Cannot Connect Websocket via Nginx when using FastAPI? Here’s the Ultimate Fix!

Are you tired of struggling with WebSockets and Nginx when building your FastAPI application? Do you find yourself scratching your head, wondering why your WebSocket connection is refused or times out? Well, fear not, dear developer! In this article, we’ll dive into the world of WebSockets, Nginx, and FastAPI, and provide you with a step-by-step guide to debugging and resolving the infamous “Cannot connect WebSocket via Nginx” issue.

Understanding the Problem

Before we dive into the solution, let’s take a step back and understand why this issue occurs in the first place. When using FastAPI, you might have a WebSocket endpoint that works perfectly fine when accessed directly. However, when you add Nginx to the mix as a reverse proxy, things start to go awry. The WebSocket connection is refused, times out, or fails to establish altogether.

Here’s what’s happening behind the scenes:

• FastAPI creates a WebSocket endpoint, which listens for incoming connections.
• Nginx acts as a reverse proxy, forwarding incoming requests to your FastAPI application.
• When a client attempts to establish a WebSocket connection, Nginx receives the request but doesn’t know how to handle it.
• Nginx tries to forward the request to your FastAPI application, but the WebSocket upgrade header is lost in translation.
• FastAPI receives the request, but without the WebSocket upgrade header, it doesn’t recognize it as a WebSocket connection.

Configuring Nginx for WebSocket Support

The first step in resolving this issue is to configure Nginx to support WebSockets. You’ll need to update your Nginx configuration file to include the following settings:

http {
    ...
    upstream backend {
        server localhost:8000;
    }

    server {
        listen 80;
        location / {
            proxy_pass http://backend;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
        }
    }
}

Breakdown of the above configuration:

• proxy_http_version 1.1; enables HTTP/1.1 support, which is necessary for WebSockets.
• proxy_set_header Upgrade $http_upgrade; passes the WebSocket upgrade header from the client to your FastAPI application.
• proxy_set_header Connection "upgrade"; sets the Connection header to “upgrade”, which is required for WebSocket connections.
• proxy_cache_bypass $http_upgrade; bypasses Nginx’s caching mechanism for WebSocket connections.

Fine-Tuning FastAPI for WebSocket Support

Now that Nginx is configured to support WebSockets, let’s focus on fine-tuning your FastAPI application. You’ll need to make a few adjustments to your code to ensure seamless WebSocket connections:

Create a new FastAPI endpoint for your WebSocket connection:

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"Received: {data}")

In the above code:

• from fastapi import FastAPI, WebSocket imports the necessary FastAPI components.
• @app.websocket("/ws") defines a WebSocket endpoint at the “/ws” path.
• async def websocket_endpoint(websocket: WebSocket): defines the WebSocket endpoint function.
• await websocket.accept() accepts the WebSocket connection.
• while True: creates an infinite loop to handle incoming WebSocket messages.

Testing WebSocket Connections

Now that you’ve configured Nginx and fine-tuned your FastAPI application, it’s time to test your WebSocket connection. You can use tools like wscat or wss to establish a WebSocket connection from the command line:

wscat -c ws://localhost/ws

This will establish a WebSocket connection to your FastAPI application. You can then send messages to the WebSocket endpoint using the wscat tool:

> hello
Received: hello

Congratulations! You’ve successfully established a WebSocket connection via Nginx using FastAPI.

Troubleshooting Common Issues

Even with the above configuration and code, you might still encounter issues. Here are some common problems and their solutions:

Issue 1: WebSocket Connection Refused

Solution:

• Check that your Nginx configuration is correct and the WebSocket endpoint is properly proxied.
• Verify that your FastAPI application is running and the WebSocket endpoint is defined correctly.

Issue 2: WebSocket Connection Times Out

Solution:

• Check that your Nginx configuration includes the proxy_read_timeout and proxy_write_timeout directives.
• Verify that your FastAPI application is not blocking or taking too long to respond to WebSocket messages.

Issue 3: WebSocket Messages Not Received

Solution:

• Check that your WebSocket endpoint is defined correctly and the WebSocket connection is properly accepted.
• Verify that your FastAPI application is handling incoming WebSocket messages correctly.

Conclusion

In this article, we’ve covered the intricacies of using WebSockets with Nginx and FastAPI. By following the steps outlined above, you should be able to establish a seamless WebSocket connection via Nginx using FastAPI. Remember to configure Nginx to support WebSockets, fine-tune your FastAPI application, and test your WebSocket connections thoroughly. Happy coding!

Configuration	Description
proxy_http_version 1.1;	Enables HTTP/1.1 support for WebSockets.
proxy_set_header Upgrade $http_upgrade;	Passes the WebSocket upgrade header from the client to your FastAPI application.
proxy_set_header Connection “upgrade”;	Sets the Connection header to “upgrade” for WebSocket connections.
proxy_cache_bypass $http_upgrade;	Bypasses Nginx’s caching mechanism for WebSocket connections.

Remember to bookmark this article for future reference and share it with your fellow developers who might be struggling with WebSockets and Nginx. Happy coding, and may the WebSocket force be with you!

Frequently Asked Question

Having trouble connecting to your FastAPI application via Nginx using websockets? Don’t worry, we’ve got you covered!