Server-Sent Events (SSE) in FastAPI
Server-Sent Events (SSE) provide a mechanism for servers to push real-time updates to clients over HTTP. SSE is particularly useful for real-time notifications, live updates, or any scenario requiring continuous data flow from server to client without repeated requests.
FastAPI, being an asynchronous web framework, is well-suited for implementing SSE due to its support for asynchronous I/O operations. This article explains SSE in FastAPI with examples, covering setup, implementation, and best practices.
What are Server-Sent Events?
SSE is a unidirectional communication mechanism where the server sends updates to the client. The client initiates the connection, and the server streams data to the client over the same HTTP connection.
Key characteristics of SSE:
Setting Up SSE in FastAPI
To implement SSE in FastAPI, you can use the Response object with a text/event-stream content type. Here's a step-by-step guide:
Step 1: Basic FastAPI Setup
First, install FastAPI and Uvicorn if you haven’t already:
pip install fastapi uvicorn
Create a basic FastAPI application:
from fastapi import FastAPI
app = FastAPI()
@app.get("/")
async def read_root():
return {"message": "Welcome to SSE example with FastAPI"}
Step 2: Creating the SSE Endpoint
SSE requires sending data in a specific format. Each message sent must be prefixed with data: and end with two newlines (\n\n). Here’s how you can set up an SSE endpoint:
from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import asyncio
app = FastAPI()
async def sse_generator():
counter = 1
while True:
# Simulating real-time updates
yield f"data: Event {counter}\n\n"
counter += 1
await asyncio.sleep(2) # Send an update every 2 seconds
@app.get("/sse")
async def sse_endpoint():
return StreamingResponse(sse_generator(), media_type="text/event-stream")
Explanation:
Step 3: Client Implementation
Here’s how a client (e.g., a browser or JavaScript application) can connect to the SSE endpoint:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>SSE Example</title>
</head>
<body>
<h1>Server-Sent Events with FastAPI</h1>
<div id="events"></div>
<script>
const eventSource = new EventSource("https://localhost:8000/sse");
eventSource.onmessage = function(event) {
const div = document.getElementById("events");
const newEvent = document.createElement("p");
newEvent.textContent = `Received: ${event.data}`;
div.appendChild(newEvent);
};
eventSource.onerror = function() {
console.error("Error with the SSE connection.");
};
</script>
</body>
</html>
Explanation:
Step 4: Testing the SSE
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uvicorn main:app --reload
Advanced Use Cases for SSE
Error Handling in SSE
When the connection is interrupted, the EventSource object will automatically attempt to reconnect. You can manage reconnections using the retry field in the SSE response:
async def sse_generator():
yield "retry: 5000\n" # Retry every 5 seconds
while True:
yield f"data: Heartbeat\n\n"
await asyncio.sleep(2)
Best Practices
Keep Connections Alive: Periodically send a heartbeat message to prevent client disconnections:
yield "data: Heartbeat\n\n"
Optimize Resource Usage: Limit the number of concurrent SSE connections to prevent resource exhaustion.
Security:
Use HTTPS to secure the connection.
Authenticate SSE requests using tokens or headers.
Browser Compatibility: Ensure fallback mechanisms for clients that do not support SSE.
Comparison: SSE vs WebSockets
Conclusion
SSE is a powerful, lightweight solution for real-time updates in scenarios where only server-to-client communication is needed. With FastAPI’s asynchronous capabilities, implementing SSE is efficient and straightforward. By following best practices, you can create robust real-time applications tailored to your needs.
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