atlas/internal/httpapp/cache_middleware.go

package httpapp

import (
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"sync"
	"time"
)

// CacheEntry represents a cached response
type CacheEntry struct {
	Body       []byte
	Headers    map[string]string
	StatusCode int
	ExpiresAt  time.Time
	ETag       string
}

// ResponseCache provides HTTP response caching
type ResponseCache struct {
	mu    sync.RWMutex
	cache map[string]*CacheEntry
	ttl   time.Duration
}

// NewResponseCache creates a new response cache
func NewResponseCache(ttl time.Duration) *ResponseCache {
	c := &ResponseCache{
		cache: make(map[string]*CacheEntry),
		ttl:   ttl,
	}

	// Start cleanup goroutine
	go c.cleanup()

	return c
}

// cleanup periodically removes expired entries
func (c *ResponseCache) cleanup() {
	ticker := time.NewTicker(1 * time.Minute)
	defer ticker.Stop()

	for range ticker.C {
		c.mu.Lock()
		now := time.Now()
		for key, entry := range c.cache {
			if now.After(entry.ExpiresAt) {
				delete(c.cache, key)
			}
		}
		c.mu.Unlock()
	}
}

// Get retrieves a cached entry
func (c *ResponseCache) Get(key string) (*CacheEntry, bool) {
	c.mu.RLock()
	defer c.mu.RUnlock()

	entry, exists := c.cache[key]
	if !exists {
		return nil, false
	}

	if time.Now().After(entry.ExpiresAt) {
		return nil, false
	}

	return entry, true
}

// Set stores a cached entry
func (c *ResponseCache) Set(key string, entry *CacheEntry) {
	c.mu.Lock()
	defer c.mu.Unlock()

	entry.ExpiresAt = time.Now().Add(c.ttl)
	c.cache[key] = entry
}

// Invalidate removes a cached entry
func (c *ResponseCache) Invalidate(key string) {
	c.mu.Lock()
	defer c.mu.Unlock()
	delete(c.cache, key)
}

// InvalidatePattern removes entries matching a pattern
func (c *ResponseCache) InvalidatePattern(pattern string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	for key := range c.cache {
		if containsPattern(key, pattern) {
			delete(c.cache, key)
		}
	}
}

// containsPattern checks if a string contains a pattern (simple prefix/suffix matching)
func containsPattern(s, pattern string) bool {
	// Simple pattern matching - can be enhanced
	return len(s) >= len(pattern) && (s[:len(pattern)] == pattern || s[len(s)-len(pattern):] == pattern)
}

// generateCacheKey creates a cache key from request
func generateCacheKey(r *http.Request) string {
	// Include method, path, and query string
	key := r.Method + ":" + r.URL.Path
	if r.URL.RawQuery != "" {
		key += "?" + r.URL.RawQuery
	}

	// Hash the key for consistent length
	hash := sha256.Sum256([]byte(key))
	return hex.EncodeToString(hash[:])
}

// generateETag generates an ETag from content
func generateETag(content []byte) string {
	hash := sha256.Sum256(content)
	return `"` + hex.EncodeToString(hash[:16]) + `"`
}

// cacheMiddleware provides response caching for GET requests
func (a *App) cacheMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Only cache GET requests
		if r.Method != http.MethodGet {
			next.ServeHTTP(w, r)
			return
		}

		// Skip caching for authenticated endpoints that may have user-specific data
		if !a.isPublicEndpoint(r.URL.Path, r.Method) {
			// Check if user is authenticated - if so, skip caching
			// In production, you might want per-user caching by including user ID in cache key
			if _, ok := getUserFromContext(r); ok {
				next.ServeHTTP(w, r)
				return
			}
		}

		// Skip caching for certain endpoints
		if a.shouldSkipCache(r.URL.Path) {
			next.ServeHTTP(w, r)
			return
		}

		// Check cache
		cacheKey := generateCacheKey(r)
				if entry, found := a.cache.Get(cacheKey); found {
			// Check If-None-Match header for ETag validation
			ifNoneMatch := r.Header.Get("If-None-Match")
			if ifNoneMatch == entry.ETag {
				w.WriteHeader(http.StatusNotModified)
				return
			}

			// Serve from cache
			for k, v := range entry.Headers {
				w.Header().Set(k, v)
			}
			w.Header().Set("ETag", entry.ETag)
			w.Header().Set("X-Cache", "HIT")
			w.WriteHeader(entry.StatusCode)
			w.Write(entry.Body)
			return
		}

		// Create response writer to capture response
		rw := &cacheResponseWriter{
			ResponseWriter: w,
			statusCode:     http.StatusOK,
			body:           make([]byte, 0),
		}

		next.ServeHTTP(rw, r)

		// Only cache successful responses
		if rw.statusCode >= 200 && rw.statusCode < 300 {
			// Generate ETag
			etag := generateETag(rw.body)

			// Store in cache
			headers := make(map[string]string)
			for k, v := range rw.Header() {
				if len(v) > 0 {
					headers[k] = v[0]
				}
			}

			entry := &CacheEntry{
				Body:       rw.body,
				Headers:    headers,
				StatusCode: rw.statusCode,
				ETag:       etag,
			}

						a.cache.Set(cacheKey, entry)

			// Add cache headers
			w.Header().Set("ETag", etag)
			w.Header().Set("X-Cache", "MISS")
		}
	})
}

// cacheResponseWriter captures response for caching
type cacheResponseWriter struct {
	http.ResponseWriter
	statusCode int
	body       []byte
}

func (rw *cacheResponseWriter) WriteHeader(code int) {
	rw.statusCode = code
	rw.ResponseWriter.WriteHeader(code)
}

func (rw *cacheResponseWriter) Write(b []byte) (int, error) {
	rw.body = append(rw.body, b...)
	return rw.ResponseWriter.Write(b)
}

// shouldSkipCache determines if a path should skip caching
func (a *App) shouldSkipCache(path string) bool {
	// Skip caching for dynamic endpoints
	skipPaths := []string{
		"/metrics",
		"/healthz",
		"/health",
		"/api/v1/system/info",
		"/api/v1/system/logs",
		"/api/v1/dashboard",
	}

	for _, skipPath := range skipPaths {
		if path == skipPath {
			return true
		}
	}

	return false
}