Would you or anyone else be kind enough to explain this interaction between Panel, HoloViews, and Bokeh?
Sure. So for a very long time HoloViews was an entirely independent project which could render to Bokeh but didn’t treat Bokeh in any special way. We had some options for deploying on Bokeh server but those were somewhat limited. In the HoloViews 1.13 release however we switched HoloViews widgets and rendering entirely over to Panel in the background so now all the widgets you see and all the rendering is done by Panel.
This also simplified the story behind interacting with Jupyter and Bokeh server, all the setting up of bi-directional communication channels could now simply be handled by Panel and HoloViews wouldn’t worry about it. So let’s look at the three scenarios you called out in your blog post:
In the Jupyter Notebook comms can be globally accessed on the
Jupyter.notebook.kernel object making it possible to easily set up bi-directional communication channels. In JupyterLab on the other hand it is not as simple and we proxy the CommManager object using the
@pyviz/jupyterlab_pyviz extension. Otherwise things work basically the same, both HoloViews and Panel set up communication channels via this mechanism.
On the bokeh server communication is handled by the Bokeh Websocket. The easiest way to set this up from HoloViews is indeed what you describe, i.e. using
pn.panel(hv_obj).servable(). This sets things up so that
panel serve ... or
bokeh serve ... will work. Alternatively you can also create a simple script which launches a server using the
pn.serve function, so you simply run
Voila currently does not follow the classic notebook API, i.e. making
Jupyter.notebook.kernel available, but also does not support JupyterLab extension which means the standard communication channels will not work out of the box. Therefore I initially prototyped
ipybokeh as part of a Jupyter dashboarding workshop last year, which eventually migrated to jupyter_bokeh. Using this it is possible to wrap a bokeh (or Panel) object in an ipywidget which is supported in Voila. You can activate this in Panel by doing:
pn.config.comms = 'ipywidget'
or explicitly calling
pn.ipywidget(hv_or_panel_obj). However currently throttling is not yet enabled so you will end up with a lot of events being generated. Starting in Panel 0.10.0 throttling will be enabled and you should be able to use Panel/Bokeh objects in Voila just like any other ipywidgets based object.