How to Prevent Blocking in a Local Flask Webhook: Handle Long-Running Tasks Concurrently with threaded=True

Learn how to keep a local Flask webhook responsive during long-running tasks by running the development server with threaded=True to handle concurrent requests.

When building a local Flask webhook endpoint, a common snag appears as soon as the handler needs to perform a long-running operation, like talking to a remote API. While that call is in-flight, you don’t want the process to stop listening for new webhooks. The question is how to keep the endpoint responsive without changing the core structure of the app.

Problem setup

The pattern below shows a minimal webhook that accepts POST requests. Inside the handler sits a time-consuming call that might monopolize request handling.

from flask import Flask, request, abort
svc = Flask(__name__)
@svc.route("/", methods=["POST"])
def intake():
    if request.method == "POST":
        slow_operation()

The concern is straightforward: while the handler is busy, will subsequent requests be processed or will they queue up behind the long-running task?

Why this blocks

A long-running task inside a request handler can prevent the process from handling additional requests unless the server is able to run multiple invocations concurrently. If only one handler can run at a time, the endpoint will feel stuck until the slow part completes.

Solution

Run the Flask development server with threading enabled. By invoking run with threaded=True, Flask’s built-in server can serve multiple requests concurrently, so a slow handler won’t block the next incoming webhook.

from flask import Flask, request, abort
svc = Flask(__name__)
@svc.route("/", methods=["POST"])
def intake():
    if request.method == "POST":
        slow_operation()
if __name__ == "__main__":
    svc.run(threaded=True)

This keeps the webhook listener responsive while the long-running code does its work.

Why it matters

Webhook endpoints are often hit in bursts. If a single slow call ties up request handling, you risk losing responsiveness at precisely the wrong moment. Enabling concurrent handling ensures that the app continues to accept and process new requests while existing work is still running. Note that using the built-in server via app.run is not meant for production code. Production servers will be multithreaded.

Takeaways

If your Flask webhook performs a long-running task, run the development server with threaded=True to avoid blocking the endpoint. This small switch lets multiple requests be served at the same time and keeps your local listener responsive. For production, rely on a multithreaded server setup rather than the built-in development server.