# MQTT proxy subscription
The proxy subscription function of EMQ X enables the client to automatically establish the user's preset subscription relationship without sending additional SUBSCRIBE messages when the connection is established.
# Create module
Open EMQ X Dashboard (opens new window), click the "Modules" tab on the left, and choose to add:
Select MQTT broker subscription module
Configure related parameters
After clicking add, the module is added
# Subscription options
Quality of Service (QoS)
The maximum QoS level of application messages that the server can send to the client.
NL (No Local)
Whether application messages can be forwarded to the client that posted this message.
-When the NL value is 0, it means that the application message can be forwarded to the client that published the message.
-When the NL value is 1, it means that the application message cannot be forwarded to the client that published the message.
RAP (Retain As Published)
When forwarding application messages to this subscription, whether to keep the RETAIN flag set when the message is published.
-When the RAP value is 0, it means that the reserved flag is set to 0 when forwarding application messages to this subscription.
-When the RAP value is 1, it means to keep the reserved flag set when the message is published when the application message is forwarded to this subscription.
RH (Retain Handling)
Whether to send a hold message when the subscription is established
-0: Send a reserved message when the subscription is established
-1: When the subscription is established, if the subscription does not currently exist, a reserved message will be sent
-2: Do not send reserved messages when the subscription is established
# Agent subscription rules
When configuring the topic subscribed by the agent, EMQ X provides two placeholders
%u for users to use. EMQ X will configure the
Replaced with the client'sClient ID
and ʻUsername respectively. It should be noted that
%u must occupy an entire topic level.
For example, after adding the rules in the above figure: configure two clients A and B, the
Client ID of client A is
testerA, and the
Client ID of client B is
The client A uses the MQTT V3.1.1 protocol to connect to EMQ X. According to the above configuration rules, the proxy subscription function will actively help the client subscribe to the two
client/testclientA with QoS 1 and ʻuser/testerA` with QoS 2. For this topic, because the connection protocol is MQTT V3.1.1, No Local, Retain As Published, and Retain Handling in the configuration are not effective.
Client B uses the MQTT V5 protocol to connect to EMQ X. According to the configuration rules above, the proxy subscription function will actively help the client to subscribe to the two topics
client/testclientB and ʻuser/testerB
, among whichclient/testclientB
The subscription options are Qos = 1, No Local, Retain As Published, and Retain Handling are all 0; the subscription options of ʻuser/testerB are Qos = 2, No Local = 1, Retain As Published = 1 and Retain Handling = 1.