golib/httpserver/doc.go

/*
 * MIT License
 *
 * Copyright (c) 2025 Nicolas JUHEL
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// Package httpserver provides production-grade HTTP/HTTPS server management with comprehensive
// lifecycle control, configuration validation, TLS support, and integrated monitoring.
//
// # Overview
//
// This package offers a robust abstraction layer for managing HTTP and HTTPS servers in Go
// applications, emphasizing production readiness through comprehensive lifecycle management,
// declarative configuration with validation, optional TLS/SSL support, handler management,
// and built-in monitoring capabilities.
//
// The package is designed for scenarios requiring multiple server instances, dynamic
// configuration updates, graceful shutdowns, and centralized management through the pool
// subpackage. It extends the standard library's http.Server with additional features while
// maintaining compatibility with existing http.Handler implementations.
//
// # Design Philosophy
//
// The httpserver package follows these core design principles:
//
// 1. Configuration-Driven Architecture: Servers are defined through declarative configuration
// structures that are validated before use, enabling early detection of configuration errors
// and supporting externalized configuration from files, environment variables, or databases.
//
// 2. Lifecycle Management: Complete control over server start, stop, and restart operations
// with context-aware cancellation and graceful shutdown support. Servers can be started,
// stopped, and restarted programmatically with proper resource cleanup.
//
// 3. Thread-Safe Operations: All operations use atomic values and proper synchronization
// primitives, ensuring safe concurrent access from multiple goroutines without external
// locking requirements.
//
// 4. Production-Ready Features: Built-in monitoring integration, structured logging with
// field context, error handling with typed error codes, port conflict detection, and health
// checking capabilities.
//
// 5. Composable Design: The pool subpackage enables orchestration of multiple server instances
// as a unified entity, supporting filtering, batch operations, and aggregated monitoring.
//
// # Key Features
//
// Complete Lifecycle Control:
//   - Context-aware Start, Stop, and Restart operations
//   - Automatic resource cleanup and graceful shutdown
//   - Uptime tracking and running state queries
//   - Error tracking and recovery mechanisms
//
// Configuration Management:
//   - Declarative configuration with struct tag validation
//   - Deep cloning for safe configuration copies
//   - Dynamic configuration updates with SetConfig
//   - TLS/HTTPS configuration with certificate management
//
// Handler Management:
//   - Dynamic handler registration via function callbacks
//   - Multiple named handlers per server instance
//   - Handler key-based routing for multi-handler scenarios
//   - Fallback BadHandler for misconfigured servers
//
// TLS/HTTPS Support:
//   - Integrated certificate management
//   - Optional and mandatory TLS modes
//   - Default TLS configuration inheritance
//   - Automatic TLS detection and configuration
//
// Monitoring and Health Checking:
//   - Built-in health check endpoints
//   - Integration with monitor package
//   - Server metrics and status information
//   - Unique monitoring identifiers per server
//
// Pool Management (via pool subpackage):
//   - Coordinate multiple server instances
//   - Unified start/stop/restart operations
//   - Advanced filtering by name, address, or URL
//   - Configuration-based pool creation
//
// Thread-Safe Operations:
//   - Atomic value storage for handlers and state
//   - Concurrent access without explicit locking
//   - Safe handler replacement during operation
//   - Context-based synchronization
//
// Port Management:
//   - Automatic port availability checking
//   - Conflict detection before binding
//   - Retry logic for port conflicts
//   - Support for all interface binding (0.0.0.0)
//
// # Architecture
//
// The package consists of three main components working together:
//
//	┌────────────────────────────────────┐
//	│          Application Layer         │
//	│      (HTTP Handlers & Routes)      │
//	└─────────────────┬──────────────────┘
//	                  │
//	        ┌─────────▼─────────┐
//	        │   httpserver      │
//	        │   Package API     │
//	        └─────────┬─────────┘
//	                  │
//	    ┌─────────────┼──────────────┐
//	    │             │              │
//	┌───▼───┐    ┌────▼────┐    ┌───▼────┐
//	│Server │    │  Pool   │    │ Types  │
//	│       │    │         │    │        │
//	│Config │◄───┤ Manager │    │Handler │
//	│Run    │    │ Filter  │    │Fields  │
//	│Monitor│    │ Clone   │    │Const   │
//	└───┬───┘    └────┬────┘    └────────┘
//	    │             │
//	    └──────┬──────┘
//	           │
//	    ┌──────▼──────┐
