caddyserver/caddy
Fast and extensible multi-platform HTTP/1-2-3 web server with automatic HTTPS
Extensible multi-platform HTTP web server with automatic HTTPS
Configuration flows from Caddyfile/JSON through adapters to module provisioning, then runtime requests flow through HTTP middleware chains to backend responses
Under the hood, the system uses 2 feedback loops, 3 data pools, 3 control points to manage its runtime behavior.
Structural Verdict
A 10-component backend api with 9 connections. 305 files analyzed. Well-connected — clear data flow between components.
How Data Flows Through the System
Configuration flows from Caddyfile/JSON through adapters to module provisioning, then runtime requests flow through HTTP middleware chains to backend responses
- Config Input — Caddyfile or JSON configuration loaded from file or admin API
- Parse & Adapt — Caddyfile parsed into tokens and adapted to JSON format
- Module Provisioning — JSON configuration unmarshaled and modules provisioned with context
- Request Processing — HTTP requests flow through middleware chain to handlers
- Response Generation — Handlers generate responses which flow back through middleware
System Behavior
How the system actually operates at runtime — where data accumulates, what loops, what waits, and what controls what.
Data Pools
Persistent storage for TLS certificates and ACME account data
Registry of available modules and their constructors
Current active configuration state
Feedback Loops
- Config Reload Loop (polling, balancing) — Trigger: Admin API config change or file watch. Action: Validate, provision modules, graceful swap. Exit: Config applied successfully or error.
- Certificate Renewal (scheduled-job, balancing) — Trigger: Certificate expiry threshold. Action: ACME challenge and certificate refresh. Exit: Certificate renewed or max retries.
Delays & Async Processing
- Graceful Shutdown (async-processing, ~configurable timeout) — Existing connections allowed to finish before stopping
- Certificate Validation (async-processing, ~network dependent) — Server startup waits for certificate acquisition
Control Points
- Admin Listen Address (env-var) — Controls: Admin API server binding
- Config File Path (env-var) — Controls: Default configuration file location
- Debug Mode (env-var) — Controls: Verbose logging and debug features
Technology Stack
Automatic HTTPS certificate management
Command-line interface framework
Structured logging
HTTP/3 protocol support
Metrics collection and exposure
Observability and tracing
Key Components
- Main (function) — Primary entry point that executes CLI commands and handles process lifecycle
cmd/main.go - App (type-def) — Core application interface that all Caddy apps must implement for lifecycle management
caddy.go - AdminConfig (class) — Manages the admin API server for runtime configuration and monitoring
admin.go - Adapter (class) — Converts Caddyfile configuration format to Caddy's native JSON format
caddyconfig/caddyfile/adapter.go - Dispenser (class) — Token-based parser for structured Caddyfile configuration parsing
caddyconfig/caddyfile/dispenser.go - Parse (function) — Parses Caddyfile input into server blocks and handles imports and environment variables
caddyconfig/caddyfile/parse.go - Module (type-def) — Base interface for all Caddy modules with registration and lifecycle management
modules.go - Context (class) — Provides module context with logger, storage, and configuration access
context.go - Replacer (class) — Template engine for variable substitution in configuration and responses
replacer.go - Storage (class) — Abstraction layer for persistent storage used by certificates and configuration
storage.go
Sub-Modules
Complete HTTP/1-3 server implementation with middleware system
TLS configuration and certificate management integration
Public Key Infrastructure for internal certificate authorities
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Frequently Asked Questions
What is caddy used for?
Extensible multi-platform HTTP web server with automatic HTTPS caddyserver/caddy is a 10-component backend api written in Go. Well-connected — clear data flow between components. The codebase contains 305 files.
How is caddy architected?
caddy is organized into 4 architecture layers: CLI Commands, Core Runtime, Configuration System, Protocol Modules. Well-connected — clear data flow between components. This layered structure enables tight integration between components.
How does data flow through caddy?
Data moves through 5 stages: Config Input → Parse & Adapt → Module Provisioning → Request Processing → Response Generation. Configuration flows from Caddyfile/JSON through adapters to module provisioning, then runtime requests flow through HTTP middleware chains to backend responses This pipeline design reflects a complex multi-stage processing system.
What technologies does caddy use?
The core stack includes CertMagic (Automatic HTTPS certificate management), Cobra (Command-line interface framework), Zap (Structured logging), QUIC-GO (HTTP/3 protocol support), Prometheus (Metrics collection and exposure), OpenTelemetry (Observability and tracing). A focused set of dependencies that keeps the build manageable.
What system dynamics does caddy have?
caddy exhibits 3 data pools (Certificate Storage, Module Registry), 2 feedback loops, 3 control points, 2 delays. The feedback loops handle polling and scheduled-job. These runtime behaviors shape how the system responds to load, failures, and configuration changes.
What design patterns does caddy use?
4 design patterns detected: Plugin Architecture, Configuration Adapters, Graceful Reloading, Template Variables.
Analyzed on March 31, 2026 by CodeSea. Written by Karolina Sarna.