Dash and data persistence

Question

I am creating an multipage app with python Dash. Being a newbie in this, I am wondering whether I can be sure that the data once loaded will stay persistent in the memory?

By this I mean when I have index.py file that acts as the page selector like this:

@app.callback(Output(component_id='page-content', component_property='children'),
              Input(component_id='url', component_property='pathname'))
def display_page(new_path):

    if new_path.startswith('/apps/Data'):
        return html.P(data.serve_layout())
    elif new_path == '/apps/Projects':
        return html.P(projects.serve_layout())
    else:
        return html.P(landing.serve_layout())

Now the page /apps/Data (data.py) has some inputs and buttons that allow user to fetch the necessary data. Data is stored in global python variable (i.e. an instance of a python class MyData), like this:

data_store = MyData()

@app.callback(
    Output('load_data','name'),
    Input('load_data', 'n_clicks'),
    prevent_initial_call=True)
def load_data (n_clicks):

    data_store.load_data()

    raise dash.exceptions.PreventUpdate

Later this data is accessed and used in the /apps/Projects (projects.py) page like this:

from apps.data import data_store

@app.callback(
    Output('show_data','name'),
    Input('show_data', 'n_clicks'),
    prevent_initial_call=True)
def show_data (n_clicks):

    print (data_store.test_show_data())

    raise dash.exceptions.PreventUpdate

Now this seems to work, but can I really trust that the data in data_store is persistent? The problem is that I do not understand the Dash memory handling deeply, so it might as well be that the data_store is suddenly deleted when some garbage collector kicks in. Or maybe it is safely inside the same application context?

Answer 1

It depends on the persistance_type you specify, although from what I understand, you want the default type. From the Dash docs...

local: uses window.localStorage. This is the default, and keeps the data indefinitely within that browser on that computer.

Dash Persistence Documentation

window.localStorage is provided by your browser. I have not tested but I would expect it to be lost if you uninstall and reinstall your browser. It also would not be available across different browsers. If you switched from Firefox to Chrome for instance. From the Monzilla Web Docs...

The localStorage read-only property of the window interface allows you to access a Storage object for the Document's origin; the stored data is saved across browser sessions.

Monzilla MDN Web Docs

Answer 2

I think this chapter of documentation explains this issue:

https://dash.plotly.com/sharing-data-between-callbacks

In there it says:

"There are three places you can store this data:

• In the user's browser session, using dcc.Store

• On the disk (e.g. in a file or database)

• In server-side memory (RAM) shared across processes and servers such as a Redis database. Dash Enterprise includes onboard, one-click Redis databases for this purpose."

I believe the stored_data instance in my example is in the server-side RAM.

I guess, if I needed to have multiple sessions at the same time, I could create a new data_store instance for each session? Not sure how, though...