/nova_chat_cluster

Primary LanguageErlang

Nova Chat

screengrab

A pubsub chat demo using websockets and Nova.

This implementation uses nova_pubsub to store all state in ets. Should the server restart, all current topics are therefore closed and users will need to reconnect.

How it works

The client POSTs to:

await fetch(`http://localhost:8080/user/${user}/subscribe`,
    { method: 'POST', body: `{"topic":"${topic}"}` }
)

Informing the backend that user wants to subscribe to topic.

%% src/controllers/nova_chat_main_controller.erl

subscribe(#{method := <<"POST">>,
            bindings := #{<<"user">> := User}} = Req) ->
    {ok, Data, _} = cowboy_req:read_body(Req),
    #{<<"topic">> := Topic} = json:decode(Data, [maps]),
    nova_pubsub:subscribe(User, Topic),
    {json, <<"Subscribed!">>};

client now connects to the backend:

socket = new WebSocket(`ws://localhost:8080/user/${user}/ws`)
%% src/nova_chat_ws.erl

websocket_init(State) ->
    #{<<"user">> := User} = State,
    ok = nova_pubsub:online(User, self()),
    {ok, State}.

This sets user as online on that socket.

When a user wants to write a message, client pushes data on the socket that gets pickuped up by the backend:

socket.send(JSON.stringify(
    { 'topic': 'erlang', 'payload': 'Hello Joe!' }))
%% src/nova_chat_ws.erl

websocket_handle({text, Message}, State) ->
    Decode = json:decode(Message, [maps]),
    #{<<"user">> := User} = State,
    #{<<"topic">> := Topic} = Decode,
    Json = json:encode(Decode#{<<"user">> => User}, 
                       [maps, binary]),
    ok = nova_pubsub:publish(Topic, Json),
    {ok, State}.

Nova looks up all the users that are subscribed to topic, and if they are online pushes this message to client:

{
    "topic": "erlang",
    "user": "nova",
    "payload":"Hello Joe!"
}