Add Jupyter Server Architecture diagram (#801)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: andreyvelich

docs/source/developers/architecture.rst

@@ -0,0 +1,73 @@
+.. _architecture:
+
+Architecture Diagrams
+=====================
+
+This page describes the Jupyter Server architecture and the main workflows.
+This information is useful for developers who want to understand how Jupyter
+Server components are connected and how the principal workflows look like.
+
+To make changes for these diagrams, use `the Draw.io <https://app.diagrams.net/>`_
+open source tool to edit the png file.
+
+
+Jupyter Server Architecture
+---------------------------
+
+The Jupyter Server system can be seen in figure below:
+
+.. image:: ../images/jupyter-server-architecture.drawio.png
+   :alt: Jupyter Server Architecture
+   :width: 100%
+   :align: center
+
+Jupyter Server contains the following components:
+
+- **ServerApp** is the main Tornado-based application which connects all
+  components together.
+
+- **Config Manager** initializes configuration for the ServerApp. You can define
+  custom classes for the Jupyter Server managers using this config and change
+  SererApp settings. Follow :ref:`the Config File Guide <other-full-config>` to
+  learn about configuration settings and how to build custom config.
+
+- **Custom Extensions** allow you to create the custom Server's REST API endpoints.
+  Follow :ref:`the Extension Guide <extensions>` to know more about extending
+  ServerApp with extra request handlers.
+
+- **Gateway Server** is a web server that, when configured, provides access to
+  Jupyter Kernels running on other hosts. There are different ways to create a
+  gateway server. If your ServerApp needs to communicate with remote Kernels
+  residing within resource-managed clusters, you can use
+  `Enterprise Gateway <https://github.com/jupyter-server/enterprise_gateway>`_,
+  otherwise, you can use
+  `Kernel Gateway <https://github.com/jupyter-server/kernel_gateway>`_, where
+  Kernels run locally to the gateway server.
+
+- **Contents Manager and File Contents Manager** are responsible for serving
+  Notebook on the file system. Session Manager uses Contents Manager to receive
+  Kernel path. Follow :ref:`the Contents API guide <contents_api>` to learn
+  about Contents Manager.
+
+- **Session Manager** processes users' Sessions. When a user starts a new Kernel,
+  Session Manager starts a process to provision Kernel for the user and generates
+  a new Session ID. Each opened Notebook has a separate Session, but different
+  Notebook Kernels can use the same Session. That is useful if the user wants to
+  share data across various opened Notebooks. Session Manager uses SQLite3
+  DataBase to store the Session information. The database is stored in memory by
+  default, but can be configured to save to disk.
+
+- **Mapping Kernel Manager** is responsible for managing the lifecycles of the
+  Kernels running within the ServerApp. It starts a new Kernel for a user's Session
+  and facilitates interrupt, restart, and shutdown operations against the Kernel.
+
+- **Jupyter Client** library is used by Jupyter Server to work with the Notebook
+  Kernels.
+
+  - **Kernel Manager** manages a single Kernel for the Notebook. To know more about
+    Kernel Manager, follow
+    `the Jupyter Client APIs documentation <https://jupyter-client.readthedocs.io/en/latest/api/manager.html#jupyter_client.KernelManager>`_.
+
+  - **Kernel Spec Manager** parses files with JSON specification for a Kernels,
+    and provides a list of available Kernel configurations. To learn about
+    Kernel Spec, check `the Jupyter Client guide <https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs>`_.

docs/source/developers/extensions.rst

@@ -1,3 +1,5 @@
+.. _extensions:
+
 =================
 Server Extensions
 =================

docs/source/developers/index.rst

@@ -8,6 +8,7 @@ These pages target people writing Jupyter Web applications and server extensions
    :maxdepth: 1
    :name: developers
 
+   architecture
    dependency
    rest-api
    extensions