Support for background callbacks on
@dash.callback
was introduced in Dash 2.6.
If you’re using an earlier version of Dash 2.x, you can uselong_callback
for long-running callbacks.
To get the most out of this page, make sure you’ve read about
Basic Callbacks in the Dash Fundamentals.
Most web servers have a 30 second timeout by default, which is an issue for callbacks that take longer to complete.
While you can increase the timeout on the web server, you risk allowing long-running callbacks to use all of your app’s
workers, preventing other requests from going through. Background callbacks offer a scalable solution for using long-running
callbacks by running them in a separate background queue. In the background queue, the callbacks are executed one-by-one
in the order that they came in by dedicated queue worker(s).
You can configure a callback to run in the background by setting background=True
on the callback.
Callbacks with background=True
use a backend configured by you to run the callback logic. There are currently two options:
A DiskCache backend that runs callback logic in a separate
process and stores the results to disk using the diskcache
library. This is the easiest backend to use for local
development, but is not recommended for production.
A Celery backend that runs callback
logic in a Celery worker and returns results to the Dash app through a Celery broker
like Redis.
This is recommended for production as, unlike Disk Cache, it queues the background callbacks, running them one-by-one in the order that they were received by dedicated Celery worker(s). Celery is a widely adopted, production-ready job queue library.
For further information on the benefits of job queues, see the Why Job Queues? section below.
Dash Enterprise makes it easy to deploy Celery and Redis for using background callbacks in production. Get Pricing or see Dash in action at our next demo session.
The following examples use the diskcache
manager when running locally. Install with:
pip install dash[diskcache]
When these examples are deployed to Dash Enterprise, they use celery
.
pip install dash[celery]
To use a background callback, you first need to configure a manager
using your chosen backend.
The @dash.callback
decorator requires this manager instance.
You can provide the manager instance to the dash.Dash
app constructor as the background_callback_manager
keyword argument,
or as the manager
argument to the @dash.callback
decorator.
In the next five examples, we’ll discuss in more detail how to implement background callbacks.
Here is a simple example of a background callback that updates an html.P
element with the number of times that a button has been clicked.
The callback uses time.sleep
to simulate a long-running operation.
import time
import os
import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html
if 'REDIS_URL' in os.environ:
# Use Redis & Celery if REDIS_URL set as an env variable
from celery import Celery
celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
background_callback_manager = CeleryManager(celery_app)
else:
# Diskcache for non-production apps when developing locally
import diskcache
cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)
app = dash.Dash(__name__)
app.layout = html.Div(
[
html.Div([html.P(id="paragraph_id", children=["Button not clicked"])]),
html.Button(id="button_id", children="Run Job!"),
]
)
@dash.callback(
output=Output("paragraph_id", "children"),
inputs=Input("button_id", "n_clicks"),
background=True,
manager=background_callback_manager,
)
def update_clicks(n_clicks):
time.sleep(2.0)
return [f"Clicked {n_clicks} times"]
if __name__ == "__main__":
app.run_server(debug=True)
Notice how in the previous example, there is no visual indication that the background callback is running.
A user might click the “Run Job!” button multiple times before the original job can complete.
You can also disable the button while the callback is running and re-enable it when the callback completes.
To do this, use the running
argument to @dash.callback
. This argument accepts a list of 3-element
tuples. The first element of each tuple must be an Output
dependency object referencing a property of a component in
the app layout. The second element is the value that the property should be set to while the callback is running, and
the third element is the value the property should be set to when the callback completes.
This example uses running
to set the disabled
property of the button to True
while the callback is running,
and False
when it completes.
Note: In this example, the background_callback_manager
is provided to the dash.Dash
app constructor instead of
the @dash.callback
decorator.
import time
import os
import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html
if 'REDIS_URL' in os.environ:
# Use Redis & Celery if REDIS_URL set as an env variable
from celery import Celery
celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
background_callback_manager = CeleryManager(celery_app)
else:
# Diskcache for non-production apps when developing locally
import diskcache
cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)
app = dash.Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
[
html.Div([html.P(id="paragraph_id", children=["Button not clicked"])]),
html.Button(id="button_id", children="Run Job!"),
]
)
@dash.callback(
output=Output("paragraph_id", "children"),
inputs=Input("button_id", "n_clicks"),
background=True,
running=[
(Output("button_id", "disabled"), True, False),
],
)
def update_clicks(n_clicks):
time.sleep(2.0)
return [f"Clicked {n_clicks} times"]
if __name__ == "__main__":
app.run_server(debug=True)
This example builds on the previous example, adding support for canceling a long-running callback using
the cancel
argument to the @dash.callback
decorator. We set the cancel
argument to a list
of Input
dependency objects that reference a property of a component in the app’s layout.
