Hi
I am trying to use pn.state.on_session_destroyed to clean up. And i received an error of “RuntimeError: Could not add session destroyed callback since no document to attach it to could be found.”
Do we have any example about how to the on_session_destroyed?
Any help will be gratefully
Kind regards
Victor
Hi @CongTang
Could you provide more detail of what you are doing like a minimum, reproducible example?
Hi Marc
I am trying to user on_session_destroyed to remove the name.
Please see the code below:
import panel as pn
import pandas as pd
import numpy as np
import string
import random
pn.extension(sizing_mode='stretch_width',comms = 'vscode',notifications=True)
if 'user_infor' not in pn.state.cache:
pn.state.cache['user_infor'] = pd.DataFrame(columns = ['Name','Role']).set_index('Name')
player_name = ''.join(random.choices(string.ascii_uppercase + string.digits, k=5))
pn.state.cache['user_infor'].loc[player_name] = 'game_role'
table = pn.pane.DataFrame(
pn.state.cache['user_infor']
)
def update():
table.object = pn.state.cache['user_infor']
game_page_cb = pn.state.add_periodic_callback(update, 5000,start =True,timeout = 60000)
def user_logout():
pn.state.cache['user_infor'].drop(player_name)
pn.state.on_session_destroyed(user_logout)
app = pn.Column(table)
app.servable();
and get an error:ValueError: Callback functions must have signature func(session_context), got func()
Any suggestion will be greatful 
Kind regards
Cong
Hi @CongTang
As I read the error message your update callback signature needs to change to def update(session_context).
If I change to that, I can get the below working.
import panel as pn
import pandas as pd
import numpy as np
import string
import random
pn.extension(sizing_mode='stretch_width',comms = 'vscode',notifications=True)
if 'user_infor' not in pn.state.cache:
pn.state.cache['user_infor'] = pd.DataFrame(columns = ['Name','Role']).set_index('Name')
player_name = ''.join(random.choices(string.ascii_uppercase + string.digits, k=5))
pn.state.cache['user_infor'].loc[player_name] = 'game_role'
table = pn.pane.DataFrame(
pn.state.cache['user_infor']
)
def update():
table.object = pn.state.cache['user_infor']
game_page_cb = pn.state.add_periodic_callback(update, 5000,start =True,timeout = 60000)
def user_logout(sessionContext):
print(sessionContext)
pn.state.cache['user_infor'].drop(player_name)
print("dropped")
pn.state.on_session_destroyed(user_logout)
app = pn.Column(table)
app.servable()
$ panel serve 'scripts\specify_value.py' --autoreload
2023-03-24 05:48:37,500 Starting Bokeh server version 2.4.3 (running on Tornado 6.2)
2023-03-24 05:48:37,501 User authentication hooks NOT provided (default user enabled)
2023-03-24 05:48:37,503 Bokeh app running at: http://localhost:5006/specify_value
2023-03-24 05:48:37,503 Starting Bokeh server with process id: 34328
2023-03-24 05:48:41,577 WebSocket connection opened
2023-03-24 05:48:41,578 ServerConnection created
2023-03-24 05:48:41,578 404 GET /favicon.ico (::1) 0.00ms
2023-03-24 05:48:50,253 WebSocket connection closed: code=1001, reason=None
2023-03-24 05:48:59,854 WebSocket connection opened
2023-03-24 05:48:59,855 ServerConnection created
2023-03-24 05:49:10,740 WebSocket connection closed: code=1001, reason=None
<bokeh.server.contexts.BokehSessionContext object at 0x0000019377B37F70>
dropped
I’ve made a PR to describe the required callback signature. Please take a look @CongTang and let me know if this improvement would have solved your issue up front.
Thanks.
Hi Marc
Really thanks for your help. That work pretty well.
Kind regards
Victor
Hi Marc
Just noticed an issue(not too sure if it is a mechanism or a bug).
The pn.state.on_session_destroyed is only working with the --atuoreload , otherwise it will end up with
<bokeh.server.contexts.BokehSessionContext object at 0x0000025CA72F3BE0>
2023-03-26 18:41:37,316 DocumentLifeCycleHandler on_session_destroyed callback <function user_logout at 0x0000025CA72EBA30> failed with following error: name 'pn' is not defined
Kind regards
Vcitor
It looks like a bug to me. Could you report it on github @CongTang ?
As a workaround you can try adding
import panel as pn
to your user_logout callback.
Sure. I will report it to GitHub.
Yes, if I import it, it will work. But it will not unable to process some parameters of the original program.
and i would see it is not a serious bug as it still works with --atuoreload
Thanks for your answer and support.
Kind regards
Victor
It happens as sessions are being unloaded from memory. I’ve encountered the same bug numerous times:
'Exception' is not defined - lol
pn.state.on_session_destroyed callback also doesn’t work when you use lifecycle hooks in a separate file (even though it’s in the state’s callbacks). However, the on_session_destroyed callback does get called from within the app_hooks.py file 
Extra FYI:
The session unloading seems to happen more aggressively when using --num-threads
This is actually a nice bug (feature? 
) as you can get around memory leaks. Even better if you periodically restart your cluster workers to release memory back to the OS:
Haven’t had issues with memory leaks since.
Ah, btw - this also makes watcher callbacks fail. Can only let them fail silently as sessions are being unloaded.
