What is Application Cache or app cache in HTML5

Answer

HTML5 introduces application cache, which means that a web application is cached, and accessible without an internet connection.

Application cache gives an application three advantages:

Offline browsing - users can use the application when they're offline
Speed - cached resources load faster
Reduced server load - the browser will only download updated/changed resources from the server

ache Manifest Basics

To enable application cache, include the manifest attribute in the document's <html> tag:

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

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

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

A manifest file needs to be served with the correct media type, which is "text/cache-manifest". Must be configured on the web server.

The Manifest File

The manifest file is a simple text file, which tells the browser what to cache (and what to never cache).

The manifest file has three sections:

CACHE MANIFEST - Files listed under this header will be cached after they are downloaded for the first time
NETWORK - Files listed under this header require a connection to the server, and will never be cached
FALLBACK - Files listed under this header specifies fallback pages if a page is inaccessible

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 file. When the manifest file is loaded, the browser will download the three files from the root directory of the web site. Then, whenever the user is not connected to the internet, the resources will still be available.

NETWORK

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

NETWORK:
login.asp

An asterisk can be used to indicate that all other resources/files require an internet connection:

NETWORK:
*

FALLBACK

The FALLBACK section below specifies that "offline.html" will be served in place of all files in the /html/ catalog, in case an internet connection cannot be established:

FALLBACK:
/html/ /offline.html

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

Updating the Cache

Once an application is cached, it remains cached until one of the following happens:

The user clears the browser's cache
The manifest file is modified (see tip below)
The application cache is programmatically updated

Example - Complete Cache Manifest File

CACHE MANIFEST
# 2012-02-21 v1.0.0
/theme.css
/logo.gif
/main.js

NETWORK:
login.asp

FALLBACK:
/html/ /offline.html

Tip: Lines starting with a "#" are comment lines, but can also serve another purpose. An application's cache is only updated when its manifest file changes. If you edit an image or change a JavaScript function, those changes will not be re-cached. Updating the date and version in a comment line is one way to make the browser re-cache your files.

Notes on Application Cache

Be careful with what you cache.

Once a file is cached, the browser will continue to show the cached version, even if you change the file on the server. To ensure the browser updates the cache, you need to change the manifest file.

Note: Browsers may have different size limits for cached data (some browsers have a 5MB limit per site).

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