Asynchronously spawn Bevy and another task

[bobbbay]

Hey Bevy!

I have an application that needs to serve a Tonic (gRPC) server and a Bevy app in one binary. They also need to be able to share data over threads (through Mutexes).

I have tried:

• Spawning each through Tokio and Futures. This doesn't work, because winit warns against spawning itself from anything but the main thread, and requires a special configuration in order to enable it. I'm not sure how to ask Bevy to enable this configuration.

• Spawning the server through a system using a thread pool and the AsyncComputeTaskPool example. A con with this is that the server never technically "ends" until the application is shut down, so I'll have a system that is infinitely running. But, even so, after trying, it doesn't seem to work - I can't connect to my server. It seems the system is erroring out because there is no Tokio reactor running.

  Here's the system I use for that:

  fn server(thread_pool: Res<AsyncComputeTaskPool>) {
      thread_pool.spawn(async {
          // Irrelevant Tonic-specific code:
          let addr = "[::1]:50061".parse().unwrap();
          let temperature = TemperatureSensor::default();
          Server::builder()
              .add_service(TemperatureServiceServer::new(temperature))
              .serve(addr)
              .await
              .unwrap();
      });
  }

  Even if this solution works out, I'm still skeptical that it's the best one, because I don't love the idea of a system never ending one call.

Thanks for all the help I can get!

[bho01]

Hello, did you ever find a solution to this? I am trying to implement grpc (client-side) into my bevy app

[BrianWiz]

hey I'm in the same boat, looking at getting grpc working to communicate with a backend.

This is the best I've come up with so far, I created a bridge between the tonic client and bevy using mpsc channels. One channel is for Bevy -> Tonic client communication, the other is for Tonic client -> Bevy communication.

Server:

use hello_world::greeter_server::{Greeter, GreeterServer};
use hello_world::{HelloReply, HelloRequest};
use tonic::{transport::Server, Request, Response, Status};

pub mod hello_world {
    tonic::include_proto!("helloworld");
}

#[derive(Debug)]
pub struct MyGreeter;

#[tonic::async_trait]
impl Greeter for MyGreeter {
    async fn say_hello(
        &self,
        request: Request<HelloRequest>,
    ) -> Result<Response<HelloReply>, Status> {
        println!("Got a request: {:?}", request);
        let reply = hello_world::HelloReply {
            message: format!("Hello {}!", request.into_inner().name),
        };

        Ok(Response::new(reply))
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let addr = "[::1]:50051".parse()?;

    Server::builder()
        .add_service(GreeterServer::new(MyGreeter {}))
        .serve(addr)
        .await?;

    Ok(())
}

Client:

Tonic receives a message from Bevy, then forwards that message off to the Tonic server. It then receives a reply from the server, and sends the reply to Bevy.

use bevy::prelude::*;
use hello_world::greeter_client::GreeterClient;
use hello_world::{HelloReply, HelloRequest};
use tokio::sync::mpsc;

pub mod hello_world {
    tonic::include_proto!("helloworld");
}

pub enum BevyMessage {
    HelloRequest(HelloRequest),
}

pub enum TonicMessage {
    HelloReply(HelloReply),
}

#[derive(Resource)]
pub struct TonicBridge {
    sender: mpsc::Sender<BevyMessage>,
    receiver: mpsc::Receiver<TonicMessage>,
}

impl TonicBridge {
    pub fn send_message_to_tonic(&self, message: BevyMessage) {
        match self.sender.try_send(message) {
            Ok(_) => {}
            Err(e) => {
                println!("Error sending message: {}", e);
            }
        }
    }

    pub fn receive_message_from_tonic(&mut self) -> Option<TonicMessage> {
        match self.receiver.try_recv() {
            Ok(message) => Some(message),
            Err(_) => None,
        }
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let (bevy_sender, mut tonic_receiver) = mpsc::channel::<BevyMessage>(100);
    let (tonic_sender, bevy_receiver) = mpsc::channel::<TonicMessage>(100);

    let mut app = App::new();

    tokio::spawn(async move {
        if let Ok(mut client) = GreeterClient::connect("http://[::1]:50051").await {
            while let Some(message) = tonic_receiver.recv().await {
                match message {
                    BevyMessage::HelloRequest(hello_request) => {
                        match client.say_hello(tonic::Request::new(hello_request)).await {
                            Ok(response) => {
                                match tonic_sender
                                    .try_send(TonicMessage::HelloReply(response.into_inner()))
                                {
                                    Ok(_) => {}
                                    Err(e) => {
                                        println!("Error sending message: {}", e);
                                    }
                                }
                            }
                            Err(e) => {
                                println!("Error sending message: {}", e);
                            }
                        }
                    }
                }
            }
        }
    });

    app.add_plugins(DefaultPlugins);
    app.add_systems(Update, send_message_to_tonic);
    app.add_systems(Update, receive_message_from_tonic);
    app.insert_resource(TonicBridge {
        sender: bevy_sender,
        receiver: bevy_receiver,
    });
    app.run();

    Ok(())
}

fn send_message_to_tonic(bridge: Res<TonicBridge>) {
    bridge.send_message_to_tonic(BevyMessage::HelloRequest(HelloRequest {
        name: "Bevy".to_string(),
    }));
}

fn receive_message_from_tonic(mut bridge: ResMut<TonicBridge>) {
    if let Some(message) = bridge.receive_message_from_tonic() {
        match message {
            TonicMessage::HelloReply(reply) => {
                println!("Received message from server: {}", reply.message);
            }
        }
    }
}

[bho01]

Hey, I found a way to do something similar to you but I leverage thebevy_tokio_tasks plugin to spawn a tokio runtime and do message passing. It's cool because you can do world queries to grab resources and entities. However, it still feels a little hacky and it would be nice to pass if there was a dedicated plugin to handle gRPC requests on a resource or an entity.