Open Source Shared Calendar Servers

Radicale is a self-hosted CalDAV and CardDAV server implemented in Python that enables calendar and contact synchronization across multiple clients using open standards. It stores all data directly on the filesystem in a simple folder hierarchy, eliminating the need for an external database, and automatically tracks every change by committing modifications to a Git repository for version history and backup. The server distinguishes itself through a modular plugin architecture that allows custom Python modules to extend authentication, storage backends, and web interface functionality at startup. Access control is managed through regex-based rules that govern read, write, and delete permissions on individual collections, while user identity is verified through multiple configurable authentication backends before granting access. All client-server communication is secured through TLS encryption. Radicale includes an event-driven notification system that triggers automated push alerts via email or message queues when calendar events are created or modified. A built-in web interface provides direct management of calendars and address books through the server URL, and the server can be deployed as a Docker container supporting both linux/amd64 and linux/arm64 platforms.

Nextcloud is a self-hosted platform designed for private cloud storage, file synchronization, and collaborative team workspaces. It provides a comprehensive suite of tools for document editing, groupware services like calendars and contacts, and secure data management, all while ensuring users maintain full control over their infrastructure and data sovereignty. The platform distinguishes itself through a decentralized federated architecture that allows independent server instances to securely share data and collaborate across a network. It features a highly modular plugin ecosystem, enabling administrators to dynamically load, activate, and manage third-party applications through a centralized registry. This extensibility is supported by an event-driven workflow engine and background job processing, which automate repetitive tasks and integrate external services without manual intervention. Beyond core storage and collaboration, the system includes robust administrative capabilities for managing user accounts, system health, and regulatory compliance. It offers extensive command-line tools for server maintenance, configuration, and performance monitoring, alongside support for integrating machine learning services to handle tasks such as language translation and speech-to-text conversion. The software is deployed as a self-hosted environment on Linux, requiring standard web server, database, and runtime configurations to operate.