYS/broker/connection/ by default. If the value of the topic is 1 the connection is active, if 0 then it is not active. See the Bridges section below for more information on bridges.

• $SYS/broker/heap/current size

The current size of the heap memory in use by mosquitto. Note that this topic may be unavailable depending on compile time options.

• $SYS/broker/heap/maximum size

The largest amount of heap memory used by mosquitto. Note that this topic may be unavailable depending on compile time options.

• $SYS/broker/load/connections/+

The moving average of the number of CONNECT packets received by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of connections received in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/bytes/received/+

The moving average of the number of bytes received by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of bytes received in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/bytes/sent/+

The moving average of the number of bytes sent by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of bytes sent in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/messages/received/+

The moving average of the number of all types of MQTT messages received by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of messages received in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/messages/sent/+

The moving average of the number of all types of MQTT messages sent by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of messages send in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/publish/dropped/+

The moving average of the number of publish messages dropped by the broker over different time intervals. This shows the rate at which durable clients that are disconnected are losing messages. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of messages dropped in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/publish/received/+

The moving average of the number of publish messages received by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of publish messages received in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/publish/sent/+

The moving average of the number of publish messages sent by the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of publish messages sent in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/load/sockets/+

The moving average of the number of socket connections opened to the broker over different time intervals. The final "+" of the hierarchy can be 1min, 5min or 15min. The value returned represents the number of socket connections in 1 minute, averaged over 1, 5 or 15 minutes.

• $SYS/broker/messages/inflight

The number of messages with QoS>0 that are awaiting acknowledgments.

• $SYS/broker/messages/received

The total number of messages of any type received since the broker started.

• $SYS/broker/messages/sent

The total number of messages of any type sent since the broker started.

• $SYS/broker/publish/messages/dropped

The total number of publish messages that have been dropped due to inflight/queuing limits. See the max_inflight_messages and max_queued_messages options in  for more information.

• $SYS/broker/publish/messages/received

The total number of PUBLISH messages received since the broker started.

• $SYS/broker/publish/messages/sent

The total number of PUBLISH messages sent since the broker started.

• $SYS/broker/retained messages/count

The total number of retained messages active on the broker.

• $SYS/broker/store/messages/count,

$SYS/broker/messages/stored (deprecated)

The number of messages currently held in the message store. This includes retained messages and messages queued for durable clients.

• $SYS/broker/store/messages/bytes

The number of bytes currently held by message payloads in the message store. This includes retained messages and messages queued for durable clients.

• $SYS/broker/subscriptions/count

The total number of subscriptions active on the broker.

• $SYS/broker/version

The version of the broker. Static.

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