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Introduction to using the application cache

This article is Ready to Use.


By Eric Bidelman
Originally published May 27, 2011

Summary

An introduction to using the appcache interface for offline application functionality.

Introduction

It's becoming increasingly important for web-based applications to be accessible offline. Yes, all browsers have caching mechanisms, but they're unreliable and don't always work as you might expect. HTML5 addresses some of the annoyances of being offline with the ApplicationCache interface.

Using the cache interface gives your application three advantages:

  1. Offline browsing - users can navigate your full site when they're offline
  2. Speed - cached resources are local, and therefore load faster.
  3. Reduced server load - the browser will only download resources from the server that have changed.

The Application Cache (or AppCache) allows a developer to specify which files the browser should cache and make available to offline users. Your app will load and work correctly, even if the user presses the refresh button while they're offline.

The cache manifest file

The cache manifest file is a simple text file that lists the resources the browser should cache for offline access.

Referencing a manifest file

To enable the application cache for an app, include the manifest attribute on the document's html tag:

<html manifest="example.appcache">
  ...
</html>

The manifest attribute should be included on every page of your web application that you want cached. The browser does not cache a page if it does not contain the manifest attribute (unless it is explicitly listed in the manifest file itself. This means that any page the user navigates to that include a manifest will be implicitly added to the application cache. Thus, there's no need to list every page in your manifest.

The manifest attribute can point to an absolute URL or relative path, but an absolute URL must be under the same origin as the web application. A manifest file can have any file extension, but needs to be served with the correct mime-type (see below).

<html manifest="http://www.example.com/example.mf">
  ...
</html>

A manifest file must be served with the mime-type text/cache-manifest. You may need to add a custom file type to your web server or .htaccess configuration.

For example, to serve this mime-type in Apache, add this line to your config file:

AddType text/cache-manifest .appcache

Or, in your app.yaml file in Google App Engine:

- url: /mystaticdir/(.*\.appcache)
  static_files: mystaticdir/\1
  mime_type: text/cache-manifest
  upload: mystaticdir/(.*\.appcache)

Structure of a manifest file

A simple manifest may look something like this:

CACHE MANIFEST
index.html
stylesheet.css
images/logo.png
scripts/main.js

This example will cache four files on the page that specifies this manifest file.

There are a couple of things to note:

  • The CACHE MANIFEST string is the first line and is required.
  • Some browsers place restrictions on the amount of storage quota available to your app. In Chrome for example, AppCache uses a shared pool of TEMPORARY storage that other offline APIs can share. If you are writing an app for the Chrome Web Store, using the unlimitedStorage removes that restriction.
  • If the manifest file or a resource specified in it fails to download, the entire cache update process fails. The browser will keep using the old application cache in the event of failure.

Lets take a look at a more complex example:

 CACHE MANIFEST
 # 2010-06-18:v2
 
 # Explicitly cached 'master entries'.
 CACHE:
 /favicon.ico
 index.html
 stylesheet.css
 images/logo.png
 scripts/main.js
 
 # Resources that require the user to be online.
 NETWORK:
 login.php
 /myapi
 http://api.twitter.com
 
 # static.html will be served if main.py is inaccessible
 # offline.jpg will be served in place of all images in images/large/
 # offline.html will be served in place of all other .html files
 FALLBACK:
 /main.py /static.html
 images/large/ images/offline.jpg
 *.html /offline.html
 

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. So for example, if you edit an image resource or change a javascript function, those changes will not be re-cached. You must modify the manifest file itself to inform the browser to refresh cached files. Creating a comment line with a generated version number, hash of your files, or timestamp is one way to ensure users have the latest version of your software. You can also programmatically update the cache once a new version is ready as discussed in the [#toc-updating-cache Updating the cache] section.

A manifest can have three distinct sections: CACHE, NETWORK, and FALLBACK.

CACHE:
This is the default section for entries. Files listed under this header (or immediately after the CACHE MANIFEST) will be explicitly cached after they're downloaded for the first time.
NETWORK:
Files listed under this section are white-listed resources that require a connection to the server. All requests to these resources bypass the cache, even if the user is offline. Wildcards may be used.
FALLBACK:
An optional section specifying fallback pages if a resource is inaccessible. The first URI is the resource, the second is the fallback. Both URIs must be relative and from the same origin as the manifest file. Wildcards may be used.

Note: These sections can be listed in any order and each section can appear more than one in a single manifest.

The following manifest defines a "catch-all" page (offline.html) that will be displayed when the user tries to access the root of the site while offline. It also declares that all other resources (e.g. those on remote a site) require an internet connection.

 CACHE MANIFEST
 # 2010-06-18:v3
 
 # Explicitly cached entries
 index.html
 css/style.css
 
 # offline.html will be displayed if the user is offline
 FALLBACK:
 / /offline.html
 
 # All other resources (e.g. sites) require the user to be online.
 NETWORK:
 *
 
 # Additional resources to cache
 CACHE:
 images/logo1.png
 images/logo2.png
 images/logo3.png
 

Note: The HTML file that references your manifest file is automatically cached. There's no need to include it in your manifest, however it is encouraged to do so.