When the value of this property changes while a callback is running, the callback is canceled.
Note that the value of the property is not significant — any change in value cancels the running job (if any).
import time
import os
import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html
if 'REDIS_URL' in os.environ:
# Use Redis & Celery if REDIS_URL set as an env variable
from celery import Celery
celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
background_callback_manager = CeleryManager(celery_app)
else:
# Diskcache for non-production apps when developing locally
import diskcache
cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)
app = dash.Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
[
html.Div([html.P(id="paragraph_id", children=["Button not clicked"])]),
html.Button(id="button_id", children="Run Job!"),
html.Button(id="cancel_button_id", children="Cancel Running Job!"),
]
)
@dash.callback(
output=Output("paragraph_id", "children"),
inputs=Input("button_id", "n_clicks"),
background=True,
running=[
(Output("button_id", "disabled"), True, False),
(Output("cancel_button_id", "disabled"), False, True),
],
cancel=[Input("cancel_button_id", "n_clicks")],
)
def update_clicks(n_clicks):
time.sleep(2.0)
return [f"Clicked {n_clicks} times"]
if __name__ == "__main__":
app.run_server(debug=True)
This example uses the progress
argument to the @dash.callback
decorator to update a progress bar while the
callback is running. We set the progress
argument to an Output
dependency grouping that references properties
of components in the app’s layout.
When a dependency grouping is assigned to the progress
argument of @dash.callback
, the decorated function
is called with a new special argument as the first argument to the function.
This special argument, named set_progress
in the example below, is a function handle that the decorated function
calls in order to provide updates to the app on its current progress. The set_progress
function accepts a single
argument, which corresponds to the grouping of properties specified in the Output
dependency grouping passed to
the progress
argument of @dash.callback
.
import time
import os
import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html
if 'REDIS_URL' in os.environ:
# Use Redis & Celery if REDIS_URL set as an env variable
from celery import Celery
celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
background_callback_manager = CeleryManager(celery_app)
else:
# Diskcache for non-production apps when developing locally
import diskcache
cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)
app = dash.Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
[
html.Div(
[
html.P(id="paragraph_id", children=["Button not clicked"]),
html.Progress(id="progress_bar", value="0"),
]
),
html.Button(id="button_id", children="Run Job!"),
html.Button(id="cancel_button_id", children="Cancel Running Job!"),
]
)
@dash.callback(
output=Output("paragraph_id", "children"),
inputs=Input("button_id", "n_clicks"),
background=True,
running=[
(Output("button_id", "disabled"), True, False),
(Output("cancel_button_id", "disabled"), False, True),
(
Output("paragraph_id", "style"),
{"visibility": "hidden"},
{"visibility": "visible"},
),
(
Output("progress_bar", "style"),
{"visibility": "visible"},
{"visibility": "hidden"},
),
],
cancel=Input("cancel_button_id", "n_clicks"),
progress=[Output("progress_bar", "value"), Output("progress_bar", "max")],
prevent_initial_call=True
)
def update_progress(set_progress, n_clicks):
total = 5
for i in range(total + 1):
set_progress((str(i), str(total)))
time.sleep(1)
return f"Clicked {n_clicks} times"
if __name__ == "__main__":
app.run_server(debug=True)
The progress
argument to the @dash.callback
decorator can be used to update arbitrary component properties.
This example creates and updates a Plotly bar graph to display the current calculation status.
This example also uses the progress_default
argument to specify a grouping of values that
should be assigned to the components specified by the progress
argument when the callback is not in progress.
