bzzz/pkg/dht/hybrid_dht.go

package dht

import (
	"context"
	"fmt"
	"sync"
	"time"

	"chorus.services/bzzz/pkg/config"
)

// HybridDHT provides a switchable interface between mock and real DHT implementations
type HybridDHT struct {
	mockDHT    DHT
	realDHT    DHT
	config     *config.HybridConfig
	
	// State management
	currentBackend string
	fallbackActive bool
	healthStatus   map[string]*BackendHealth
	
	// Synchronization
	mu sync.RWMutex
	
	// Monitoring
	metrics *HybridMetrics
	logger  Logger
}

// BackendHealth tracks health status of DHT backends
type BackendHealth struct {
	Backend     string        `json:"backend"`
	Status      HealthStatus  `json:"status"`
	LastCheck   time.Time     `json:"last_check"`
	ErrorCount  int          `json:"error_count"`
	Latency     time.Duration `json:"latency"`
	Consecutive int          `json:"consecutive_failures"`
}

type HealthStatus string

const (
	HealthStatusHealthy  HealthStatus = "healthy"
	HealthStatusDegraded HealthStatus = "degraded"
	HealthStatusFailed   HealthStatus = "failed"
)

// HybridMetrics tracks hybrid DHT performance and behavior
type HybridMetrics struct {
	mu sync.RWMutex
	
	MockRequests      uint64        `json:"mock_requests"`
	RealRequests      uint64        `json:"real_requests"`
	FallbackEvents    uint64        `json:"fallback_events"`
	RecoveryEvents    uint64        `json:"recovery_events"`
	
	MockLatency       time.Duration `json:"mock_latency_avg"`
	RealLatency       time.Duration `json:"real_latency_avg"`
	
	MockErrorRate     float64       `json:"mock_error_rate"`
	RealErrorRate     float64       `json:"real_error_rate"`
	
	TotalOperations   uint64        `json:"total_operations"`
	LastMetricUpdate  time.Time     `json:"last_update"`
}

// Logger interface for structured logging
type Logger interface {
	Info(msg string, fields ...interface{})
	Warn(msg string, fields ...interface{})
	Error(msg string, fields ...interface{})
	Debug(msg string, fields ...interface{})
}

// NewHybridDHT creates a new hybrid DHT instance
func NewHybridDHT(config *config.HybridConfig, logger Logger) (*HybridDHT, error) {
	hybrid := &HybridDHT{
		config:       config,
		logger:       logger,
		healthStatus: make(map[string]*BackendHealth),
		metrics:      &HybridMetrics{},
	}
	
	// Initialize mock DHT (always available)
	mockDHT := NewMockDHT()
	hybrid.mockDHT = mockDHT
	hybrid.healthStatus["mock"] = &BackendHealth{
		Backend:   "mock",
		Status:    HealthStatusHealthy,
		LastCheck: time.Now(),
	}
	
	// Initialize real DHT if enabled
	if config.IsRealDHTEnabled() {
		realDHT, err := NewRealDHT(config)
		if err != nil {
			logger.Warn("Failed to initialize real DHT, falling back to mock", "error", err)
			hybrid.currentBackend = "mock"
			hybrid.fallbackActive = true
		} else {
			hybrid.realDHT = realDHT
			hybrid.currentBackend = "real"
			hybrid.healthStatus["real"] = &BackendHealth{
				Backend:   "real",
				Status:    HealthStatusHealthy,
				LastCheck: time.Now(),
			}
		}
	} else {
		hybrid.currentBackend = "mock"
	}
	
	// Start health monitoring
	go hybrid.startHealthMonitoring()
	go hybrid.startMetricsCollection()
	
	logger.Info("Hybrid DHT initialized", 
		"backend", hybrid.currentBackend, 
		"fallback_enabled", config.IsFallbackEnabled())
	
	return hybrid, nil
}

// PutValue stores a key-value pair using the current backend
func (h *HybridDHT) PutValue(ctx context.Context, key string, value []byte) error {
	start := time.Now()
	backend := h.getCurrentBackend()
	
	var err error
	switch backend {
	case "mock":
		err = h.mockDHT.PutValue(ctx, key, value)
		h.updateMetrics("mock", start, err)
	case "real":
		err = h.realDHT.PutValue(ctx, key, value)
		h.updateMetrics("real", start, err)
		
		// Handle fallback on error
		if err != nil && h.config.IsFallbackEnabled() {
			h.logger.Warn("Real DHT PutValue failed, trying fallback", "key", key, "error", err)
			h.recordBackendError("real")
			
