My personal understanding of the MQTT protocol and emQTTD Open Source Project (22) – on the subject of client on-line automatic subscription

You can do this by modifying the configuration file.

Emq v1.1.3, rel \ emqttd \ etc \ emqttd config

The ‘Subscription’ extension allows you to automatically subscribe or resume subscribing to certain topics when a client goes live:

%% Modules
    {modules, [
        %% Client presence management module.
        %% Publish messages when client connected or disconnected
        {presence, [{qos, 0}]},

        %% Subscribe topics automatically when client connected
        {subscription, [

            %% $c will be replaced by clientid
            %% {"$queue/clients/$c", 1},
            {"$c/w", 1}

            %% Static subscriptions from backend
            %%backend
        ]}

        %% Rewrite rules
        %% {rewrite, [{file, "etc/rewrite.config"}]}
    ]},
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In emQ v2.3.11, the software architecture has been adjusted to integrate functions into the EMq_modules module. /data/loaded_plugins load emq_modules by default.

We only need to change the configuration file emqx-rel-2.3.11/deps/emq_modules/etc/emq_modules.conf

##--------------------------------------------------------------------
## Subscription Module
##--------------------------------------------------------------------

## Enable Subscription Module.
##
## Value: on | off
module.subscription = on

## Subscribe the Topics automatically when client connected.
module.subscription.1.topic = %c/w
## Qos of the subscription: 0 | 1 | 2
module.subscription.1.qos = 1
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Set Module. subscription to on,

The module. Subscription. 1. The topic is set to % c/w, % c is wildcard, will automatically switch to clientId. Note that it is %c, not $c.