Radicale is a really small piece of software, but understanding it is not as easy as it seems. But don’t worry, reading this short page is enough to understand what a CalDAV/CardDAV server is, and how Radicale’s code is organized.

General Architecture

Here is a simple overview of the global architecture for reaching a calendar or an address book through network:

Part Layer Protocol or Format
Server Calendar/Contact Storage iCal/vCard
Calendar/Contact Server CalDAV/CardDAV Server
Transfer Network CalDAV/CardDAV (HTTP + TLS)
Client Calendar/Contact Client CalDAV/CardDAV Client
GUI Terminal, GTK, Web interface, etc.

Radicale is only the server part of this architecture.

Please note that:

Radicale being a CalDAV/CardDAV server, it also can be seen as a special WebDAV and HTTP server.

Radicale is not the client part of this architecture. It means that Radicale never draws calendars, address books, events and contacts on the screen. It only stores them and give the possibility to share them online with other people.

If you want to see or edit your events and your contacts, you have to use another software called a client, that can be a “normal” applications with icons and buttons, a terminal or another web application.

Code Architecture

The radicale package offers 9 modules.

The main module provides a simple function called run. Its main work is to read the configuration from the configuration file and from the options given in the command line; then it creates a server, according to the configuration.
This is the core part of the module, with the code for the CalDAV/CardDAV server. The server inherits from a WSGIServer server class, which relies on the default HTTP server class given by Python. The code managing the different HTTP requests according to the CalDAV/CardDAV normalization is written here.
This part gives a dict-like access to the server configuration, read from the configuration file. The configuration can be altered when launching the executable with some command line options.
The functions defined in this module are mainly called by the CalDAV/CardDAV server class to read the XML part of the request, read or alter the calendars, and create the XML part of the response. The main part of this code relies on ElementTree.
The start function provided by this module starts a logging mechanism based on the default Python logging module. Logging options can be stored in a logging configuration file.
This module provides a default authentication manager equivalent to Apache’s htpasswd. Login + password couples are stored in a file and used to authenticate users. Passwords can be encrypted using various methods. Other authentication methods can inherit from the base class in this file and be provided as plugins.
This module is a set of Access Control Lists, a set of methods used by Radicale to manage rights to access the calendars. When the CalDAV/CardDAV server is launched, an Access Control List is chosen in the set, according to the configuration. The HTTP requests are then filtered to restrict the access depending on who is authenticated. Other configurations can be written using regex-based rules. Other rights managers can also inherit from the base class in this file and be provided as plugins.
In this module are written the classes representing collections and items in Radicale, and the class storing these collections and items in your filesystem. Other storage classes can inherit from the base class in this file and be provided as plugins.
This module contains the web interface.