Key In Http Protocol (一)：'Cache-Control'

Brief

HTTP (Hypertext Transfer Protocol) is perhaps the most popular application protocol used in the Internet (or The WEB).

HTTP is an asymmetric request-response client-server protocol as illustrated. An HTTP client sends a request message to an HTTP server. The server, in turn, returns a response message.

In other words, HTTP is a pull protocol, the client pulls information from the server (instead of server pushes information down to the client).

HTTP is a client-server application-level protocol.

① http response message

② http request message

‘Cache-Control’ Key

Cache-Control header was defined as part of the HTTP/1.1 specification and supersedes previous headers (e.g. Expires) used to define response caching policies. All modern browsers support Cache-Control, hence that is all we will need.

Each resource can define its caching policy via Cache-Control HTTP header Cache-Control directives control who can cache the response, under which conditions, and for how long.

Defining optimal Cache-Control policy

浏览器发出的所有 HTTP 请求会首先被路由到浏览器的缓存，以查看是否缓存了可以用于实现请求的有效响应。如果有匹配的响应，会直接从缓存中读取响应，这样就避免了网络延迟以及传输产生的数据成本。然而，如果我们希望更新或废弃已缓存的响应，该怎么办？

例如，假设我们已经告诉访问者某个 CSS 样式表缓存长达 24 小时 (max-age=86400)，但是设计人员刚刚提交了一个更新，我们希望所有用户都能使用。我们该如何通知所有访问者缓存的 CSS 副本已过时，需要更新缓存？ 很简单，在资源内容更改时，我们可以更改资源的网址，强制用户下载新响应。通常情况下，可以通过在文件名中嵌入文件的指纹码（或版本号）来实现 - 例如 style_x234dff.css。 图片：

上述方案的具体实践，可参考百度的FIS3。

Caching checklist:

1. Use consistent URLs
2. Ensure the server provides a validation token (ETag)
3. Identify which resources can be cached by intermediaries
4. Determine the optimal cache lifetime for each resource
5. Determine the best cache hierarchy for your site
6. Minimize churn

附录

• Google HTTP Caching
• Baidu FIS3