//	    │  Go stdlib  │
//	    │ http.Server │
//	    └─────────────┘
//
// Component Responsibilities:
//
//   - Server: Core HTTP server implementation with lifecycle management
//   - Config: Declarative configuration with validation and cloning
//   - Pool: Multi-server orchestration and batch operations
//   - Types: Shared type definitions and constants
//   - Monitor: Health checking and metrics collection
//   - Handler: Dynamic handler registration and management
//
// Data Flow:
//  1. Configuration is created and validated
//  2. Handlers are registered via function callbacks
//  3. Server instance is created from configuration
//  4. Start operation binds to port and begins serving
//  5. Handlers process incoming HTTP requests
//  6. Monitoring tracks health and metrics
//  7. Stop operation gracefully shuts down with timeout
//
// # Thread Safety Architecture
//
// The package uses multiple synchronization mechanisms for thread safety:
//
//	Component           | Mechanism           | Concurrency Model
//	--------------------|---------------------|----------------------------------
//	Server State        | atomic.Value        | Lock-free reads, atomic writes
//	Handler Registry    | atomic.Value        | Lock-free handler swapping
//	Logger              | atomic.Value        | Thread-safe logging
//	Runner              | atomic.Value        | Lifecycle synchronization
//	Config Storage      | context.Config      | Context-based atomic storage
//	Pool Map            | sync.RWMutex        | Multiple readers, exclusive writes
//
// All public methods are safe for concurrent use from multiple goroutines without
// external synchronization. Internal state updates use atomic operations to prevent
// data races.
//
// # Basic Usage
//
// Creating and starting a simple HTTP server:
//
//	cfg := httpserver.Config{
//	    Name:   "api-server",
//	    Listen: "127.0.0.1:8080",
//	    Expose: "http://localhost:8080",
//	}
//
//	cfg.RegisterHandlerFunc(func() map[string]http.Handler {
//	    mux := http.NewServeMux()
//	    mux.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
//	        w.WriteHeader(http.StatusOK)
//	        w.Write([]byte("OK"))
//	    })
//	    return map[string]http.Handler{"": mux}
//	})
//
//	if err := cfg.Validate(); err != nil {
//	    log.Fatal(err)
//	}
//
//	srv, err := httpserver.New(cfg, nil)
//	if err != nil {
//	    log.Fatal(err)
//	}
//
//	ctx := context.Background()
//	if err := srv.Start(ctx); err != nil {
//	    log.Fatal(err)
//	}
//	defer srv.Stop(ctx)
//
// # Configuration
//
// The Config structure defines all server parameters:
//
//	type Config struct {
//	    Name             string           // Server identifier (required)
//	    Listen           string           // Bind address host:port (required)
//	    Expose           string           // Public URL (required)
//	    HandlerKey       string           // Handler map key (optional)
//	    Disabled         bool             // Disable flag for maintenance
//	    Monitor          moncfg.Config    // Monitoring configuration
//	    TLSMandatory     bool             // Require valid TLS
//	    TLS              libtls.Config    // TLS/certificate configuration
//	    ReadTimeout      libdur.Duration  // Request read timeout
//	    ReadHeaderTimeout libdur.Duration // Header read timeout
//	    WriteTimeout     libdur.Duration  // Response write timeout
//	    MaxHeaderBytes   int              // Maximum header size
//	    // ... HTTP/2 configuration fields ...
//	}
//
// Configuration Validation:
//
// All configurations must pass validation before server creation:
//   - Name: Must be non-empty string
//   - Listen: Must be valid hostname:port format
//   - Expose: Must be valid URL with scheme
//   - TLS: Must be valid if TLSMandatory is true
//
// Configuration Methods:
//   - Validate(): Comprehensive validation with detailed errors
//   - Clone(): Deep copy of configuration
//   - RegisterHandlerFunc(): Set handler function
//   - SetDefaultTLS(): Set default TLS provider
//   - SetContext(): Set parent context provider
//   - Server(): Create server instance from config
//
// # Handler Management
//
// Handlers are registered via callback functions returning handler maps:
//
//	handlerFunc := func() map[string]http.Handler {
//	    return map[string]http.Handler{
//	        "":      defaultHandler,  // Default handler
//	        "api":   apiHandler,       // Named handler for API
//	        "admin": adminHandler,     // Named handler for admin
//	    }
//	}
//
//	cfg.RegisterHandlerFunc(handlerFunc)
//
// Handler Keys:
//
// The HandlerKey configuration field selects which handler from the map to use:
//   - Empty string or "": Uses default handler
//   - "api": Uses handler registered with "api" key
//   - Custom keys: Application-specific handler selection
//
// Multiple servers can share the same handler function but use different keys
// to serve different handlers on different ports.
