How to create a streaming HTTP interface in Python?

Hi Andreas,

Do you have special requirements?

With HTTP 1.1 you can stream data using chunked transfer encoding, which means you can send the header Transfer-Encoding: chunked and the data you want. You can see an example on MDN.

You can also stream with other protocols: HTTP 2, websockets, grpc and so on.

So if you control both the client and the server you can choose how to stream your data.

Falcon unfortunately doesn't support that header.

The actual reason why is that WSGI itself (the interface under pretty much all Python servers) does not support Transfer-Encoding:

However, because WSGI servers and applications do not communicate via HTTP, what RFC 2616 calls "hop-by-hop" headers do not apply to WSGI internal communications. WSGI applications must not generate any "hop-by-hop" headers [4], attempt to use HTTP features that would require them to generate such headers, or rely on the content of any incoming "hop-by-hop" headers in the environ dictionary. WSGI servers must handle any supported inbound "hop-by-hop" headers on their own, such as by decoding any inbound Transfer-Encoding, including chunked encoding if applicable.

This because WSGI servers are middlewares and streaming would break the pattern.

Flask does not support it either but they worked around it using generators. Here it's my example:

import time
from datetime import datetime
from flask import Flask, Response

app = Flask(__name__)


@app.route('/time')
def doyouhavethetime():
    def generate():
        while True:
            yield "{}\n".format(datetime.now().isoformat())
            time.sleep(1)
    return Response(generate(), mimetype='text/plain')

and the client:

As you can see it's not using "Transfer-Encoding", just iterating on the generator and sending data.

Another option you have is to use aiohttp which is not based on WSGI and works well with chunked streaming.

You can find an example in this article though there's a bug on line 8 of his example, the rest works :-)

Replace:

interval = int(request.GET.get('interval', 1))

with

interval = int(request.query.get('interval', 1))

The headers sent by the server:

< HTTP/1.1 200 OK
< Content-Type: text/plain
< Transfer-Encoding: chunked
< Server: Python/3.6 aiohttp/3.0.9

As you can see it supports chunked streaming.

Use SSE with aiohttp, you can use websockets or Transfer-Encoding: chunked as well. Don't use any framework on top of WSGI unless you are building server for few/one concurrent clients