Force update of AppCache with PHP

Posted on

September 27, 2011

AppCache can be hard to deal with. Browsers seem unpredictable in updating the AppCache. This example of a PHP-script (for PHP 5) makes sure the AppCache gets updated when pages have been changed.

Important

This script relies heavily on http-headers sent dynamically by PHP. Headers that are defined in .htaccess can never be overruled by PHP, so don’t put these headers (especially Last-Modified and Expires) in .htaccess: the script won’t work then.

The Process: two modi

The script will be executed on the server in two modi: from the manifest and from a regular webpage on your site.

To clarify this let’s name the first mode “manifest-mode” and the second mode “page-mode”. Both modi use different parts of the PHP-script in manifest-loader.php.

In manifest-mode the script will be activated directly by .htaccess. In this mode the script will echo the contents of manifest.appcache to the browser, but not before it has searched for pages that are more recent than the manifest, in all the folders that are listed in a special file named folders.txt. Since almost every page of your site may link to the manifest, every time one of these pages is visited, the script will start its search for modificated pages.
In page-mode if the script is included in the page, it will only check if the current webpage is more recent than the manifest. In page-mode the script sends cache-control-headers (Cache-Control: max-age, Last-Modified, Expires) for the current webpage to the browser.

More about the manifest-mode

The script will investigate if the script itself or one of your regular webpages is more recent than the manifest.
If so, the manifest on the server will be updated with an new version-number and modification-time. Then it will be sent to the browser, along with some headers that tell the browser that the manifest has expired and will have to be refreshed from the browser during the next request of a webpage.
The update of the online manifest forces the visitor’s browser to update its local AppCache.
In case there are no webpages more recent than the manifest, the script checks if maybe your stylesheet is more recent than the manifest. In that case the manifest also will be updated.
There may be a delay before you see updates of the manifest. This delay is determined by:
1. The number of files in the appcache that have to be checked for updates (and the time it takes to download modificated files).
2. On the max-age send sent by the script for each file while in page-mode.

More about the page mode

If the requested webpage on the server is more recent than the manifest, the script will update the manifest on the server with a new version-number and modification-time.
The script also checks if the requested page is in a folder that is not yet listed in folders.txt. If so, this file will be updated with the new folder. This list of folders enables the script to check this new folder for modificated pages when it’s in manifest-mode.

The Files

You need four files, each of which you can find below:

The manifest that is called manifest.appcache.
The .htaccess file
The script that is called manifest-loader.php
A register for the folders on your site folders.txt

Also you need to include some extra PHP-code in your regular webpages, as you can see in the samples below.

All files mentioned above are available in an archive: .

Don’t forget to make manifest.appcache and folders.txt writable! Without this permission the script will not be able to write/change/add anything in these files.

The Manifest

This is an example of the manifest thats is needed to enable AppCache. Modify according to your requirements, but don’t change the first two occurrences of lines that start with “#”.

0001CACHE MANIFEST

0003# Don't touch the following two lines; they will be updated dynamically by manifest-loader.php:

0004# Version: 58

0005# Updated: 2011-09-05, 16:26:57

0007CACHE:

0009/page.php

0010/css/css.css

0012#If you want to use fallback pages for pages that haven't been downloaded yet of if you want to define pages that have to be always fetched online, remove the corresponding hashtags below

0014#FALLBACK:

0015#/ /offline.php

0017#NETWORK:

0018#*

.htaccess

Only the bare minimum of rules needed for this setup are demonstrated here. This file must reside in the root directory of your site.

0001RewriteEngine On

0003#send the manifest:

0004rewriteRule ^manifest\.appcache$ appcache/manifest-loader.php?sendmanifest = true [L]

0006<IfModule mod_headers.c>

0007#make sure (with Etag and max-age) that modified static content will be reloaded after the manifest has been updated:

0008 <FilesMatch ".(html|htm|xml|txt|css)$">

0009  FileETag MTime Size

0011  Header set Cache-Control "max-age = 120"

0012 </FilesMatch>

0014</IfModule>

folders.txt

In this file manifest-loader.php maintains a list of folders on your site. This list enables manifest-loader.php while in manifest-mode to crawl the folders of your site in search of files that are more recent than the manifest.

0001a:1:{i:0;s:1:"/";}

manifest-loader.php

This is the script that does all the hard work. It has to be included in the head of your pages (see the sample below).

0001<?php

0002class AppCache {

0004 //first we define some properties of the class:

0006/** root folder of your site; dynamically filled in the constructor

0007* @var string */

0008 private $_root_folder = "";

0010/** The folder that contains this script and the manifest; dynamically filled in the constructor

0011* @var string */

0012 private $_appcache_folder = "";

0014/** Path to the manifest; dynamically filled in the constructor

0015* @var string */

0016 private $_manifest = "";

0018/** Contents of the manifest, fetched from manifest.appcache and sent to the browser

0019* @var string */

0020 private $_manifest_contents = "";

0022/** Will be true if and when the manifest source-file manifest.appcache has been updated. This update is executed by AppCache::_manifest_update()

0023* @var boolean */

0024 private $_manifest_updated = false;

0026/** The delay in seconds it will take before you will see updated content in your browser. 2 Minutes (120 seconds) is a good value for this: you don't have to wait overly long for updated content and your pages will still load very fast.

