[BUG] A progress callable of a bg callback sometimes doesn't trigger.

[jonnyrobbie]

python 3.10.13
dash 2.16.0

• if frontend related, tell us your Browser, Version and OS

  • OS: macos 14.4
  • Browser firefox
  • Version 124.0.2

Describe the bug

A function callable of a callback with a progress param doesn't seem to get hooked immediately. And calls to this callable seem to be ignored sometimes - especially fraction of a seconds after the callback itself has been triggered.

Expected behavior

MVE:

from dash import html, Input, Output, Dash, callback, DiskcacheManager
import diskcache
import time

cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)

app = Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
    [html.Div(id="result"), html.Div(id="progress"), html.Button("Run", id="run")]
)


@callback(
    [Output("result", "children")],
    [Input("run", "n_clicks")],
    prevent_initial_call=True,
    background=True,
    running=[(Output("run", "disabled"), True, False)],
    progress=[Output("progress", "children")],
)
def run(set_progress, n):
    # time.sleep(1)
    set_progress("running")
    time.sleep(1)
    set_progress("still running")
    time.sleep(1)  # long running function
    set_progress("finished")
    return (f"Run clicked {n} times",)


if __name__ == "__main__":
    app.run(debug=True)

Run this app and click the button.

Expected behaviour:

• it shows "running immediately after pressing the button. Then after a second, "still sunning" is shown and after yet another second "finished" is displayed and stays on the screen.

Observed behaviour:

• after clicking a button, nothing changes for a second, and then it shows "still running". The first call to set_progress at the beginning of the callback seems to be ignored.
• if you uncomment the time.sleep call, then the first call to set_progress seems to go through.

[T4rk1n]

it shows "running immediately after pressing the button. Then after a second, "still sunning" is shown and after yet another second "finished" is displayed and stays on the screen.

If you have a sleep for 1 second, before setting "running" it is never going to appear right with the default settings.
The background callbacks operates a chains of requests, it wait for interval argument which by default is 1000 ms. You can set the interval to a lower amount to ensure faster retrieval of progress values.

[jonnyrobbie]

The documentation is extremely unclear of the inner workings. The https://dash.plotly.com/background-callbacks sections does not mention the behaviour you mentioned at all and does not mention any limitations for the bg callback progress callable.
API reference just says "interval - Time to wait between the long callback update requests.", which explains very little.