Interface-Based Module Registries — Plugin Architectures

Caddy is an extensible, modular web server platform designed for high-performance traffic management and automated security. At its core, it functions as a dynamic HTTP gateway that handles request routing, static asset delivery, and reverse proxying through a chain of configurable handler modules. The system is built on a modular architecture that allows developers to extend server functionality by registering custom components, all managed through a unified lifecycle and provisioning framework. What distinguishes Caddy is its focus on automated infrastructure and zero-downtime operations. It provides native, automated HTTPS management by handling the entire lifecycle of TLS certificates, including issuance and renewal via public or private certificate authorities. The server state is managed through a JSON-driven configuration schema that supports atomic, background validation and swapping, enabling real-time updates to routing rules and server settings without interrupting active connections. The platform offers a comprehensive suite of tools for observability and control, including a dedicated administrative API for managing server state and inspecting metrics. It supports complex traffic filtering through flexible request matching, allowing for granular control over how incoming traffic is processed. Developers can define server behavior using a declarative configuration syntax, which the system validates and converts into its native JSON format for deployment.