Event Topics
The Rule of Data Integrations provides several event topics available for FROM clause.
| Event topic name | Explanation |
|---|---|
| $events/client_connected | Client connect |
| $events/client_disconnected | Client disconnect |
| $events/message_delivered | Message delivery |
| $events/message_acked | Message acknowledged |
| $events/message_dropped | Message dropped when routing |
| $events/delivery_dropped | Message dropped when delivering |
| $events/session_subscribed | Subscribe |
| $events/session_unsubscribed | Unsubscribe |
This article will introduce the usage of event topics, and the meaning of each field from three aspects: client connect and disconnect events, message events, topic subscribe and unsubscribe events.
Client connect and disconnect events
Client connect
The topic of client connect is $events/client_connected, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/client_connected"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clean_start | MQTT clean_start |
| clientid | Client ID |
| connected_at | Terminal connection completion time (s) |
| event | Event type, fixed at "client.connected" |
| expiry_interval | MQTT Session Expiration time |
| is_bridge | Whether it is MQTT bridge connection |
| keepalive | MQTT keepalive interval |
| mountpoint | Mountpoint for bridging messages |
| node | Node name of the trigger event |
| peername | IPAddress and Port of terminal |
| proto_name | Protocol name |
| proto_ver | Protocol version |
| sockname | IPAddress and Port listened by emqx |
| timestamp | Event trigger time (ms) |
| username | Current MQTT username |
Client disconnect
The topic of client disconnect is $events/client_disconnected, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/client_disconnected"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clientid | Client ID |
| disconnected_at | Terminal disconnection completion time (s) |
| event | Event type, fixed at "client.disconnected" |
| node | Node name of the trigger event |
| expiry_interval | MQTT Session Expiration time |
| sockname | IPAddress and Port listened by emqx |
| timestamp | Event trigger time (ms) |
| username | Current MQTT username |
| peername | IPAddress and Port of terminal |
| reason | Reason for disconnection of terminal |
Here are some common reasons for client disconnections.
| types | reason |
|---|---|
| normal | MQTT client disconnects normally |
| kicked | Using the REST API to kick out clients |
| keepalive_timeout | MQTT keepalive timeout |
| not_authorized | Authentication fails, or Pub/Sub without permission will actively disconnect the client when acl_nomatch = disconnect |
| tcp_closed | The TCP connection is actively closed by the client |
| receive_maximum_exceeded | When a client sends a qos2 message, a large number of messages pile up on the MQTT Broker because it does not reply to the PUBREL in time. When the number of message pileups reaches the upper limit, EMQX will actively disconnect |
| internal_error | Malformed packet parsing error |
| einval | EMQX wants to send a message to the client, but the Socket has been disconnected |
| function_clause | MQTT packet format error |
| etimedout | TCP send timeout (no TCP ACK response received) |
| proto_unexpected_c | MQTT connection request received repeatedly when there is already an MQTT connection |
| idle_timeout | 15s after the TCP connection is established, the connect message has not been received |
| others | Please refer to MQTT Reason Code and ACK |
Message events
Message delivery
The topic of client disconnect is $events/message_delivered, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/message_delivered"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clientid | Client ID |
| event | Event type, fixed at "message.delivered" |
| flags | MQTT Message flags |
| from_clientid | Message source client ID |
| from_username | Message source username |
| id | MQTT message ID |
| node | Node name of the trigger event |
| payload | MQTT payload |
| peerhost | Client IPAddress |
| publish_received_at | Time when PUBLISH message reaches Broker (ms) |
| qos | Enumeration of message QoS 0,1,2 |
| timestamp | Event trigger time (ms) |
| topic | MQTT topic |
| username | Current MQTT username |
Message acknowledged
The topic of client disconnect is $events/message_acked, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/message_acked"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clientid | Client ID |
| event | Event type, fixed at "message.acked" |
| flags | MQTT message flags |
| from_clientid | Message source client ID |
| from_username | Message source username |
| id | MQTT message ID |
| node | Node name of the trigger event |
| payload | MQTT payload |
| peerhost | Client IPAddress |
| publish_received_at | Time when PUBLISH message reaches Broker (ms) |
| qos | Enumeration of message QoS 0,1,2 |
| timestamp | Event trigger time (ms) |
| topic | MQTT topic |
| username | Current MQTT username |
Message dropped
The topic of a message dropped is $events/message_dropped, which triggers the trigger rule when a message has no subscribers. Click on Rules on the left menu bar, click on Create, and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/message_dropped"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| Field | Explanation |
|---|---|
| id | MQTT message id |
| reason | Reasons of dropping, possible reasons: no_subscribers: no clients subscribes the topic receive_maximum_exceeded: awaiting_rel queue is full packet_identifier_inuse: send a qos2 message with unreleased packet ID |
| clientid | Client ID of the sender |
| username | Username of the sender |
| payload | MQTT payload |
| peerhost | Client IPAddress |
| topic | MQTT topic |
| qos | Enumeration of message QoS 0,1,2 |
| flags | Flags |
| pub_props | The PUBLISH Properties (MQTT 5.0 only) |
| timestamp | Event trigger time(millisecond) |
| publish_received_at | Time when PUBLISH message reaches Broker (ms) |
| node | Node name of the trigger event |
Delivery_dropped
The topic of delivery_dropped is `$events/delivery_dropped, trigger rule when subscriber's message queue is full. Click on Rules on the left menu bar, click on Create, and enter the following rule to match the SQL statement:
SELECT
*
FROM
"$events/delivery_dropped"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| Field | Explanation |
|---|---|
| id | MQTT message id |
| reason | Reasons of dropping, possible reasons: queue_full: the message queue is full(QoS>0) no_local: it's not allowed for the client to received messages published by themselves expired: the message or the session is expired qos0_msg: the message queue is full(QoS0) |
| clientid | Client ID of the sender |
| from_clientid | Client ID of the sender |
| from_username | Username of the sender |
| username | Username of the sender |
| payload | MQTT payload |
| peerhost | Client IPAddress |
| topic | MQTT topic |
| qos | Enumeration of message QoS 0,1,2 |
| flags | Flags |
| pub_props | The PUBLISH Properties (MQTT 5.0 only) |
| timestamp | Event trigger time(millisecond) |
| publish_received_at | Time when PUBLISH message reaches Broker (ms) |
| node | Node name of the trigger event |
Topic subscribe and unsubscribe events
Subscribe
The topic of client disconnect is $events/session_subscribed, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/session_subscribed"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clientid | Client ID |
| event | Event type, fixed at "session.subscribed" |
| node | Node name of the trigger event |
| peerhost | Client IPAddress |
| qos | Enumeration of message QoS 0,1,2 |
| timestamp | Event trigger time (ms) |
| topic | MQTT topic |
| username | Current MQTT username |
Unsubscribe
The topic of client disconnect is $events/session_unsubscribed, click on Rules on the left menu bar → Rules, click on Create and enter the following rule to match the SQL statement.
SELECT
*
FROM
"$events/session_unsubscribed"You can test the SQL, view the return field after firing this rule. 
The fields are explained as follows.
| field | Explanation |
|---|---|
| clientid | Client ID |
| event | Event type, fixed at "session.unsubscribed" |
| node | Node name of the trigger event |
| peerhost | Client IPAddress |
| qos | Enumeration of message QoS 0,1,2 |
| timestamp | Event trigger time (ms) |
| topic | MQTT topic |
| username | Current MQTT username |