Why is FastAPI's StreamingResponse Not Streaming with Generator Functions?

FastAPI StreamingResponse Not Streaming with Generator Function

FastAPI's StreamingResponse is intended to stream data back to the client as it becomes available. However, there have been reports of StreamingResponse not working as expected when using generator functions. This article will investigate the potential causes of this issue and provide a solution.

Blocking Operations and Generator Functions

Generator functions in Python can define a sequence of values that are yielded one at a time. However, if a blocking operation (such as time.sleep()) is performed within a generator function, it can block the event loop, preventing FastAPI from streaming data to the client.

Def vs. Async Def

FastAPI handles StreamingResponse differently based on whether the generator function uses the def or async def syntax. If the generator function is defined using the async def syntax, FastAPI assumes it's an asynchronous generator and executes it in a thread pool or task pool. However, if the generator function uses the def syntax, FastAPI recognizes it as a blocking generator and uses iterate_in_threadpool() to run it in a separate thread.

Recommended Approach

To avoid blocking operations and ensure proper streaming, it's recommended to use an asynchronous generator function (async def). If necessary, any blocking operations should be performed in an external thread pool and awaited to avoid disrupting the event loop.

Response Media Type

In some cases, browsers may buffer text/plain responses to check for MIME type. To prevent this, it's advisable to specify a different media type, such as text/event-stream, application/json, or set the X-Content-Type-Options header to nosniff.

Example

Here's an example of a working FastAPI app with a generator function for streaming data:

from fastapi import FastAPI, StreamingResponse, Request
from fastapi.responses import HTMLResponse
import asyncio

app = FastAPI()

@app.get("/stream")
async def streaming_data(request: Request):
    def generate_data():
        for i in range(10):
            yield b'some fake data\n\n'
            await asyncio.sleep(0.5)
    return StreamingResponse(generate_data(), media_type="text/event-stream")

Conclusion

By avoiding blocking operations, using asynchronous generator functions, and specifying the appropriate media type, you can ensure that FastAPI StreamingResponse works as intended, allowing you to stream data efficiently to clients.

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