How do you implement HTTP response caching in a Go service?

HTTP caching reduces load and improves response times. Go services implement caching at multiple levels: HTTP Cache-Control headers (browser/CDN caching), application-level caching (Redis/in-memory), and conditional requests (ETag/Last-Modified).

// HTTP Cache-Control headers
func publicDataHandler(w http.ResponseWriter, r *http.Request) {
    // Cache in browser and CDN for 60s, stale for 10s
    w.Header().Set("Cache-Control", "public, max-age=60, stale-while-revalidate=10")
    w.Header().Set("Vary", "Accept-Encoding") // vary by encoding
    // ... serve data
}

func privateDataHandler(w http.ResponseWriter, r *http.Request) {
    // No CDN caching — only browser may cache, private to user
    w.Header().Set("Cache-Control", "private, max-age=30")
    // ... serve user-specific data
}

// ETag-based conditional caching
func userHandler(w http.ResponseWriter, r *http.Request) {
    user, err := svc.GetUser(r.Context(), r.PathValue("id"))
    if err != nil { /* handle error */ return }

    // Compute ETag (hash of the content)
    data, _ := json.Marshal(user)
    etag := fmt.Sprintf(`"%x"`, md5.Sum(data))

    w.Header().Set("ETag", etag)
    w.Header().Set("Cache-Control", "private, must-revalidate")

    // Check if client has current version
    if r.Header.Get("If-None-Match") == etag {
        w.WriteHeader(http.StatusNotModified) // 304: send no body
        return
    }

    w.Header().Set("Content-Type", "application/json")
    w.Write(data)
}

// In-memory LRU cache middleware
func cacheMiddleware(cache *lru.Cache, ttl time.Duration) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            if r.Method != http.MethodGet { next.ServeHTTP(w, r); return }
            key := r.URL.RequestURI()
            if val, ok := cache.Get(key); ok {
                w.Header().Set("X-Cache", "HIT")
                w.Write(val.([]byte)); return
            }
            crw := &capturingResponseWriter{ResponseWriter: w}
            next.ServeHTTP(crw, r)
            cache.Add(key, crw.body)
        })
    }
}

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