			// Try mock fallback
			fallbackErr := h.mockDHT.PutValue(ctx, key, value)
			h.updateMetrics("mock", start, fallbackErr)
			
			if fallbackErr == nil {
				h.triggerFallback("real", "mock")
				return nil
			}
			return fmt.Errorf("both real and mock DHT failed: real=%w, mock=%v", err, fallbackErr)
		}
	}
	
	if err != nil {
		h.recordBackendError(backend)
	} else {
		h.recordBackendSuccess(backend)
	}
	
	return err
}

// GetValue retrieves a value by key using the current backend
func (h *HybridDHT) GetValue(ctx context.Context, key string) ([]byte, error) {
	start := time.Now()
	backend := h.getCurrentBackend()
	
	var value []byte
	var err error
	
	switch backend {
	case "mock":
		value, err = h.mockDHT.GetValue(ctx, key)
		h.updateMetrics("mock", start, err)
	case "real":
		value, err = h.realDHT.GetValue(ctx, key)
		h.updateMetrics("real", start, err)
		
		// Handle fallback on error
		if err != nil && h.config.IsFallbackEnabled() {
			h.logger.Warn("Real DHT GetValue failed, trying fallback", "key", key, "error", err)
			h.recordBackendError("real")
			
			// Try mock fallback
			fallbackValue, fallbackErr := h.mockDHT.GetValue(ctx, key)
			h.updateMetrics("mock", start, fallbackErr)
			
			if fallbackErr == nil {
				h.triggerFallback("real", "mock")
				return fallbackValue, nil
			}
			return nil, fmt.Errorf("both real and mock DHT failed: real=%w, mock=%v", err, fallbackErr)
		}
	}
	
	if err != nil {
		h.recordBackendError(backend)
	} else {
		h.recordBackendSuccess(backend)
	}
	
	return value, err
}

// Provide announces that this node provides a value for the given key
func (h *HybridDHT) Provide(ctx context.Context, key, providerId string) error {
	start := time.Now()
	backend := h.getCurrentBackend()
	
	var err error
	switch backend {
	case "mock":
		err = h.mockDHT.Provide(ctx, key, providerId)
		h.updateMetrics("mock", start, err)
	case "real":
		err = h.realDHT.Provide(ctx, key, providerId)
		h.updateMetrics("real", start, err)
		
		// Handle fallback on error
		if err != nil && h.config.IsFallbackEnabled() {
			h.logger.Warn("Real DHT Provide failed, trying fallback", "key", key, "error", err)
			h.recordBackendError("real")
			
			// Try mock fallback
			fallbackErr := h.mockDHT.Provide(ctx, key, providerId)
			h.updateMetrics("mock", start, fallbackErr)
			
			if fallbackErr == nil {
				h.triggerFallback("real", "mock")
				return nil
			}
			return fmt.Errorf("both real and mock DHT failed: real=%w, mock=%v", err, fallbackErr)
		}
	}
	
	if err != nil {
		h.recordBackendError(backend)
	} else {
		h.recordBackendSuccess(backend)
	}
	
	return err
}

// FindProviders finds providers for the given key
func (h *HybridDHT) FindProviders(ctx context.Context, key string) ([]string, error) {
	start := time.Now()
	backend := h.getCurrentBackend()
	
	var providers []string
	var err error
	
	switch backend {
	case "mock":
		providers, err = h.mockDHT.FindProviders(ctx, key)
		h.updateMetrics("mock", start, err)
	case "real":
		providers, err = h.realDHT.FindProviders(ctx, key)
		h.updateMetrics("real", start, err)
		
		// Handle fallback on error
		if err != nil && h.config.IsFallbackEnabled() {
			h.logger.Warn("Real DHT FindProviders failed, trying fallback", "key", key, "error", err)
			h.recordBackendError("real")
			
			// Try mock fallback
			fallbackProviders, fallbackErr := h.mockDHT.FindProviders(ctx, key)
			h.updateMetrics("mock", start, fallbackErr)
			
			if fallbackErr == nil {
				h.triggerFallback("real", "mock")
				return fallbackProviders, nil
			}
			return nil, fmt.Errorf("both real and mock DHT failed: real=%w, mock=%v", err, fallbackErr)
		}
	}
	
	if err != nil {
		h.recordBackendError(backend)
	} else {
		h.recordBackendSuccess(backend)
	}
	
	return providers, err
}

// GetStats returns statistics from the current backend
func (h *HybridDHT) GetStats() DHTStats {
	backend := h.getCurrentBackend()
	
	switch backend {
	case "mock":
		return h.mockDHT.GetStats()
	case "real":
		if h.realDHT != nil {
			return h.realDHT.GetStats()
		}
		fallthrough
	default:
		return h.mockDHT.GetStats()
	}
}

