cgi.d : cgi_no_threads no supported by websocket server

[andre2007]

In my scenario, I have a WebSocket server which provides access to a third party library which has a state. WebSocket requests will will be triggered only by 1 client and in sequence.

As far as I can see the version cgi_no_threads is only for debugging and also has no effect on a WebSocket server.

Is there any possibility to force a single threaded WebSocket server?

[adamdruppe]

On Tue, Mar 24, 2020 at 10:30:44PM -0700, andre2007 wrote:

As far as I can see the version cgi_no_threads is only for debugging and also has no effect on a WebSocket server.

did you try it? it might just work.

but indeed i don't really recommend it, nothing in there is eally designed for single threading, it makes the server liable to hang ups by one user handle.

[adamdruppe]

there's also a variable processPoolSize if you are on Linux and specifically using -version=embedded_httpd_processes

you can set that to 1 before calling cgiMainImpl and kinda get the same thing

or just modify your copy of the code, line 4112 has the thread pool declaration and it can be downsized.

[andre2007]

Thanks Adam, setting processPoolSize to 1 has the desired effect.

Yes I tested cgi_no_threads with this code and it has no effect. The text "initialize map" should only written once to console but is written 8 times (processPoolSize).

/+ dub.sdl:
    name "server"
    dependency "arsd-official:cgi" version="6.0.2"
    versions "cgi_no_threads"
+/

import std;
import arsd.cgi;

void main() 
{
    writeln("Listening on port 5000");
    cgiMainImpl!((cgi) { websocketEcho(cgi); }, Cgi, defaultMaxContentLength)(["--port", "5000"]);
}

static string[string] myMap;

void websocketEcho(Cgi cgi) {
    assert(cgi.websocketRequested(), "i want a web socket!");
    auto websocket = cgi.acceptWebsocket();

    while(websocket.recvAvailable(300.seconds)) 
    {
        auto msg = websocket.recv();
        if (msg.opcode == WebSocketOpcode.close) break;
        if (myMap.keys.length == 0)
        {
            writeln("initialize map");
            myMap["foo"] = "bar";
        }

        websocket.send(msg.data);

    }
    websocket.close();
}

client.py

import asyncio
import websockets

async def hello():
    data = b'\x00\x00\x00'
    uri = "ws://localhost:5000"

    for x in range(0, 50):
        async with websockets.connect(uri) as websocket:
            await websocket.send(data)
            recv_buffer = await websocket.recv()
            assert(recv_buffer == data)

asyncio.get_event_loop().run_until_complete(hello())

[adamdruppe]

On Wed, Mar 25, 2020 at 09:56:00PM -0700, andre2007 wrote:

Yes I tested cgi_no_threads with this code and it has no effect. The text "initialize map" should only written once to console but is written 8 times (processPoolSize).

no_threads only applies when paired with embedded_httpd_threads.

the default on linux is embedded_httpd_processes which uses processPoolSize instead. On Windows it defaults to the threads implementation. So difference there on platform but you can specify with -version... not sure if the dub configurations will fully respect it though, i have never tried.

(none of this is really documented since it is all really internal implementation details in my mind. well, the httpd_processes vs httpd_threads is documented, but none of the hacky tweaks.)

[andre2007]

Thanks for the explanation. You are right, the SDL versions statement does not work. I created a DUB issue: https://github.com/dlang/dub/issues/1909

I forgot I can't use embedded_httpd_threads at the moment as lambdas are not accepted (https://github.com/adamdruppe/arsd/issues/241#issuecomment-589219904)