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Webkit's Research on Application Cache Implementation of HTML 5_html/css_WEB-ITnose

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2016-06-24 11:49:071013browse

Due to work, I have been studying ApplicationCache for a while. First, let’s introduce some basic concepts of SQL DB Table related to it,

  • CacheGroups: It is a set of resources, such as html, css, js, image, they are related to a cache manifest Association, each cache item contained in the CacheGroups table has its own ID and corresponding manifest URL.
  • CacheEntries, CacheResources: These two tables store some resource metadata (metadta), such as HTTP header, mime type, etc.
  • CacheResourcesData: This table stores each resource in BLOB format.
  • Caches : This table stores the size of each resource.
    • In the Webkit/Source/WebCore/Loader/AppCache/ directory, there is its implementation: ApplicationCacheStorageXXX. When we compile Webkit, turn on the Offline Cache option, it will enable the HTML5 App Cache function and generate a SQL DB file named ApplicationCache.db in the cache directory specified by the user, so that when we put the manifest file on the Web side, it will be used To describe which files need to be cached, and when those files do not need to be cached, Webkit's Loader will decide whether to store the resource in this file based on the manifest description, and whether to load resources such as images from this DB, so as to avoid requests from the Network. , which can greatly save costs.


    For more introduction, please refer to the article from w3school.com.cn:

    Using HTML5, through Create a cache manifest file to easily create an offline version of your web application.

    What is Application Cache?

    HTML5 introduces application caching, which means web applications can be cached and accessed without an Internet connection.

    Application caching brings three benefits to apps:

  • Offline browsing - users can use apps while they are offline
  • Speed ​​- cached resources load faster
  • Reduce server load - the browser will only download updated or changed resources from the server.

    • Browser Support

    Application caching is supported by all major browsers except Internet Explorer.

    HTML5 Cache Manifest Example

    The following example shows an HTML document with cache manifest (for offline browsing):

    Example

    <!DOCTYPE HTML><html manifest="demo.appcache"><body>The content of the document......</body></html>

    Try it yourself

    Cache Manifest Basics

    To enable application caching, include the manifest in the document's tag Attributes: 

    <!DOCTYPE HTML><html manifest="demo.appcache">...</html>

    Each page with a specified manifest will be cached when the user accesses it. If the manifest attribute is not specified, the page will not be cached (unless it is specified directly in the manifest file).

    The recommended file extension for manifest files is: ".appcache".

    Please note that the manifest file needs to be configured with the correct MIME-type, i.e. "text/cache-manifest". Must be configured on the web server.

    Manifest file

    Manifest files are simple text files that tell the browser what is cached (and what is not cached).

    The manifest file can be divided into three parts:

  • CACHE MANIFEST - files listed under this heading will be cached after the first download
  • NETWORK - under this heading Outgoing files require a connection to the server and will not be cached
  • FALLBACK - The files listed under this heading specify the fallback page when the page is inaccessible (such as a 404 page)

    • CACHE MANIFEST

    The first line, CACHE MANIFEST, is required: 

    CACHE MANIFEST/theme.css/logo.gif/main.js

    The manifest file above lists three resources: a CSS file, a GIF image, and a JavaScript document. When the manifest file loads, the browser downloads these three files from the root directory of the website. Then, whenever the user disconnects from the Internet, these resources are still available.

    NETWORK

    The following NETWORK section specifies that the file "login.asp" will never be cached and is not available offline: 

    NETWORK:login.asp

    You can use Star number to indicate that all other resources/files require an Internet connection: 

    NETWORK:*

    FALLBACK

    The FALLBACK subsection below specifies that if an Internet connection cannot be established, "offline.html" is used instead of /html5 / All files in the directory: 

    FALLBACK:/html5/ /404.html

    Note: The first URI is the resource, the second is the fallback.

    Update cache

    Once an app is cached, it remains cached until:

  • 用户清空浏览器缓存
  • manifest 文件被修改（参阅下面的提示）
  • 由程序来更新应用缓存

    • 实例 - 完整的 Manifest 文件

    CACHE MANIFEST# 2012-02-21 v1.0.0/theme.css/logo.gif/main.jsNETWORK:login.aspFALLBACK:/html5/ /404.html

    重要的提示：以 "#" 开头的是注释行，但也可满足其他用途。应用的缓存会在其 manifest 文件更改时被更新。如果您编辑了一幅图片，或者修改了一个 JavaScript 函数，这些改变都不会被重新缓存。更新注释行中的日期和版本号是一种使浏览器重新缓存文件的办法。

    关于应用程序缓存的注释

    请留心缓存的内容。

    一旦文件被缓存，则浏览器会继续展示已缓存的版本，即使您修改了服务器上的文件。为了确保浏览器更新缓存，您需要更新 manifest 文件。

    注释：浏览器对缓存数据的容量限制可能不太一样（某些浏览器设置的限制是每个站点 5MB）。


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