What's the relationship between the jupyterlab and jupyterlab_server

[Song Liu]

Inside the jupyterlab/jupyterlab, it seems that the app is constructed from notebook app:

```

from notebook.notebookapp import NotebookApp

...

class LabApp(NotebookApp):

```

some questions:

- is this LabApp just the frontend server (Web) ?

- is jupyterlab_server the real backend server ?

- if yes, how does this LabApp interact with the jupyterlab_server ?

Could someone help clarify this ? many thanks.