前端緩存策略：從瀏覽器到CDN

有效的前端緩存策略需分層實施：1. 瀏覽器緩存通過Cache-Control等HTTP頭控制靜態資源本地存儲，對帶哈希的文件設置長max-age（如一年）並使用immutable，HTML文件則用no-cache確保重驗證；2. Service Worker實現高級緩存邏輯，如緩存優先、陳舊更新或網絡優先策略，適用於PWA和第三方資源，但需謹慎選擇緩存內容避免存儲浪費；3. CDN緩存通過全球邊緣節點加速內容分發，依賴正確的Cache-Control頭、版本化URL及必要時的主動清除API來管理緩存有效性；4. 緩存失效是關鍵挑戰，推薦使用版本化URL作為最安全方式，結合CDN的標籤清除功能和動態內容的短TTL，避免僅依賴時間過期。整體策略需HTML不緩存、靜態資源長期緩存、API響應條件緩存，並通過服務工作線程支持離線訪問，同時確保構建流程、部署管道與緩存配置協同自動化，最終實現快速可靠的用戶體驗。

When we talk about front-end performance, caching is one of the most powerful tools in our arsenal. A well-designed caching strategy can drastically reduce load times, decrease server load, and improve user experience—especially for repeat visitors. But effective caching isn't just about setting a header and calling it a day. It spans multiple layers: from the browser all the way to the CDN. Let's break down the key strategies at each level.

1. Browser Caching: Control How Assets Are Stored Locally

Browser caching allows users to store static assets (like CSS, JS, images) locally, so they don't need to be re-downloaded on every visit. This is controlled primarily through HTTP headers.

Key Headers:

• Cache-Control : The main directive for modern caching behavior.
  • max-age : How long (in seconds) the browser should use the cached version.
  • public / private : Indicates whether the response can be stored in shared caches (eg, CDN) or only in the user's browser.
  • no-cache or no-store : Forces revalidation or prevents caching entirely (use sparingly).
• ETag or Last-Modified : Used for conditional requests. If the file hasn't changed, the server responds with 304 (Not Modified), saving bandwidth.

Best Practices:

• Use long max-age (eg, max-age=31536000 ) for versioned static assets (eg, app.a1b2c3d.js ).
• Avoid caching HTML files directly—use no-cache so the browser revalidates on each visit.
• Use cache-busting techniques: include hashes in filenames (eg, via Webpack or Vite) so updated files get new URLs.

Example:
Cache-Control: public, max-age=31536000, immutable
This tells the browser and intermediaries to cache the asset for a year and never revalidate—safe for hashed files.

2. Service Worker Caching: Advanced Control on the Client Side

Service workers let you intercept network requests and implement custom caching logic—going beyond what HTTP headers can do.

Common Strategies:

• Cache First, Network Fallback : Great for static assets. Serve from cache if available; if not, fetch from network.
• Stale-While-Revalidate : Return cached version immediately, then update it in the background.
• Network First, Cache Fallback : Useful for dynamic content. Try network first, fall back to cache if offline.

Use Cases:

• Progressive Web Apps (PWAs) that work offline.
• Caching third-party assets (eg, fonts, APIs) that aren't under your CDN control.
• Fine-grained control over expiration and updates.

Tip: Don't cache everything. Be selective—large media files or infrequently used routes can waste storage.

3. CDN Caching: Speed Up Delivery Globally

A Content Delivery Network (CDN) caches your content on servers distributed around the world. When a user requests a file, they get it from the nearest edge location.

How It Works:

• First user requests style.css → CDN fetches from origin, caches it, and serves it.
• Subsequent users in the same region get it from the edge cache—much faster.

Optimization Tips:

• Set appropriate Cache-Control headers from your origin server—CDNs respect these by default.
• Use versioned URLs (eg, /v1.2.0/app.js ) to avoid cache invalidation issues.
• Leverage CDN purge/invalidate APIs when you need to force an update.
• Cache at multiple levels: static assets at the edge, dynamic content with short TTLs or not at all.

Advanced: Some CDNs support cache key customization (eg, ignore certain query params) and origin shielding to reduce origin load.

4. Cache Invalidation: The Hardest Part

No caching strategy is complete without a solid invalidation plan. Caching is easy—knowing when to stop caching is hard.

Strategies:

• Versioned URLs : The safest method. Change the filename or path when content changes.
• Cache tags or purge APIs : Some CDNs (eg, Cloudflare, Fastly) let you tag assets and purge all with a given tag (eg, "homepage").
• Short TTLs for dynamic content : Accept that some content will be slightly stale, but refresh frequently.

Never rely on time-based expiration alone for critical updates. Combine with versioning.

Putting It All Together

A robust front-end caching strategy layers these approaches:

• HTML : No caching or short TTL revalidation (via ETag ).
• JS/CSS/Images with hashes : Long-term browser and CDN caching ( max-age=1yr , immutable ).
• API responses : Conditional caching with ETag or short TTLs.
• Offline support : Service worker for core app shell and critical assets.

The key is alignment: your build process, headers, CDN config, and deployment pipeline must all work together. Automate cache-busting in your CI/CD, and monitor cache hit ratios on your CDN.

Caching isn't one-size-fits-all, but with the right mix of browser policies, service workers, and CDN intelligence, you can deliver fast, reliable experiences—every time.

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