How to Handle Streaming Responses Effectively in FastAPI?

Handling Streaming Responses with FastAPI

FastAPI provides the StreamingResponse class to stream data to clients during API calls. While this functionality is intended for streaming data in a non-blocking manner, issues can arise when using a generator function with blocking operations or improper usage.

Enhanced Response for Streaming Data

To ensure successful streaming, consider the following:

• GET Request: Use GET requests instead of POST for fetching data.
• Secure Credentials: Avoid sending credentials via the query string; use headers or cookies instead.
• Non-Blocking Operations: Define generator functions as def (not async def) if they contain blocking operations. Alternately, use async def with asynchronous blocking operations.
• Suitable Media Type: Specify a different media type for the response (e.g., application/json or text/event-stream) instead of text/plain, or disable MIME sniffing by setting the X-Content-Type-Options header to nosniff. This ensures browsers do not buffer the response, allowing data to stream in real-time.

Example Implementation

Consider the following Python code:

# app.py
from fastapi import FastAPI, StreamingResponse
from fastapi.responses import StreamingResponse
import asyncio

app = FastAPI()

async def fake_data_streamer():
    for i in range(10):
        yield b'some fake data\n\n'
        await asyncio.sleep(0.5)

@app.get('/')
async def main():
    return StreamingResponse(fake_data_streamer(), media_type='text/event-stream')
    # or, use:
    '''
    headers = {'X-Content-Type-Options': 'nosniff'}
    return StreamingResponse(fake_data_streamer(), headers=headers, media_type='text/plain')
    '''

# test.py (using httpx)
import httpx

url = 'http://127.0.0.1:8000/'

with httpx.stream('GET', url) as r:
    for chunk in r.iter_raw():  # or, for line in r.iter_lines():
        print(chunk)

This code demonstrates how to stream data from a generator function in a FastAPI application and consume it using the httpx library.

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