Key Concepts#


Deployments are the central concept in Ray Serve. A deployment contains business logic or an ML model to handle incoming requests and can be scaled up to run across a Ray cluster. At runtime, a deployment consists of a number of replicas, which are individual copies of the class or function that are started in separate Ray Actors (processes). The number of replicas can be scaled up or down (or even autoscaled) to match the incoming request load.

To define a deployment, use the @serve.deployment decorator on a Python class (or function for simple use cases). Then, bind the deployment with optional arguments to the constructor to define an application. Finally, deploy the resulting application using (or the equivalent serve run CLI command, see Development Workflow for details).

from ray import serve
from ray.serve.handle import DeploymentHandle

class MyFirstDeployment:
    # Take the message to return as an argument to the constructor.
    def __init__(self, msg):
        self.msg = msg

    def __call__(self):
        return self.msg

my_first_deployment = MyFirstDeployment.bind("Hello world!")
handle: DeploymentHandle =
assert handle.remote().result() == "Hello world!"


An application is the unit of upgrade in a Ray Serve cluster. An application consists of one or more deployments. One of these deployments is considered the “ingress” deployment, which handles all inbound traffic.

Applications can be called via HTTP at the specified route_prefix or in Python using a DeploymentHandle.

DeploymentHandle (composing deployments)#

Ray Serve enables flexible model composition and scaling by allowing multiple independent deployments to call into each other. When binding a deployment, you can include references to other bound deployments. Then, at runtime each of these arguments is converted to a DeploymentHandle that can be used to query the deployment using a Python-native API. Below is a basic example where the Ingress deployment can call into two downstream models. For a more comprehensive guide, see the model composition guide.

from ray import serve
from ray.serve.handle import DeploymentHandle

class Hello:
    def __call__(self) -> str:
        return "Hello"

class World:
    def __call__(self) -> str:
        return " world!"

class Ingress:
    def __init__(self, hello_handle: DeploymentHandle, world_handle: DeploymentHandle):
        self._hello_handle = hello_handle
        self._world_handle = world_handle

    async def __call__(self) -> str:
        hello_response = self._hello_handle.remote()
        world_response = self._world_handle.remote()
        return (await hello_response) + (await world_response)

hello = Hello.bind()
world = World.bind()

# The deployments passed to the Ingress constructor are replaced with handles.
app = Ingress.bind(hello, world)

# Deploys Hello, World, and Ingress.
handle: DeploymentHandle =

# `DeploymentHandle`s can also be used to call the ingress deployment of an application.
assert handle.remote().result() == "Hello world!"

Ingress deployment (HTTP handling)#

A Serve application can consist of multiple deployments that can be combined to perform model composition or complex business logic. However, one deployment is always the “top-level” one that is passed to to deploy the application. This deployment is called the “ingress deployment” because it serves as the entrypoint for all traffic to the application. Often, it then routes to other deployments or calls into them using the DeploymentHandle API, and composes the results before returning to the user.

The ingress deployment defines the HTTP handling logic for the application. By default, the __call__ method of the class is called and passed in a Starlette request object. The response will be serialized as JSON, but other Starlette response objects can also be returned directly. Here’s an example:

import requests
from starlette.requests import Request

from ray import serve

class MostBasicIngress:
    async def __call__(self, request: Request) -> str:
        name = (await request.json())["name"]
        return f"Hello {name}!"

app = MostBasicIngress.bind()
assert (
    requests.get("", json={"name": "Corey"}).text
    == "Hello Corey!"

After binding the deployment and running, it is now exposed by the HTTP server and handles requests using the specified class. We can query the model using requests to verify that it’s working.

For more expressive HTTP handling, Serve also comes with a built-in integration with FastAPI. This allows you to use the full expressiveness of FastAPI to define more complex APIs:

import requests
from fastapi import FastAPI
from fastapi.responses import PlainTextResponse

from ray import serve

fastapi_app = FastAPI()

class FastAPIIngress:
    async def say_hi(self, name: str) -> str:
        return PlainTextResponse(f"Hello {name}!")

app = FastAPIIngress.bind()
assert requests.get("").text == "Hello Corey!"

What’s next?#

Now that you have learned the key concepts, you can dive into these guides: