Cal.com

Cal.com (formerly Calendso) is an open‑source, self‑hosted booking and scheduling platform that aims to replace proprietary services like Calendly. From a developer’s standpoint, the project is designed for extensibility and performance: it exposes a rich GraphQL API, supports webhooks, and can be integrated into existing authentication or billing pipelines. The core of the application is a full‑stack React/Next.js frontend coupled with an Express/Node backend that communicates with a PostgreSQL database via Prisma ORM. The system is built to be modular; most features—such as time‑zone handling, availability rules, and email notifications—are implemented as pluggable modules that can be overridden or replaced without touching the core codebase.

• Frontend: Next.js 13 with React Server Components, TypeScript, and Tailwind CSS. The UI is a single‑page application that consumes the GraphQL API over HTTP/2, leveraging server‑side rendering for SEO and performance.
• Backend: Node.js 20 with Express, TypeScript, and Apollo Server. Business logic is split into services (e.g., scheduling, notifications) and controllers, following a clean‑architecture pattern that facilitates unit testing.
• Data Layer: PostgreSQL 14+ accessed through Prisma. The schema is versioned with Prisma migrations, and the database supports advanced features such as JSONB for flexible event metadata.
• Authentication: The platform integrates with OAuth providers (Google, GitHub, Microsoft) and supports JWT‑based session handling. Custom authentication can be injected via the auth plugin interface.
• Containerization: Docker Compose files are provided for local development, and a Helm chart is available for Kubernetes deployments. The container image is built on node:20-alpine and serves both the API and UI, reducing runtime overhead.

• GraphQL API: CRUD operations for users, teams, event types, bookings, and availability. The schema is extensible; developers can add custom fields to the Event type via a plugin hook.
• Webhooks & Callbacks: Triggered on booking creation, cancellation, or rescheduling. The payload is JSON and can be consumed by external services (Zapier, custom microservices).
• Email & SMS: Out‑of‑the‑box integration with SendGrid, Postmark, and Twilio. The notification service is pluggable; you can swap providers by implementing the IEmailProvider interface.
• Recurrence & Availability: Support for complex recurrence rules (iCal RFC 5545) and time‑zone aware calendars. Availability is defined per user or team, allowing shared booking links that automatically respect each participant’s calendar.
• Plugin System: A cal.com/plugins directory lets developers drop in JavaScript modules that register routes, middlewares, or GraphQL resolvers. The system uses a simple dependency injection container to wire plugins at runtime.

Cal.com is designed for self‑hosting on any platform that supports Docker or Kubernetes. Key requirements:

• CPU/RAM: Minimum 2 vCPU and 4 GB RAM for a small team; production clusters scale horizontally via the Kubernetes Helm chart.
• Storage: Persistent PostgreSQL volumes are required; optional S3-compatible storage can be used for media uploads.
• CI/CD: The repository includes GitHub Actions that build Docker images, run tests, and push to Docker Hub. Developers can integrate these workflows into their own pipelines.
• Scaling: The API layer is stateless; load balancers can route traffic to multiple replicas. The database scales via read replicas or sharding if needed.

• OAuth & SSO: Built‑in support for SAML, OIDC, and custom OAuth providers. Developers can expose a /auth endpoint that redirects to any identity provider.
• Custom Domains & SSL: The platform can serve multiple subdomains, each mapped to a team or user. Let’s Encrypt certificates are automatically provisioned via Certbot in the Docker image.
• Webhook Callbacks: Developers can register custom endpoints that receive events, enabling real‑time integration with CRMs or ticketing systems.
• SDKs & Client Libraries: While the primary API is GraphQL, a lightweight TypeScript SDK is available in the packages/sdk folder for client applications.

• Documentation: The repo contains a comprehensive docs/ folder with architecture diagrams, API reference, and plugin guides. The README is kept concise but links to deeper resources.
• Community: Active discussions on GitHub, a dedicated Discord channel, and regular community calls. The project follows the Contributor Covenant, making it welcoming for newcomers.
• Testing: Jest and Playwright are used for unit and end‑to‑end tests. The test suite covers core scheduling logic, ensuring that new features do not regress existing behavior.
• Licensing: AGPLv3 guarantees that any derivative work must also be open source, which aligns with the community‑driven ethos of Cal.com.

1. Enterprise Scheduling – A SaaS company can host Cal.com to offer its customers a self‑hosted booking widget that integrates with their existing identity and billing systems.
2. Consulting Firms – Teams can share custom event types (e.g.,