how to make server_send() synchronous about simple-websocket-server

[eidheim]

Consider this simpler example, send_stream1 and send_stream2 are here sent synchronously:

    //Example: Echo twice
    //  Sending received messages two times to connected client
    //  Test with the following JavaScript:
    //    var ws=new WebSocket("ws://localhost:8080/echo_twice");
    //    ws.onmessage=function(evt){console.log(evt.data);};
    //    ws.send("test");
    auto& echo_twice=server.endpoint["^/echo_twice/?$"];
    echo_thrice.onmessage=[&server](shared_ptr<WsServer::Connection> connection, shared_ptr<WsServer::Message> message) {
        auto message_str=message->string();

        auto send_stream1=make_shared<WsServer::SendStream>();
        *send_stream1 << message_str;
        //server.send is an asynchronous function
        server.send(connection, send_stream1, [&server, connection, message_str](const boost::system::error_code& ec) {
            if(!ec) {
                //send_stream1 has now been received by the client without error
                auto send_stream2=make_shared<WsServer::SendStream>();
                *send_stream2 << message_str;
                server.send(connection, send_stream2); //send_stream2 is now being sent
            }
        });
    };

edit: updated comments

[eidheim]

I would think, however, that creating some kind of synchronisation between the server and client would be a good idea. For instance:
Client: send me some data (for instance frame 0-4)
Server: here is data
Client: thank you, I need some more data now (for instance frame 5-9)
Server: here is some more data
Client: thank you, I need some more data now (for instance frame 10-14)
Server: here is some more data
and so on

Especially when creating a stream application, since you might want the client to buffer some of the stream, but only up to a point, and this would depend on data transferred and data shown on the client.

Edit: in the above case I would only use one server.send on the server side for each "Server: ....."

[alanywlee]

Thanks for your instruction about synchronous data transfer. I doubt the delay I encountered is probably there are some limitation on size of binary data to send synchronously. Regarding to the simple modification shown in #23, I encountered more than 5 seconds delay even if I send whole binary file in one chunk.

[eidheim]

I saw now in #23 that you are using several client.send(send_stream, nullptr, 130);, that is with the same send_stream. This will lead to unexpected results, and is most likely causing the strange delay. Try to do client.send(send_stream, nullptr, 130); only once.

A new SendStream object needs to be created when doing server.send if one does not know if a previous server.send using a SendStream object has completed.

Edit: or better yet, create a new SendStream object for each server.send.

[alanywlee]

Regarding to your instruction to send synchronous, it seems not able to send fix size (e.g., 80 pixels per chunk) of binary chunks using a for loop. Hard code 10 - 20 server.send(){} by hand seems a little bit wired.

[eidheim]

Avoid for loops when working with an asynchronous protocol like websocket. If you need to send several messages in one event, you should most likely implement it as a recursive function. Also you need to make sure your file stays open and is accessible in the callback (send_message2). I suggest though that you implement it as the Client-Message example I wrote in my second-last post. That is, with one server.send for each onmessage for instance. I think this would be simplest.