Dash callback stops working when interacting with sqlite

Dash Python
1

Hi,
I have a relatively simple app as follows:

import dash
import dash_core_components as dcc
import dash_html_components as html
import dash_bootstrap_components as dbc
from dash.dependencies import Input, Output
from layout_constructor import create_header
import dao
import sqlite3
from sqlalchemy import create_engine
import pandas as pd 

conn = create_engine('sqlite:///G:/pyplicate/attunity.db')


def query(q):
    df = pd.read_sql_query(sql=q, con=conn)
    return df 

def get_summary_server_details():
    server_details_query = f'''SELECT cdc_inserts_count, cdc_update_count, cdc_delete_count, cdc_ddl_count 
        FROM taskDetails ORDER BY status_timestamp DESC LIMIT 1'''

    return query(server_details_query)

external_stylesheets = [
    'https://codepen.io/chriddyp/pen/bWLwgP.css', dbc.themes.SUPERHERO]

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([
    create_header(),
    html.Div(
        [
            dcc.Interval(
                id='interval-component',
                interval=1*1000,
                n_intervals=0
            )
        ]
    )
])


@app.callback(
    Output(component_id='agg-running-task', component_property='children'),
    [Input(component_id='interval-component', component_property='n_intervals')]
)
def update_output_div(n):
    #sql_result = pd.read_sql_query('SELECT cdc_inserts_count, cdc_update_count, cdc_delete_count, cdc_ddl_count FROM taskDetails ORDER BY status_timestamp DESC LIMIT 1', con=conn)

    return 'Ok!'
    # return 'You\'ve entered "{}"'.format(input_value)


if __name__ == '__main__':
    app.run_server(debug=True)

For now the callback returns ‘Ok’. The moment i uncomment the “sql_result” line i cant get the callback to return anything. Not even “Ok”. Does anyone have a clue why uncommenting this one line breaks the whole app?

Any help is highly appreciated.

Best Regards,
Simon

2

Do you get any error messages either in the terminal or the JavaScript console? If you run the sql_result like outside of a callback does it work as expected? How long does it take? If it’s running for a long time the callback could time out (we’re talking 30-60 seconds before that starts to be a problem which is unlikely for a select query unless you have a lot of data).

3

Hi,
No errors messages in either terminal nor console. If i add a print(sql_result) my pandas dataframe is printed with the correct data every second as expected.

4

Too add to this, the “return ‘ok!’” breaks (and never returns anything to my H5) even tho data is comming throgh in the callback every second.

5

Try reducing the rate that interval fires at? It’s possible that you’re just permanently stuck in an updating state because the query takes about a second and by the time its complete the callback has already triggered again.

2 Likes
6

Jesus. That worked :man_facepalming: Time to change database / optimize the database :slight_smile:

Thanks a lot!

2 Likes