If progress_default
is not provided, all the dependency properties specified in progress
are set to None
when the callback is not running. In this case, progress_default
is set to a figure with a zero width bar.
import time
import os
import dash
from dash import DiskcacheManager, CeleryManager, Input, Output, html, dcc
import plotly.graph_objects as go
if 'REDIS_URL' in os.environ:
# Use Redis & Celery if REDIS_URL set as an env variable
from celery import Celery
celery_app = Celery(__name__, broker=os.environ['REDIS_URL'], backend=os.environ['REDIS_URL'])
background_callback_manager = CeleryManager(celery_app)
else:
# Diskcache for non-production apps when developing locally
import diskcache
cache = diskcache.Cache("./cache")
background_callback_manager = DiskcacheManager(cache)
def make_progress_graph(progress, total):
progress_graph = (
go.Figure(data=[go.Bar(x=[progress])])
.update_xaxes(range=[0, total])
.update_yaxes(
showticklabels=False,
)
.update_layout(height=100, margin=dict(t=20, b=40))
)
return progress_graph
app = dash.Dash(__name__, background_callback_manager=background_callback_manager)
app.layout = html.Div(
[
html.Div(
[
html.P(id="paragraph_id", children=["Button not clicked"]),
dcc.Graph(id="progress_bar_graph", figure=make_progress_graph(0, 10)),
]
),
html.Button(id="button_id", children="Run Job!"),
html.Button(id="cancel_button_id", children="Cancel Running Job!"),
]
)
@dash.callback(
output=Output("paragraph_id", "children"),
inputs=Input("button_id", "n_clicks"),
background=True,
running=[
(Output("button_id", "disabled"), True, False),
(Output("cancel_button_id", "disabled"), False, True),
(
Output("paragraph_id", "style"),
{"visibility": "hidden"},
{"visibility": "visible"},
),
(
Output("progress_bar_graph", "style"),
{"visibility": "visible"},
{"visibility": "hidden"},
),
],
cancel=[Input("cancel_button_id", "n_clicks")],
progress=Output("progress_bar_graph", "figure"),
progress_default=make_progress_graph(0, 10)
)
def update_progress(set_progress, n_clicks):
total = 10
for i in range(total):
time.sleep(0.5)
set_progress(make_progress_graph(i, 10))
return [f"Clicked {n_clicks} times"]
if __name__ == "__main__":
app.run_server(debug=True)
When your app is deployed in production, a finite number of CPUs serve requests for that app.
Callbacks that take longer than 30 seconds often experience timeouts when deployed in production.
And even callbacks that take less than 30 seconds can tie up all available server resources when multiple
users access your app at the same time. When all CPUs are processing callbacks, new visitors to your app see a
blank screen and eventually a “Server Timed Out” message.
Job queues are a solution to these timeout issues. Like the web processes serving your Dash app, job queues run
with a dedicated number of CPU workers. These workers go through the jobs one at a time and aren’t subject to timeouts.
While the job queue workers are processing the data, the web processes serving the Dash app and the regular callbacks
display informative loading screens, progress bars, and the results of the job queues.
End users never see a timeout and always see a responsive app.
Number of workers
In production apps, you can tune the number of workers you want to process your web requests versus process background
jobs in the queue using command line flags in Gunicorn and Celery.
Here is an example of a Procfile with 4 CPUs dedicated to regular Dash callbacks and 2 CPUs dedicated to
processing background callbacks in a queue.
web: gunicorn app:server --workers 4
queue: celery -A app:celery_app worker --loglevel=INFO --concurrency=2
The ratio of Gunicorn web workers to Celery queue workers will depend on your app.
You’ll want enough web workers that your app remains responsive to new users opening your app and enough
background queue workers so tasks don’t wait too long in the queue.
If your regular callbacks respond
quickly (less than 500ms), consider configuring fewer web gunicorn workers.
For deploying to Dash Enterprise, you’ll also need to update your DOKKU_SCALE file:
web=1
queue=1
Background callbacks address the following limitations of long callbacks:
ALL
, MATCH
, or ALL_SMALLER
.dash.callback_context
is not supported.Input/State/Output
dependencies do not exist when the app starts (if they reference components that aresuppress_callback_exceptions=True
does not prevent Dash from raising callbackIt was not possible to fix these issues without introducing backwards incompatible changes to long_callback
.
So, this feature was re-architected in a way that fixed these limitations without changing and
breaking long_callback
.