// GetHybridMetrics returns hybrid-specific metrics
func (h *HybridDHT) GetHybridMetrics() *HybridMetrics {
	h.metrics.mu.RLock()
	defer h.metrics.mu.RUnlock()
	
	// Return a copy to avoid concurrent access issues
	metrics := *h.metrics
	return &metrics
}

// GetBackendHealth returns health status for all backends
func (h *HybridDHT) GetBackendHealth() map[string]*BackendHealth {
	h.mu.RLock()
	defer h.mu.RUnlock()
	
	// Return a deep copy
	health := make(map[string]*BackendHealth)
	for k, v := range h.healthStatus {
		healthCopy := *v
		health[k] = &healthCopy
	}
	
	return health
}

// SwitchBackend manually switches to a specific backend
func (h *HybridDHT) SwitchBackend(backend string) error {
	h.mu.Lock()
	defer h.mu.Unlock()
	
	switch backend {
	case "mock":
		if h.mockDHT == nil {
			return fmt.Errorf("mock DHT not available")
		}
		h.currentBackend = "mock"
		h.logger.Info("Manually switched to mock DHT")
		
	case "real":
		if h.realDHT == nil {
			return fmt.Errorf("real DHT not available")
		}
		h.currentBackend = "real"
		h.fallbackActive = false
		h.logger.Info("Manually switched to real DHT")
		
	default:
		return fmt.Errorf("unknown backend: %s", backend)
	}
	
	return nil
}

// Close shuts down the hybrid DHT
func (h *HybridDHT) Close() error {
	h.logger.Info("Shutting down hybrid DHT")
	
	var errors []error
	
	if h.realDHT != nil {
		if closer, ok := h.realDHT.(interface{ Close() error }); ok {
			if err := closer.Close(); err != nil {
				errors = append(errors, fmt.Errorf("real DHT close error: %w", err))
			}
		}
	}
	
	if h.mockDHT != nil {
		if closer, ok := h.mockDHT.(interface{ Close() error }); ok {
			if err := closer.Close(); err != nil {
				errors = append(errors, fmt.Errorf("mock DHT close error: %w", err))
			}
		}
	}
	
	if len(errors) > 0 {
		return fmt.Errorf("errors during close: %v", errors)
	}
	
	return nil
}

// Private methods

func (h *HybridDHT) getCurrentBackend() string {
	h.mu.RLock()
	defer h.mu.RUnlock()
	return h.currentBackend
}

func (h *HybridDHT) triggerFallback(from, to string) {
	h.mu.Lock()
	defer h.mu.Unlock()
	
	if h.currentBackend != to {
		h.currentBackend = to
		h.fallbackActive = true
		
		h.metrics.mu.Lock()
		h.metrics.FallbackEvents++
		h.metrics.mu.Unlock()
		
		h.logger.Warn("Fallback triggered", "from", from, "to", to)
	}
}

func (h *HybridDHT) recordBackendError(backend string) {
	h.mu.Lock()
	defer h.mu.Unlock()
	
	if health, exists := h.healthStatus[backend]; exists {
		health.ErrorCount++
		health.Consecutive++
		health.LastCheck = time.Now()
		
		// Update status based on consecutive failures
		if health.Consecutive >= 3 {
			health.Status = HealthStatusFailed
		} else if health.Consecutive >= 1 {
			health.Status = HealthStatusDegraded
		}
	}
}

func (h *HybridDHT) recordBackendSuccess(backend string) {
	h.mu.Lock()
	defer h.mu.Unlock()
	
	if health, exists := h.healthStatus[backend]; exists {
		health.Consecutive = 0 // Reset consecutive failures
		health.LastCheck = time.Now()
		health.Status = HealthStatusHealthy
		
