Browser HTML cache

Last Update:2014-08-05 Source: Internet

Author: User

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The browser cache saves the file on the client, and a good caching strategy can reduce the network bandwidth footprint, improve access speed, improve the user experience, and reduce the burden on the Server. therefore, it is necessary to understand the principle of its implementation, to improve the performance of the Site. When a client requests a Web server, the content of the request can be obtained from the following Places: server, browser cache, or cache server. This depends on the page information of the Server-side Output. The paging file has three cache States. 1. Latest: choose not to cache the page and get the latest content from the server on each Request. 2. Not expired: cache in a given time, if the user refresh or page expires to the server request, otherwise the local cache will be read, which can improve the browsing Speed. 3. expired: that is, stale pages, when requesting this page, must be Re-acquired. The state of the Page's cache is determined by the HTTP header, a browser request information, and a server response Message. Mainly include pragma:no-cache, cache-control, Expires, last-modified, if-modified-since. Among them Pragma:no-cache by http/1.0 stipulation, Cache-control by http/1.1 Stipulation. Main parameters of Cache-control The Cache-control:private/public public response is cached and shared among multiple Users. Private responses can only be used as private caches and can no longer be shared among Users. Cache-control:no-cache: do not cache Cache-control:max-age=x: cache time in seconds Cache-control:must-revalidate: If the page is out of date, go to the server to get it. Expires: Display the Settings page expiration time Last-modified: The last modification time of the request object is used to determine whether the cache expiration is usually generated by the File's time information If-modified-since: the information supplied with the client sending request refers to the last modification date of the browser cache request object For comparison with Server-side last-modified In the case of ie, there are four ways () of "every visit page check", The user uses reload or exceeds the expiration date, The browser will think this page is stale (it will send a if-modified-since information. if the page does not change, The server side responds with a 304 status not Modified and does not send the entire page, so it will be fast, but the server must generate a valid last-modified headers and the server time must be VALID. A server-side response that does not cache http/1.0 OK Content-type:text/html content-length:19662 Pragma:no-cache Cache-control:no-cache server:roxen/2.1.185 Accept-ranges:bytes expires:wed, Jan 2001 00:18:55 GMT Sometimes just setting Pragma:no-cache Cache-control:no-cache or not, it is necessary to set the expiration time to the past time to ensure that the object is not Cached. A server-side response that allows caching http/1.1 OK date:tue, 2001 14:50:31 GMT server:apache/1.3.12 cache-control:max-age=43200 expires:wed, 2001 02:50:31 GMT last-modified:sun, Geneva 23:52:56 GMT ETag: "1cbf3-dfd-3a2adcd8" Accept-ranges:bytes content-length:3581 Connection:close Content-type:text/html CACHE-CONTROL:MAX-AGE=43200 means cache for 12 hours Let's look at a specific example of a browser cache Request a file for the first time Request: Get/file.html http/1.1 accept:image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-powerpoint, application/vnd.ms-excel , application/msword, application/x-comet, */* Accept-language:en-us accept-encoding:gzip, deflate user-agent:mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host:24.5.203.101 Connection:keep-alive Response: http/1.1 OK date:tue, 2001 20:00:22 GMT Server:apache cache-control:max-age=604800 last-modified:wed, 15:28:38 GMT ETag: "1df-28f1-3a2520a6" Accept-ranges:bytes content-length:10481 keep-alive:timeout=5, max=100 Connection:keep-alive Content-type:text/html This returns the Last-modified and etag, both of which are used to compare the current browser cache file with the Server-side file, and if it is not always up-to-date, the local cache is Read. Second request Request: Get/file.html http/1.1 Accept: */* Accept-language:en-us accept-encoding:gzip, deflate if-modified-since:wed, 15:28:38 GMT If-none-match: "1df-28f1-3a2520a6" user-agent:mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host:24.5.203.101 Connection:keep-alive Response: http/1.1 304 Not Modified date:tue, 2001 20:01:07 GMT Server:apache Connection:keep-alive keep-alive:timeout=5, max=100 ETag: "1df-28f1-3a2520a6" cache-control:max-age=604800 The If-none-match information in the request is the etag of the first response to verify that the etag is consistent with the current Response. The server returns not Modified, and the browser reads the local cache. We can also use dedicated cache servers to improve Performance. His principle is the same as the browser caching principle, all browser requests will be responded by the cache server, the cache server can use its own cache files or obtain new files in response to user Requests. therefore, with the cache server, it will greatly improve the performance of the Site.

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