Pyramid stream response body

Question

I'm trying to stream Server-Sent Events from my Pyramid application, but I can't figure out how to stream the response body from my view. Here's the test view I'm using (it totally doesn't implement SSE, it's just to work out the streaming portion):

@view_config(route_name='iter_test')
def iter_test(request):
    import time
    def test_iter():
        i = 0
        while True:
            i += 1
            if i == 5:
                raise StopIteration
            yield str(time.time())
            print time.time()
            time.sleep(1)

    return test_iter()

This produces ValueError: Could not convert return value of the view callable function pdiff.views.iter_test into a response object. The value returned was <generator object test_iter at 0x3dc19b0>.

I've tried return Response(app_iter=test_iter()) instead, which at least doesn't error out, but it doesn't stream the response - it waits until the generator has completed before returning the response to my browser.

I recognize that could simply return a single event per request and allow the clients to reconnect after each event, but I'd prefer to preserve the realtime nature of Server-Sent Events by streaming multiple events from a single request, without the reconnection delay. How can I do this with Pyramid?

Answer 1

I've found the issue. Turns out my application code is fine, and the problem lies with Waitress and nginx:

1. Waitress, the default web server Pyramid uses, buffers all output in 18000-byte chunks (see this issue for details).

2. The source of the problem was hidden from me by nginx, the web server I put in front of my Pyramid application, which also buffers responses.

(1) can be solved by either:

• Configuring waitress with send_bytes = 1 in your .ini file. This fixes the streaming problem, but makes your entire app super slow. As @Zitrax mentioned, you can recover some speed with higher values, but any value higher than 1 risks messages getting stuck in the buffer.

• Switching to gunicorn. I don't know whether gunicorn just uses a smaller buffer, or if it behaves better with app_iter, but it worked, and kept my app fast.

(2) can be solved by configuring nginx to disable buffering for your stream routes.

You need to set proxy_buffering off in your nginx conf. This setting applies to sites hosted via proxy_pass. If you're not using proxy_pass you may need a different setting.

• You may configure nginx to dynamically enable/disable buffering for each response based on request headers, as shown in this question on the topic (a good solution for EventSource/Server-Sent Events)

• You may alternatively configure this in a location block in your nginx conf. This is good if you're using something besides EventSource and you're not expecting to receive a particular header, or if you are using EventSource, but want to debug the response from a plain browser tab, where you can't send the Accept header in your request.