//
// # TLS/HTTPS Configuration
//
// Servers support optional and mandatory TLS/SSL:
//
//	cfg := httpserver.Config{
//	    Name:         "secure-api",
//	    Listen:       "0.0.0.0:8443",
//	    Expose:       "https://api.example.com",
//	    TLSMandatory: true,
//	    TLS: libtls.Config{
//	        CertPEM: "/path/to/cert.pem",
//	        KeyPEM:  "/path/to/key.pem",
//	        // Additional TLS options...
//	    },
//	}
//
// TLS Modes:
//
//   - TLSMandatory = false: TLS is optional, server starts without certificates
//   - TLSMandatory = true: Valid TLS config required, server fails to start without it
//
// TLS Configuration Inheritance:
//
// Servers can inherit from a default TLS configuration:
//
//	cfg.SetDefaultTLS(func() libtls.TLSConfig {
//	    return defaultTLSConfig
//	})
//	cfg.TLS.InheritDefault = true
//
// # Lifecycle Management
//
// Servers implement the libsrv.Runner interface for lifecycle control:
//
//	// Start server
//	err := srv.Start(ctx)
//	if err != nil {
//	    log.Fatal(err)
//	}
//
//	// Check if running
//	if srv.IsRunning() {
//	    log.Println("Server is running")
//	}
//
//	// Get uptime
//	uptime := srv.Uptime()
//	log.Printf("Server uptime: %v", uptime)
//
//	// Stop gracefully
//	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
//	defer cancel()
//	err = srv.Stop(ctx)
//
//	// Restart (stop then start)
//	err = srv.Restart(ctx)
//
// Graceful Shutdown:
//
// The Stop method performs graceful shutdown:
//  1. Stops accepting new connections
//  2. Waits for active requests to complete (up to timeout)
//  3. Closes server resources and listeners
//  4. Returns error if shutdown exceeds timeout
//
// # Server Information
//
// Servers provide read-only access to configuration and state:
//
//	name := srv.GetName()           // Server identifier
//	bind := srv.GetBindable()       // Listen address
//	expose := srv.GetExpose()       // Public URL
//	disabled := srv.IsDisable()     // Disabled flag
//	hasTLS := srv.IsTLS()          // TLS enabled check
//	cfg := srv.GetConfig()          // Full configuration
//
// # Monitoring and Health Checking
//
// Servers integrate with the monitor package for health checks and metrics:
//
//	// Get monitoring identifier
//	monitorName := srv.MonitorName()
//
//	// Get monitor with health checks and metrics
//	monitor, err := srv.Monitor(versionInfo)
//	if err != nil {
//	    log.Printf("Monitor error: %v", err)
//	}
//
// Health checks verify:
//   - Server is running (runner state check)
//   - Port is bound and accepting connections
//   - No fatal errors in server operation
//   - TCP connection can be established to bind address
//
// Monitor provides:
//   - Health check status and last error
//   - Server configuration and runtime information
//   - Uptime and running state
//   - Custom metrics from monitoring configuration
//
// # Port Management
//
// The package includes port conflict detection and resolution:
//
//	// Check if port is available
//	err := httpserver.PortNotUse(ctx, "127.0.0.1:8080")
//	if err == nil {
//	    // Port is available
//	}
//
//	// Check if port is in use
//	err = httpserver.PortInUse(ctx, "127.0.0.1:8080")
//	if err == nil {
//	    // Port is in use
//	}
//
// Automatic Port Conflict Handling:
//
// Servers automatically check for port conflicts before binding:
//   - Retries up to 5 times with delays
//   - Returns ErrorPortUse if port remains unavailable
//   - Configurable retry count via RunIfPortInUse
//
// # Error Handling
//
// The package defines typed errors with diagnostic codes:
//
//	Error Code             | Description
//	-----------------------|------------------------------------------
//	ErrorParamEmpty        | Required parameter missing
//	ErrorInvalidInstance   | Invalid server instance
//	ErrorHTTP2Configure    | HTTP/2 configuration failed
//	ErrorServerValidate    | Configuration validation failed
//	ErrorServerStart       | Server failed to start or listen
//	ErrorInvalidAddress    | Bind address format invalid
//	ErrorPortUse           | Port is already in use
//
// Error checking:
//
//	if err := srv.Start(ctx); err != nil {
//	    var liberr errors.Error
