What is Jupyter Server doing in the stack of JupyterLab / Kernel / user code ?

[janpfeifer]

Apologies for the noob question!

From reading and the introductory text of the documentation it is not very clear what Jupyter Server actually does -- what are its inputs (clients) and outputs (maybe also clients) ? I'm missing a diagram somewhere :)

I'm trying to figure out to whom a Jupyter kernel is talking to using ZeroMQ and the Jupyter Messaging Protocol, Is it the Jupyter Server ? (it documents a REST API, but since the kernel is using ZeroMQ...)

A second question, which may or may not be related to Jupyter Server (although I think it's set in Jupyter Server): what is the purpose of the exported environment variable "JPY_SESSION_NAME", considering the user may rename the notebook, and the variable becomes stale ? -- I'm asking from the perspective of a kernel developer, maybe the exported env variable was not meant to be used by Kernels ?

many thanks!