		// Trigger recovery if we were in fallback mode
		if h.fallbackActive && backend == "real" && h.config.IsRealDHTEnabled() {
			h.currentBackend = "real"
			h.fallbackActive = false
			
			h.metrics.mu.Lock()
			h.metrics.RecoveryEvents++
			h.metrics.mu.Unlock()
			
			h.logger.Info("Recovery triggered, switched back to real DHT")
		}
	}
}

func (h *HybridDHT) updateMetrics(backend string, start time.Time, err error) {
	h.metrics.mu.Lock()
	defer h.metrics.mu.Unlock()
	
	latency := time.Since(start)
	h.metrics.TotalOperations++
	h.metrics.LastMetricUpdate = time.Now()
	
	switch backend {
	case "mock":
		h.metrics.MockRequests++
		h.metrics.MockLatency = h.updateAverageLatency(h.metrics.MockLatency, latency, h.metrics.MockRequests)
		if err != nil {
			h.metrics.MockErrorRate = h.updateErrorRate(h.metrics.MockErrorRate, true, h.metrics.MockRequests)
		} else {
			h.metrics.MockErrorRate = h.updateErrorRate(h.metrics.MockErrorRate, false, h.metrics.MockRequests)
		}
		
	case "real":
		h.metrics.RealRequests++
		h.metrics.RealLatency = h.updateAverageLatency(h.metrics.RealLatency, latency, h.metrics.RealRequests)
		if err != nil {
			h.metrics.RealErrorRate = h.updateErrorRate(h.metrics.RealErrorRate, true, h.metrics.RealRequests)
		} else {
			h.metrics.RealErrorRate = h.updateErrorRate(h.metrics.RealErrorRate, false, h.metrics.RealRequests)
		}
	}
}

func (h *HybridDHT) updateAverageLatency(currentAvg, newLatency time.Duration, count uint64) time.Duration {
	if count <= 1 {
		return newLatency
	}
	
	// Exponential moving average with weight based on count
	weight := 1.0 / float64(count)
	return time.Duration(float64(currentAvg)*(1-weight) + float64(newLatency)*weight)
}

func (h *HybridDHT) updateErrorRate(currentRate float64, isError bool, count uint64) float64 {
	if count <= 1 {
		if isError {
			return 1.0
		}
		return 0.0
	}
	
	// Exponential moving average for error rate
	weight := 1.0 / float64(count)
	errorValue := 0.0
	if isError {
		errorValue = 1.0
	}
	
	return currentRate*(1-weight) + errorValue*weight
}

func (h *HybridDHT) startHealthMonitoring() {
	ticker := time.NewTicker(h.config.DHT.HealthCheckInterval)
	defer ticker.Stop()
	
	for range ticker.C {
		h.performHealthChecks()
	}
}

func (h *HybridDHT) startMetricsCollection() {
	ticker := time.NewTicker(h.config.Monitoring.MetricsInterval)
	defer ticker.Stop()
	
	for range ticker.C {
		h.collectAndLogMetrics()
	}
}

func (h *HybridDHT) performHealthChecks() {
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	
	// Health check for real DHT
	if h.realDHT != nil {
		start := time.Now()
		_, err := h.realDHT.GetValue(ctx, "health-check-key")
		
		h.mu.Lock()
		if health, exists := h.healthStatus["real"]; exists {
			health.LastCheck = time.Now()
			health.Latency = time.Since(start)
			
			if err != nil {
				health.ErrorCount++
				health.Consecutive++
				if health.Consecutive >= 3 {
					health.Status = HealthStatusFailed
				} else {
					health.Status = HealthStatusDegraded
				}
			} else {
				health.Consecutive = 0
				health.Status = HealthStatusHealthy
			}
		}
		h.mu.Unlock()
	}
	
	// Health check for mock DHT (should always be healthy)
	h.mu.Lock()
	if health, exists := h.healthStatus["mock"]; exists {
		health.LastCheck = time.Now()
		health.Status = HealthStatusHealthy
		health.Latency = 1 * time.Millisecond // Mock is always fast
	}
	h.mu.Unlock()
}

func (h *HybridDHT) collectAndLogMetrics() {
	metrics := h.GetHybridMetrics()
	health := h.GetBackendHealth()
	
	h.logger.Info("Hybrid DHT metrics",
		"current_backend", h.getCurrentBackend(),
		"fallback_active", h.fallbackActive,
		"mock_requests", metrics.MockRequests,
		"real_requests", metrics.RealRequests,
		"fallback_events", metrics.FallbackEvents,
		"recovery_events", metrics.RecoveryEvents,
		"mock_latency_ms", metrics.MockLatency.Milliseconds(),
		"real_latency_ms", metrics.RealLatency.Milliseconds(),
		"mock_error_rate", metrics.MockErrorRate,
		"real_error_rate", metrics.RealErrorRate,
		"total_operations", metrics.TotalOperations)
	
	// Log health status
	for backend, healthStatus := range health {
		h.logger.Debug("Backend health",
			"backend", backend,
			"status", healthStatus.Status,
			"error_count", healthStatus.ErrorCount,
			"consecutive_failures", healthStatus.Consecutive,
			"latency_ms", healthStatus.Latency.Milliseconds())
	}
}