CDN 缓存控制标头

Welcome to CDN Cache Control Headers! HTTP headers are like instructions attached to web content. They tell browsers and Content Delivery Networks (CDNs) exactly how to handle caching.

Understanding these headers is crucial for optimizing your CDN's performance, ensuring content freshness, and managing user privacy.

The Cache-Control header is the most powerful and flexible HTTP header for dictating caching behavior. It allows you to specify directives that apply to both private caches (like a user's browser) and shared caches (like CDNs).

It gives you fine-grained control over how long content should be stored and under what conditions.

One of the most common Cache-Control directives is max-age. It tells the cache (CDN or browser) how long, in seconds, the resource is considered fresh and can be served without revalidating with the origin server.

For example, max-age=3600 means the content is fresh for one hour.

Here are other key Cache-Control directives:

• public: The response can be cached by any cache, including shared CDNs.
• private: The response is intended for a single user and can only be cached by private (browser) caches. CDNs should not cache it.
• no-cache: The cache must revalidate with the origin server before serving the content, even if it appears fresh.
• no-store: The cache must not store any part of the client request or server response. This is for highly sensitive data.

The Expires header specifies an absolute date and time after which the response is considered stale. It's an older header, largely superseded by Cache-Control: max-age.

If both Cache-Control and Expires are present, Cache-Control usually takes precedence. CDNs will respect Expires if no Cache-Control header is set.

An ETag (Entity Tag) is a unique identifier for a specific version of a resource. It's like a fingerprint or a hash of the content.

CDNs use ETag for efficient revalidation. When a cached resource becomes stale, the CDN sends an If-None-Match header with the stored ETag to the origin server. If the resource hasn't changed, the origin responds with a lightweight 304 Not Modified.

Similar to ETag, the Last-Modified header provides a timestamp indicating when the resource was last changed on the origin server.

CDNs can use this for revalidation by sending an If-Modified-Since header with the stored timestamp. If the resource hasn't been modified since that time, the origin responds with 304 Not Modified.

When a CDN receives a request for content it has cached, it first checks if the content is still fresh based on Cache-Control: max-age or Expires.

If the content is stale, or if no-cache is specified, the CDN will contact the origin server. It sends a conditional request using If-None-Match (with ETag) or If-Modified-Since (with Last-Modified). This prevents re-downloading the entire content if it hasn't changed, saving bandwidth and speeding up delivery.

Which of these Cache-Control directives prevent a CDN from storing a response, or require revalidation with the origin server before serving?

In this lesson, we explored how HTTP cache control headers guide CDNs and browsers. You learned about:

• The powerful Cache-Control header and its directives like max-age, public, private, no-cache, and no-store.
• The older Expires header for absolute expiry dates.
• ETag and Last-Modified for efficient content revalidation.

Mastering these headers is key to fine-tuning your CDN for optimal performance, ensuring content freshness, and handling sensitive data securely.