Note: HTTP cache headers and the caching restrictions imposed on pages served over SSL are overridden by cache manifests. Thus, pages served over https can be made to work offline.

Updating the cache

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

  1. The user clears their browser's data storage for your site.
  2. The manifest file is modified. Note: updating a file listed in the manifest doesn't mean the browser will re-cache that resource. The manifest file itself must be altered.
  3. The app cache is programatically updated.

Cache status

The window.applicationCache object is your programmatic access the browser's app cache. Its status property is useful for checking the current state of the cache:

var appCache = window.applicationCache;

switch (appCache.status) {
  case appCache.UNCACHED: // UNCACHED == 0
    return 'UNCACHED';
    break;
  case appCache.IDLE: // IDLE == 1
    return 'IDLE';
    break;
  case appCache.CHECKING: // CHECKING == 2
    return 'CHECKING';
    break;
  case appCache.DOWNLOADING: // DOWNLOADING == 3
    return 'DOWNLOADING';
    break;
  case appCache.UPDATEREADY:  // UPDATEREADY == 4
    return 'UPDATEREADY';
    break;
  case appCache.OBSOLETE: // OBSOLETE == 5
    return 'OBSOLETE';
    break;
  default:
    return 'UKNOWN CACHE STATUS';
    break;
};

To programmatically update the cache, first call applicationCache.update(). This will attempt to update the user's cache (which requires the manifest file to have changed). Finally, when the applicationCache.status is in its UPDATEREADY state, calling applicationCache.swapCache() will swap the old cache for the new one.

var appCache = window.applicationCache;

appCache.update(); // Attempt to update the user's cache.

...

if (appCache.status == window.applicationCache.UPDATEREADY) {
  appCache.swapCache();  // The fetch was successful, swap in the new cache.
}

Note: Using update() and swapCache() like this does not serve the updated resources to users. This flow simply tells the browser to check for a new manifest, download the updated content it specifies, and repopulate the app cache. Thus, it takes two page reloads to server new content to users, one to pull down a new app cache, and another to refresh the page content.

The good news: you can avoid this double reload headache. To update users to the newest version of your site, set a listener to monitor the updateready event on page load:

// Check if a new cache is available on page load.
window.addEventListener('load', function(e) {

  window.applicationCache.addEventListener('updateready', function(e) {
    if (window.applicationCache.status == window.applicationCache.UPDATEREADY) {
      // Browser downloaded a new app cache.
      // Swap it in and reload the page to get the new hotness.
      window.applicationCache.swapCache();
      if (confirm('A new version of this site is available. Load it?')) {
        window.location.reload();
      }
    } else {
      // Manifest didn't changed. Nothing new to server.
    }
  }, false);

}, false);

AppCache events

As you may expect, additional events are exposed to monitor the cache's state. The browser fires events for things like download progress, updating the app cache, and error conditions. The following snippet sets up event listeners for each type of cache event:

function handleCacheEvent(e) {
  //...
}

function handleCacheError(e) {
  alert('Error: Cache failed to update!');
};

// Fired after the first cache of the manifest.
appCache.addEventListener('cached', handleCacheEvent, false);

// Checking for an update. Always the first event fired in the sequence.
appCache.addEventListener('checking', handleCacheEvent, false);

// An update was found. The browser is fetching resources.
appCache.addEventListener('downloading', handleCacheEvent, false);

// The manifest returns 404 or 410, the download failed,
// or the manifest changed while the download was in progress.
appCache.addEventListener('error', handleCacheError, false);

// Fired after the first download of the manifest.
appCache.addEventListener('noupdate', handleCacheEvent, false);

// Fired if the manifest file returns a 404 or 410.
// This results in the application cache being deleted.
appCache.addEventListener('obsolete', handleCacheEvent, false);

// Fired for each resource listed in the manifest as it is being fetched.
appCache.addEventListener('progress', handleCacheEvent, false);

// Fired when the manifest resources have been newly redownloaded.
appCache.addEventListener('updateready', handleCacheEvent, false);

If the manifest file or a resource specified in it fails to download, the entire update fails. The browser will continue to use the old application cache in the event of such a failure.

References



Compatibility

Desktop

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic Support 20
12
4 -moz
10
12.0
5.1

Mobile

Feature Android BlackBerry Chrome for mobile Firefox Mobile (Gecko) IE Mobile Opera Mobile Opera Mini Safari Mobile
Basic Support 2.1
?
?
?
4 -moz
?
Unsupported
?
3.2

See also

Related articles

Off-line Storage












  • tutorials/appcache beginner


Attribution

This article contains content originally from external sources.

Portions of this content come from HTML5Rocks! article