using EMQTT to create clients in Erlang/OTP does not work.

[SaveMe0]

The snippet of code that was posted by William Yang on on Dec 20, 2024 does not work as the emqtt functions I am using in my com2.erl module and contained in this discussion are the same. The only difference between the minimal stuff and my code is the host name.

I know I can communicate with emqtt because my command line subscriptions and publications work perfectly. So the problem can only exist in emqtt!

here is the minimal stuff you want, assuming you are connecting to "127.0.0.1:14567"
{ok, Pid} = emqtt:start_link([{host, {127,0,0,1}},
                           {port, 14567}
                          ]),
 _ =emqtt:quic_connect(Pid),
emqtt:subscribe(Pid, <<"a/b/c">>, qos0).

Here is my code. It is using the same minimal emqtt function above. The only thing difference is the host IP address. I am running the code on Debain 12 hosting Erlang/OTP 27.

-module(com2).
-behaviour(gen_statem).
-define(NAME, com2).

-export([start/0, stop/0]).
-export([cnct/0, sub/0]).
-export([cnctState/3, subState/3]).

-export([init/1,callback_mode/0, handle_event/3, terminate/3, code_change/4]).

start() ->
    {ok, Pid} = gen_statem:start_link({local,?NAME}, ?MODULE, [], []),
    io:format("My proccess Pid ~p~n", [Pid]).

cnct() ->
    gen_statem:cast(?NAME, cnct).

sub() ->
    %gen_statem:call(?NAME, sub).
    gen_statem:cast(?NAME, sub).

stop() ->
    gen_statem:stop(?NAME).

-record(comm, {connPid, emqttMsg}).

callback_mode() ->
    [state_functions,state_enter].

init(_Args) ->
    process_flag(trap_exit, true),
    Properties = #{'Session-Expiry-Interval' => 300,
                    'Server-Keep-Alive' => 30,
                    'Retain-Available' => 1
                },

    Options = [ {host, "192.168.178.33"},
                {port, 14567}
                %%{keepalive, 30},
                %{clientid, <<"alb65McbrideBCCanada">>}, 
                %%{proto_ver, v5},
                %%{reconnect_timeout, 1000}
                %{quic_opts, {quicer:conn_opts(), quicer:stream_opts()}},
                %%{properties, Properties}
            ],

    {ok, ConnPid} = emqtt:start_link(Options),
    io:format("ConnPid from emqtt:start_link/1 ~p~n", [ConnPid]),
    State = cnctState, Data = #comm{connPid = ConnPid},
    %io:format("Data ~p~n", [Data]),
    {ok, State, Data}.


cnctState(cast, cnct, Data) ->
    ConnPid = Data#comm.connPid,
	io:format("ConnPid for emqtt:quic_connect/1 ~p~n", [ConnPid]),
    ConnPid = Data#comm.connPid,
    emqtt:quic_connect(ConnPid),
    %emqtt:open_quic_connection(ConnPid),
    %emqtt:quic_mqtt_connect(ConnPid),
    {next_state, subState, Data};
cnctState(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data).   

%subState({call, From}, sub, Data) ->
subState(cast, sub, Data) ->
    ConnPid = Data#comm.connPid,
    io:format("CommPid for emqtt:subscribe/3 ~p~n", [ConnPid]),
    %Properties = #{},
    Properties = #{ %'Maximum-Packet-Size' => 1048576,
                    %'Receive-Maximum' => 32,
                    %'Retain-Available' => 1,
                    %'Shared-Subscription-Available' => 1,
                    %'Subscription-Identifier-Available' => 1,
                    %'Topic-Alias-Maximum' => 65535,
                    %'Wildcard-Subscription-Available' => 1,
                    %%'Message-Expiry-Interval' => 10,
                    %'Content-Type' => <<"Json">>,
                    %%'Payload-Format-Indicator' => 1,
                    %%'Content-Type' => <<"application/json">>,
                    %%'Server-Keep-Alive' => 30
                    %'Maximum-QoS' => 1
                    },
   SubOpts = [qos1],
    Subscriptions = [{<<"EK9160/EK-98EFB6/Stream1/Json/Tx/Data">>, SubOpts}],
    _ = emqtt:subscribe(ConnPid, Properties, Subscriptions),
    %io:format("Subscription~p~n", [Reply]),
    %{keep_state,Data, [{noreply, From, Data}]};
    {next_state, subState, Data};
subState(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data). 

handle_event(_, _, Data) ->
    %% Ignore all other events
    {keep_state, Data}.

terminate(_Reason, _State, _Data) ->
    ok.

code_change(_Vsn, State, Data, _Extra) ->
{ok, State, Data}.

When I run the com2.erl module shown above, I get the following result

1> com2:start().
ConnPid from emqtt:start_link/1 <0.209.0>
My proccess Pid <0.208.0>
ok
2> com2:cnct().
ConnPid for emqtt:quic_connect/1 <0.209.0>
ok
3> com2:sub().
CommPid for emqtt:subscribe/3 <0.209.0>
ok
4>  

[SaveMe0]

The sentence
"I know I can communicate with emqtt because my command line subscriptions and publications work perfectly. So the problem can only exist in emqtt!" above does not read correctly.

What I should have written is
"With emqtt It is that it is possible to subscribe and publish through the command line/Linux Shell but it is not possible to create Erlang/OTP clients using emqtt for what it has been designed for."