I have a flask REST endpoint that does some cpu-intensive image processing and takes a few seconds to return. Often, this endpoint gets called, then aborted by the client. In these situations I would like to cancel processing. How can I do this in flask?
In node.js, I would do something like:
req.on('close', function(){ //some handler });
I was expecting flask to have something similar, or a synchronous method (request.isClosed()) that I could check at certain points during my processing and return if it's closed, but I can't find one.
I thought about sending something to test that the connection is still open, and catching the exception if it fails, but it seems Flask buffers all outputs so the exception isn't thrown until the processing completes and tries to return the result:
An established connection was aborted by the software in your host machine
How can I cancel my processing half way through if the client aborts their request?
Flask comes with a handy abort() function that aborts a request with an HTTP error code early. It will also provide a plain black and white error page for you with a basic description, but nothing fancy. Depending on the error code it is less or more likely for the user to actually see such an error.
When the Flask application handles a request, it creates a Request object based on the environment it received from the WSGI server. Because a worker (thread, process, or coroutine depending on the server) handles only one request at a time, the request data can be considered global to that worker during that request.
By default, a route only answers to GET requests. You can use the methods argument of the route() decorator to handle different HTTP methods. If GET is present, Flask automatically adds support for the HEAD method and handles HEAD requests according to the HTTP RFC.
There is a potentially... hacky solution to your problem. Flask has the ability to stream content back to the user via a generator. The hacky part would be streaming blank data as a check to see if the connection is still open and then when your content is finished the generator could produce the actual image. Your generator could check to see if processing is done and return None
or ""
or whatever if it's not finished.
from flask import Response @app.route('/image') def generate_large_image(): def generate(): while True: if not processing_finished(): yield "" else: yield get_image() return Response(generate(), mimetype='image/jpeg')
I don't know what exception you'll get if the client closes the connection but I'm willing to bet its error: [Errno 32] Broken pipe
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