JupyterHub is a production-grade multi-user server for Jupyter notebooks. It provides authentication, spawning of single-user notebook servers, and proxying. Built on Tornado, it uses SQLAlchemy for persistence, supports pluggable Authenticators and Spawners, and includes an OAuth2 provider, RBAC scopes, a REST API, and a web UI. The system is designed for deployment in education, research, and enterprise settings, managing user sessions, isolating notebook environments, and routing traffic through a configurable HTTP proxy. The architecture follows a hub-and-spoke model: the Hub (Tornado process) handles authentication, authorization, and state; the ConfigurableHTTPProxy routes requests to per-user single-user servers; and Spawners manage server lifecycle. Key entry points include the `jupyterhub` CLI command, the `jupyterhub_config.py` configuration file, and the REST API at `/hub/api`. External dependencies include Tornado, SQLAlchemy, Alembic, oauthlib, prometheus_client, and aiohttp. The codebase is well-structured with clear abstractions for authentication, spawning, proxying, and authorization, making it a strong example of a complex web application framework.