Out-of-band Communication

Typically, Ray actor communication is done through actor method calls and data is shared through the distributed object store. However, in some use cases out-of-band communication can be useful.

Wrapping Library Processes

Many libraries already have mature, high-performance internal communication stacks and they leverage Ray as a language-integrated actor scheduler. The actual communication between actors is mostly done out-of-band using existing communication stacks. For example, Horovod-on-Ray uses NCCL or MPI-based collective communications, and RayDP uses Spark’s internal RPC and object manager. See Ray Distributed Library Patterns for more details.

Ray Collective

Ray’s collective communication library (ray.util.collective) allows efficient out-of-band collective and point-to-point communication between distributed CPUs or GPUs. See Ray Collective for more details.

HTTP Server

You can start a http server inside the actor and expose http endpoints to clients so users outside of the ray cluster can communicate with the actor.

import ray
import asyncio
import requests
from aiohttp import web


@ray.remote
class Counter:
    async def __init__(self):
        self.counter = 0
        asyncio.get_event_loop().create_task(self.run_http_server())

    async def run_http_server(self):
        app = web.Application()
        app.add_routes([web.get("/", self.get)])
        runner = web.AppRunner(app)
        await runner.setup()
        site = web.TCPSite(runner, "localhost", 8080)
        await site.start()

    async def get(self, request):
        return web.Response(text=str(self.counter))

    async def increment(self):
        self.counter = self.counter + 1


ray.init()
counter = Counter.remote()
[ray.get(counter.increment.remote()) for i in range(5)]
r = requests.get("http://localhost:8080/")
# Should print "5"
print(r.text)

Similarly, you can expose other types of servers as well (e.g., gRPC servers).

Limitations

When using out-of-band communication with Ray actors, keep in mind that Ray does not manage the calls between actors. This means that functionality like distributed reference counting will not work with out-of-band communication, so you should take care not to pass object references in this way.