Bokeh; storing data into session_context or otherwise? - bokeh

Question

I have some data that I would like to store for each user (session). I tried injecting it into curdoc().session_context but it appears to not be a simple data structure like a dict, it's a SessionContext object, and moreover, from reading the docs, I can't tell how to manipulate it (or it appears it is not meant to be): https://docs.bokeh.org/en/latest/docs/reference/document.html
How can I store some arbitrary local data for each user that gets destroyed when the session is destroyed?

Answer 1

I think you are overthinking this, then. If you want to store data between callbacks that is contained in one session, you don't need to do anything special. You could (for example) define a module level variable data = {} in the Bokeh app then callbacks can read and write to the dict however you need:
data = {}
def button_callback(attr, old, new):
# write some to session "data store"
data['foo'] = 'bar'
def button_callback():
# read some data from session "data store"
print(data['foo'])
Every session will get its own instance of data inaccessible to any other session, that will disappear whenever the session ends.
Obviously this is fairly simplistic. A more sophisticated version might define a class with a well-defined API for storing/retrieving data, and make a module level instance of the class for callbacks to use, instead of a bare dict.

Answer 2

With bokeh serve using Bokeh 2.1.1, I find that Python instance variables are unique to each session, so they can store session-specific data. For example, run this Bokeh app in three browser windows and click the Test button in window 1 then 2 then 3 and again in window 1 then 2 then 3 - the console messages show that each session has its own unique mySession value:
class InstanceVar():
def __init__(self, title):
self.testButton = Button(label="Test", button_type="primary")
self.testButton.on_click(self._test)
self.mySession = id(curdoc().session_context)
print("Set mySession={}".format(self.mySession))
def _test(self):
print("mySession={}".format(self.mySession))
But Python class variables seem to be shared by all sessions; when I run the following app in the same way, all three sessions print the mySession value of the most recently started session since they are all using the same class variable in memory:
class ClassVar():
mySession = None
def __init__(self, title):
self.testButton = Button(label="Test", button_type="primary")
self.testButton.on_click(self._test)
ClassVar.mySession = id(curdoc().session_context)
print("Set mySession={}".format(ClassVar.mySession))
def _test(self):
print("mySession={}".format(ClassVar.mySession))
So I assume from this that Bokeh Server only imports each class once and therefore all sessions/users share the class variables.