Flask request waiting for asynchronous background job
Tornado would do the trick.
Flask is not designed for asynchronization. A Flask instance processes one request at a time in one thread. Therefore, when you hold the connection, it will not proceed to next request.
I have an HTTP API using Flask and in one particular operation clients use it to retrieve information obtained from a 3rd party API. The retrieval is done with a celery task. Usually, my approach would be to accept the client request for that information and return a 303 See Other response with an URI that can be polled for the response as the background job is finished.
However, some clients require the operation to be done in a single request. They don't want to poll or follow redirects, which means I have to run the background job synchronously, hold on to the connection until it's finished, and return the result in the same response. I'm aware of Flask streaming, but how to do such long-pooling with Flask?