//	    if errors.As(err, &liberr) {
//	        switch liberr.Code() {
//	        case httpserver.ErrorPortUse:
//	            log.Println("Port already in use")
//	        case httpserver.ErrorServerValidate:
//	            log.Println("Invalid configuration")
//	        default:
//	            log.Printf("Server error: %v", err)
//	        }
//	    }
//	}
//
// # Performance Characteristics
//
// Server Operations:
//
//	Operation               | Time         | Memory    | Notes
//	------------------------|--------------|-----------|------------------------
//	Config Validation       | ~100ns       | O(1)      | Field validation only
//	Server Creation         | <1ms         | ~5KB      | Includes initialization
//	Start Server            | 1-5ms        | ~10KB     | Port binding overhead
//	Stop Server (graceful)  | <5s          | O(1)      | Default timeout
//	Handler Execution       | Variable     | Variable  | Depends on handler
//	Info Methods            | <100ns       | O(1)      | Atomic reads
//	Configuration Update    | <1ms         | O(1)      | Validation + atomic swap
//
// Throughput:
//   - HTTP Requests: Limited by Go's http.Server (~50k+ req/s typical)
//   - HTTPS/TLS: ~20-30k req/s depending on cipher suite and hardware
//   - Overhead: Minimal (<1% vs standard http.Server)
//
// Memory Usage:
//   - Single Server: ~10-15KB baseline + handler memory
//   - With Monitoring: +5KB per server
//   - Pool Overhead: ~1KB per server in pool
//
// Scalability:
//   - Supports hundreds of server instances per process
//   - Linear memory growth with server count
//   - No global locks on request path
//
// # Use Cases
//
// ## Microservices Architecture
//
// Run multiple API versions simultaneously:
//
//	// API v1 on port 8080
//	cfgV1 := httpserver.Config{
//	    Name:       "api-v1",
//	    Listen:     "0.0.0.0:8080",
//	    Expose:     "http://api.example.com/v1",
//	    HandlerKey: "v1",
//	}
//
//	// API v2 on port 8081
//	cfgV2 := httpserver.Config{
//	    Name:       "api-v2",
//	    Listen:     "0.0.0.0:8081",
//	    Expose:     "http://api.example.com/v2",
//	    HandlerKey: "v2",
//	}
//
// ## Multi-Tenant Systems
//
// Dedicated server per tenant with isolated configuration:
//
//	for _, tenant := range tenants {
//	    cfg := httpserver.Config{
//	        Name:   tenant.ID,
//	        Listen: fmt.Sprintf("0.0.0.0:%d", tenant.Port),
//	        Expose: tenant.Domain,
//	        TLS:    tenant.TLSConfig,
//	    }
//	    srv, _ := httpserver.New(cfg, nil)
//	    srv.Start(ctx)
//	}
//
// ## Development and Testing
//
// Dynamic server creation for integration tests:
//
//	func TestAPI(t *testing.T) {
//	    port := getFreePort()
//	    cfg := httpserver.Config{
//	        Name:   "test-server",
//	        Listen: fmt.Sprintf("127.0.0.1:%d", port),
//	        Expose: fmt.Sprintf("http://localhost:%d", port),
//	    }
//	    cfg.RegisterHandlerFunc(testHandler)
//	    srv, _ := httpserver.New(cfg, nil)
//	    srv.Start(context.Background())
//	    defer srv.Stop(context.Background())
//	    // Run tests...
//	}
//
// ## API Gateway
//
// Route traffic to multiple backend servers:
//
//	pool := pool.New(nil, gatewayHandler)
//	for _, backend := range backends {
//	    cfg := httpserver.Config{
//	        Name:   backend.Name,
//	        Listen: backend.Address,
//	        Expose: backend.URL,
//	    }
//	    pool.StoreNew(cfg, nil)
//	}
//	pool.Start(ctx)
//
// ## Production Deployments
//
// Graceful shutdown during rolling updates:
//
//	// Handle SIGTERM for graceful shutdown
//	sigChan := make(chan os.Signal, 1)
//	signal.Notify(sigChan, syscall.SIGTERM)
//	go func() {
//	    <-sigChan
//	    ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
//	    defer cancel()
//	    if err := srv.Stop(ctx); err != nil {
//	        log.Printf("Shutdown error: %v", err)
//	    }
//	}()
//
// # Limitations
//
// 1. Handler Replacement During Operation: Handlers can be replaced while the server
// is running, but active requests continue using the previous handler. New requests
// use the updated handler. This may cause inconsistent behavior during handler updates.
