Why Isn't My FastAPI StreamingResponse Working with a Generator Function?

StreamingResponse Not Streaming with Generator Function

FastAPI offers the StreamingResponse class to send responses back to clients incrementally. However, in some cases, this functionality may not be working as expected.

Root Cause Investigation

After analyzing the provided FastAPI code and the issue description, we have identified several potential causes:

1. Use of POST Request for Data Request:
The use of a POST request is not suitable for requesting data from a server. Instead, it is recommended to use a GET request for this purpose.

2. Use of Query Parameters for Authentication:
Sending sensitive credentials, such as the auth_key, through query parameters is not secure. Consider using headers or cookies for authentication instead.

3. Blocking Generator Function:
The generator function in the StreamingResponse is defined with def (not async def), which can lead to blocking issues within the FastAPI event loop.

4. Line-based Chunking:
Requests' iter_lines() iterates over the response data one line at a time. If no line breaks exist in the response, data will not be printed incrementally.

5. MIME Sniffing:
Some browsers (e.g., Chrome) may buffer text/plain responses to check for plaintext content before displaying them. This can hinder streaming.

Recommended Fixes:

1. Use GET Request:
Refactor the code to use a GET request for fetching data.

2. Secure Authentication:
Use headers or cookies to send authentication credentials securely.

3. Async Generator Function:
Define the generator function for the StreamingResponse with async def. If any blocking operations are necessary within the generator, use an external thread pool to execute them.

4. Chunk-based Chunking:
Use iter_content() instead of iter_lines() to iterate over the response data in chunks. Specify an appropriate chunk size.

5. Disable MIME Sniffing:
Disable MIME Sniffing by specifying a different media type (e.g., application/json or text/event-stream) for the StreamingResponse or by setting the X-Content-Type-Options header to nosniff.

Working Example:

The following code demonstrates a working FastAPI app with streaming capability:

from fastapi import FastAPI, 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')

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