what is `modified_timestamp` used for in plotly store component

Question

I am reading about the first example on https://dash.plot.ly/dash-core-components/store

why

@app.callback(Output('{}-clicks'.format(store), 'children'),
              [Input(store, 'modified_timestamp')],
              [State(store, 'data')])
def on_data(ts, data):
    if ts is None:
        raise PreventUpdate

    data = data or {}

    return data.get('clicks', 0)

instead of

@app.callback(Output('{}-clicks'.format(store), 'children'),
              [Input(store, 'data')])
def on_data(data):
    data = data or {}
    return data.get('clicks', 0)

I don't understand the reason for using modified_timestamp.

Answer 1

From the docs page you shared:

Retrieving the initial store data

If you use the data prop as an output, you cannot get the initial data on load with the data prop. To counter this, you can use the modified_timestamp as Input and the data as State.

This limitation is due to the initial None callbacks blocking the true data callback in the request queue.

See https://github.com/plotly/dash-renderer/pull/81 for further discussion.