//
// 2. TLS Certificate Rotation: Updating TLS certificates requires server restart.
// The SetConfig method does not apply TLS changes to running servers.
//
// 3. Port Binding Limitations: Cannot bind to privileged ports (<1024) without
// appropriate system permissions (CAP_NET_BIND_SERVICE on Linux or root access).
//
// 4. Listen Address Changes: Changing the Listen address via SetConfig requires
// a server restart to take effect. The new address is not applied to running servers.
//
// 5. HTTP/2 Configuration: HTTP/2 settings cannot be changed after server creation
// without recreating the server instance.
//
// 6. Concurrent Stop Calls: Multiple concurrent Stop calls may result in redundant
// shutdown attempts. Use external synchronization if multiple goroutines may call Stop.
//
// 7. Context Cancellation: Server operations respect context cancellation, but
// forcefully cancelled contexts (e.g., exceeded deadline during graceful shutdown)
// may result in abrupt connection termination.
//
// 8. Monitoring Overhead: Each server maintains monitoring state. For deployments
// with hundreds of servers, consider disabling monitoring for non-critical instances.
//
// # Best Practices
//
// DO:
//   - Always validate configuration before creating servers
//   - Use context.WithTimeout for Stop operations to prevent hanging
//   - Register handlers before calling New() when possible
//   - Use defer srv.Stop(ctx) to ensure cleanup
//   - Check IsRunning() before calling Stop to avoid errors
//   - Set appropriate timeouts (ReadTimeout, WriteTimeout, IdleTimeout)
//   - Use the pool subpackage for managing multiple servers
//   - Monitor server health in production deployments
//   - Use unique server names for debugging and monitoring
//
// DON'T:
//   - Don't create servers without validating configuration
//   - Don't ignore errors from Start, Stop, or SetConfig
//   - Don't assume port availability without checking
//   - Don't call Stop without context timeout
//   - Don't modify Config after passing to New() (use Clone first)
//   - Don't rely on handler updates to active connections
//   - Don't bind to 0.0.0.0 without considering security implications
//   - Don't start servers in goroutines without proper error handling
//
// # Integration with Standard Library
//
// The package integrates seamlessly with Go's standard library:
//
//	Standard Library         | Integration Point
//	-------------------------|------------------------------------------
//	net/http.Server          | Wrapped and managed by this package
//	net/http.Handler         | Compatible with all standard handlers
//	context.Context          | Used throughout for cancellation
//	net.Listener             | Managed internally for binding
//	crypto/tls               | TLS configuration via certificates package
//
// # Related Packages
//
// This package works with other golib packages:
//
//   - github.com/nabbar/golib/httpserver/pool: Multi-server orchestration
//   - github.com/nabbar/golib/httpserver/types: Shared type definitions
//   - github.com/nabbar/golib/certificates: TLS/SSL certificate management
//   - github.com/nabbar/golib/monitor: Health checking and metrics
//   - github.com/nabbar/golib/logger: Structured logging integration
//   - github.com/nabbar/golib/runner: Lifecycle management primitives
//   - github.com/nabbar/golib/context: Typed context storage
//
// External Dependencies:
//
//   - github.com/go-playground/validator/v10: Configuration validation
//   - golang.org/x/net/http2: HTTP/2 support
//
// # Testing
//
// The package includes comprehensive testing using Ginkgo v2 and Gomega:
//
//   - Configuration validation tests
//   - Server lifecycle tests (start, stop, restart)
//   - Handler management tests
//   - Monitoring integration tests
//   - Concurrency and race detection tests
//   - Integration tests with actual HTTP servers
//
// Run tests:
//
//	go test -v ./...
//	go test -race -v ./...
//	go test -cover -v ./...
//
// For detailed testing documentation, see TESTING.md in the package directory.
//
// # Examples
//
// See example_test.go for comprehensive usage examples covering:
//   - Basic HTTP server creation and lifecycle
//   - HTTPS server with TLS configuration
//   - Multiple handlers with handler keys
//   - Server pool management
//   - Dynamic configuration updates
//   - Graceful shutdown patterns
//   - Monitoring integration
//   - Error handling strategies
package httpserver