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FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.7+ based on standard Python type hints. It is designed to be easy to use and to provide high performance, on par with NodeJS and Go, thanks to its asynchronous capabilities. FastAPI is particularly useful for building web applications that require real-time data exchange, such as chat applications, live notifications, and more.
When working with FastAPI, you might encounter a situation where a WebSocket connection cannot be established. This issue typically manifests as a failure to connect to the WebSocket endpoint, resulting in errors in the client application. The error message might look something like this:
WebSocket connection to 'ws://example.com/ws' failed: Error during WebSocket handshake: Unexpected response code: 404
The WebSocket connection failure often occurs due to misconfiguration of the WebSocket endpoint or issues with the client setup. WebSockets are a protocol for full-duplex communication channels over a single TCP connection, and they require a specific handshake process to establish a connection. If this handshake fails, the connection cannot be established.
Common causes include incorrect endpoint URLs, server-side misconfigurations, or client-side issues such as incorrect headers or protocols.
To resolve WebSocket connection issues in FastAPI, follow these steps:
Ensure that the WebSocket endpoint is correctly defined in your FastAPI application. Check the URL path and make sure it matches the client-side configuration. For example:
from fastapi import FastAPI, WebSocket
app = FastAPI()
@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
await websocket.accept()
while True:
data = await websocket.receive_text()
await websocket.send_text(f"Message text was: {data}")
Ensure that the client is configured to connect to the correct WebSocket URL. The URL should start with ws://
for non-secure connections or wss://
for secure connections. Example in JavaScript:
const socket = new WebSocket('ws://localhost:8000/ws');
Check the network logs in your browser's developer tools to see if there are any errors during the WebSocket handshake. Additionally, inspect the server logs to identify any server-side issues that might be causing the connection failure.
Use a WebSocket client tool like WebSocket Echo Test to test the connection independently of your application. This can help isolate whether the issue is with the client or the server.
For more information on setting up WebSockets with FastAPI, refer to the FastAPI WebSockets documentation. Additionally, consider reviewing the MDN WebSockets API documentation for a deeper understanding of WebSocket protocols and usage.
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