Radicale is a production-grade CalDAV/CardDAV server written in Python, providing calendar and contact synchronization over WebDAV. It operates as a standalone HTTP server (with optional WSGI compatibility) and includes a built-in web UI for managing collections, sharing, and user settings. The architecture is highly modular, with pluggable backends for authentication (htpasswd, LDAP, IMAP, OAuth2, PAM, etc.), storage (multifilesystem with caching and locking), authorization (owner-only, file-based rules), sharing (token-based, map-based, group/realm), and hooks (email, RabbitMQ). The server handles the full CalDAV/CardDAV protocol suite, including PROPFIND, REPORT, PUT, DELETE, MKCALENDAR, MKCOL, MOVE, PROPPATCH, and sync-collection, with support for recurring events, timezone handling, and free-busy queries. Key entry points include the CLI runner (`radicale/__main__.py`), the WSGI application factory (`radicale/__init__.py`), and the configuration system (`radicale/config.py`). Data flows from HTTP requests through the application layer (mixin-based HTTP method handlers) to the storage backend, which persists data as iCalendar (.ics) and vCard (.vcf) files on the filesystem. The sharing subsystem adds a REST API (`/.sharing/v1/`) for managing shares, property overlays, and on-the-fly conversions (e.g., VCF→ICS for birthdays). External dependencies include the `vobject` library for iCalendar/vCard parsing, and optional packages for LDAP, IMAP, PAM, RabbitMQ, and bcrypt/argon2 password hashing. The codebase demonstrates strong engineering practices: comprehensive test suites (unit, integration with Playwright, CalDAV compatibility), extensive CI/CD (Python 3.9–3.14, PyPy, multiple filesystems), and a well-documented plugin system. It is actively maintained with frequent releases and a responsive issue tracker. The modular design and clean separation of concerns make it a solid reference for building extensible server applications.