การใช้งาน WebSockets ใน FastAPI

Welcome to implementing WebSockets with FastAPI! WebSockets provide a persistent, full-duplex communication channel between a client and a server.

• Full-duplex: Both client and server can send and receive messages simultaneously.
• Real-time: Ideal for applications needing instant updates, like chat, live dashboards, or gaming.

FastAPI has excellent built-in support for WebSockets, leveraging Python's async/await features.

Just like HTTP endpoints, you define WebSocket endpoints using a decorator. Instead of @app.get() or @app.post(), you use @app.websocket().

Your endpoint function must be async def and accept a websocket: WebSocket parameter. This WebSocket object is your primary tool for interaction.

The first step inside your function is always to await websocket.accept() to establish the connection.

Let's create our first WebSocket endpoint. This simple example accepts a connection, sends a welcome message, and then closes the connection.

Notice the @app.websocket("/ws") decorator and the async def function, which are key for WebSocket handling.

Try running this example and observe the connection:

WebSocket connections can be interrupted for various reasons (client closes tab, network error). It's crucial to handle these disconnections gracefully.

FastAPI raises a WebSocketDisconnect exception when a client disconnects. You can catch this exception to perform cleanup tasks, like removing the client from a list of active connections.

Wrap your WebSocket communication logic in a try...except WebSocketDisconnect block.

Once connected, your server can receive messages from the client. The WebSocket object provides methods for this:

• await websocket.receive_text(): For receiving string data.
• await websocket.receive_bytes(): For receiving binary data.
• await websocket.receive_json(): For receiving JSON data (requires python-multipart).

These operations are asynchronous, so always use await.

This example demonstrates a simple 'echo' server. It accepts a connection, then continuously receives messages from the client and sends them back.

The while True loop keeps the connection open, and the try...except WebSocketDisconnect handles when the client leaves.

Similarly, your server can send messages to the client. The WebSocket object offers:

• await websocket.send_text("your message"): To send string data.
• await websocket.send_bytes(b"your bytes"): To send binary data.
• await websocket.send_json({"key": "value"}): To send JSON data.

Remember to await these calls. It's common to send a response right after receiving a message.

For structured data, sending and receiving JSON is preferred. FastAPI's WebSocket object handles serialization/deserialization for you.

• Sending: Pass a Python dict to send_json().
• Receiving: receive_json() returns a Python dict.

This simplifies data exchange compared to manually parsing strings.

For applications with multiple clients, you'll need to manage active connections. A common pattern is to use a class to keep track of all connected WebSocket objects.

This manager can then be used to add/remove connections and send messages to specific clients or broadcast to all.

You've learned the basics of setting up and interacting with WebSocket connections in FastAPI.

Let's check your understanding of accepting new connections.

You've successfully learned how to implement WebSockets in FastAPI!

• Use @app.websocket() to define endpoints.
• Always await websocket.accept() to establish the connection.
• Handle disconnections with try...except WebSocketDisconnect.
• Use await websocket.receive_text() and await websocket.send_text() for communication.
• For structured data, send_json() and receive_json() are your friends.

Next, you'll build a real-time chat application to put these concepts into practice!