Using a WebSocket endpoint allows responses to be streamed to the client, enabling real-time communication and efficient data transfer.

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Required changes

To set up a WebSocket endpoint, you need to configure your app to use a custom runtime. Below is an example of the required changes in your cerebrium.toml configuration file: 
[cerebrium.runtime.custom]
port = 5000
entrypoint = "uvicorn main:app --host 0.0.0.0 --port 5000"
healthcheck_endpoint = "/health"
readycheck_endpoint = "/ready"
Explanation:
  • port: The port your app will listen on inside the container
  • entrypoint: The command to start your app. In this example, we’re using Uvicorn to run a FastAPI app located in main.py
  • healthcheck_endpoint: The endpoint used to confirm instance health. If unspecified, defaults to a TCP ping on the configured port. If the health check registers a non-200 response, it will be considered unhealthy, and be restarted should it not recover timely.
  • readycheck_endpoint: The endpoint used to confirm if the instance is ready to receive. If unspecified, defaults to a TCP ping on the configured port. If the ready check registers a non-200 response, it will not be a viable target for request routing.

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Things to note

  • Custom Runtime Required: A custom runtime is necessary to set up a WebSocket endpoint because it allows you to define how your app is run inside the container
  • WebSocket URL: Requests need to be made to a WebSocket URL starting with wss://. Ensure that your client supports secure WebSocket connections

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Making a request

You can test your WebSocket endpoint using websocat, a command-line utility for connecting to WebSocket servers: 
websocat wss://api.aws.us-east-1.cerebrium.ai/v4/<your-project-id>/<your-app-name>/<your-websocket-function-name>

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Implementing the WebSocket Endpoint

Here’s an example of how you can implement a WebSocket endpoint using FastAPI: 
# In main.py:
from fastapi import FastAPI, WebSocket

app = FastAPI()

@app.websocket("/your-websocket-function-name")
async def websocket_endpoint(websocket: WebSocket):
    await websocket.accept()
    await websocket.send_text("Hello, WebSocket!")
    await websocket.close()

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Additional Info

Client-side Implementation: When connecting from a client app, ensure you handle the WebSocket connection properly, including error handling and reconnection logic if necessary. 
// Example using JavaScript in a browser
const socket = new WebSocket(
  "wss://api.aws.us-east-1.cerebrium.ai/v4/<your-project-id>/<your-app-name>/<your-websocket-function-name>",
);

socket.onopen = function (event) {
  console.log("WebSocket is open now.");
};

socket.onmessage = function (event) {
  console.log("Received data: " + event.data);
};

socket.onclose = function (event) {
  console.log("WebSocket is closed now.");
};

socket.onerror = function (error) {
  console.error("WebSocket error observed:", error);
};