set_props only updating last output in background callbacks

[faulty13]

Describe your context
Mac OS, occurring in Safari & Chrome

dash                 2.18.2
dash-core-components 2.0.0
dash-html-components 2.0.0
dash-table           5.0.0

Describe the bug

When updating two different components via set_props, in a background callback with running, only the last output is updated. No console errors encountered.

Further testing:

• If I remove the background, and running, then both outputs are updated as expected.
• The issue persists if I replace running with cancel.
• If I swap the order of the set_props lines, the other component updates instead.

See reproducible example below:

from dash import Dash, Output, Input, html, set_props, DiskcacheManager
import diskcache
import dash

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

app = Dash()

app.layout = [
    html.Button("start", id="start"),
    html.Div("initial", id="output"),
    html.Div("initial 2", id="output-2"),
]

@app.callback(
    Input("start", "n_clicks"),
    background=True,
    running=[
        (Output("start", "disabled"), True, False)
    ],
    manager=background_callback_manager,
)
def on_click(_):
    set_props("output", {"children": "changed"})
    set_props("output-2", {"style": {"background": "red"}})

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

Expected behavior

All outputs should update, not just the last one.

[T4rk1n]

This is bug with the Diskcache manager, it only set the last value here:

dash/dash/long_callback/managers/diskcache_manager.py

Lines 181 to 182 in 7ba267b

	def _set_props(_id, props):
	cache.set(f"{result_key}-set_props", {_id: props})

This need to be check if there is an old value first.

[olgavoz1971]

I have the same problem with set_props in the background callback with Redis+Celery

[Raahul-Singh]

Hi, I've opened an MR (#3182) fixing this. Thanks a ton @T4rk1n for the heads up!