0027* @var numeric */

0028 private $_appcache_refresh_delay = 120;

0030/** Folders that will be checked by the dynamic manifest. Folder list will be stored in the file folders.txt in the folder appcache and will be automatically updated. Initially only the root folder of the site - "/" - is stored

0031* @var array */

0032 private $_folders = Array("/");

0034/** txt-file that stores the serialized list op folders of your site

0035* @var string */

0036 private $_folder_index = "";

0038/** Folders that contain the stylesheet(s) of your site. Has a stylesheet been updated, the manifest will also be updated, to force an update of the AppCache in the browser

0039* @var array */

0040 private $_css_folders = Array("/css/");

0042/** File modification time of the manifest, needed for computations to determine if the manifest needs an update

0043* @var numeric */

0044 private $_manifest_filemtime = 0;

0046/** True if this file has been referenced from .htaccess (when the manifest has to be echoed). If the current class has been instantiated from an include on a page, this property will stay false.

0047* @var boolean */

0048 private $_is_manifest = false;

0050/**

0051 * Constructor of the class. It will echo the manifest or updated it, if required

0052 *

0053 * @return    void

0054 */

0055 public function __construct () {

0057  //determine if this file has been loaded as stand-alone (to echo the manifest), of as an include on a page:

0058  if (isset($_GET, $_GET['sendmanifest']))$this->_is_manifest = true;

0060  //initialise the locations of the folders:

0061  $this->_root_folder = $_SERVER['DOCUMENT_ROOT'];

0062  $this->_appcache_folder = $this->_root_folder . "/appcache";

0064  $this->_folder_index = $this->_appcache_folder . "/folders.txt";

0066  //initialize some properties:

0067  $this->_folders = unserialize(file_get_contents($this->_folder_index));

0069  $this->_manifest = $this->_appcache_folder . "/manifest.appcache";

0071  $this->_manifest_filemtime = filemtime($this->_manifest);

0073  //if in manifest mode, echo the contents of manifest.appcache:

0074  if ($this->_is_manifest)$this->_echo_manifest();

0076  //otherwise: if this file has been loaded from a regular php-page:

0077  else {

0078   //determine the file modification time of the page that has called this file/class:

0079   $pagetime = filemtime(CALLER);

0081   //if the page is newer then the manifest, update the manifest:

0082   if ($pagetime > $this->_manifest_filemtime)$this->_manifest_update();

0084   //send some headers for cache-control of the page (don't use the second argument true for header(), or you'll get internal server errors):

0085   header("Cache-Control: max-age = " . $this->_appcache_refresh_delay);

0086   header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT", $pagetime);

0088   $expires = time() + 60 * 60 * 24 * 14;

0089   header("Expires: ".gmdate("D, d M Y H:i:s", $expires)." GMT"); // Far future expiration header set to 14 days

0091   //optionally update the list of folders that have to be checked for changes in the php-pages (list will be stored in appcache/folders.txt):

0092   $this->_update_folders();

0093  }

0095 } //end of __construct()

0097/**

0098 * Echoes the manifest and before that conditionally updates the manifest if pages or resources of the site have been updated

0099 *

0100 * @return    void

0101 */

0102 private function _echo_manifest () {

0104  // if this class / file has been updated, update the manifest:

0105  if (filemtime(__FILE__) > $this->_manifest_filemtime)$this->_manifest_update();

0107  //if the manifest hasn't been updated by the previous line, check the folders of the site for changes in de php-pages.

0108  //if any are found, the manifest will then be also updated, in order to force a refresh of the local AppCache of the browser

0109  if (!$this->_manifest_updated) {

0110   //will contain folders that are listed in folders.txt but don't exist anymore:

0111   $delete_folders = Array();

0113   foreach ($this->_folders as $folder) {

0114    $folder_to_check = $this->_root_folder . substr($folder, 0, -1);

0116    //if a folder doesn't exist anymore, add it to a list. The folders in this list will be purged from folders.txt later on:

0117    if (!file_exists($folder_to_check) || !is_dir($folder_to_check)) {

0118     $delete_folders[] = $folder;

0119     continue;

0120    }

0122    //determine the most recent file modification time in a folder:

0123    $most_recent_page = $this->_get_most_recent_file_in_folder($folder_to_check);

0125    //if the most recent page in a folder is more recent than the manifest, update the manifest:

0126    if ($most_recent_page > $this->_manifest_filemtime) {

0127     $this->_manifest_update();

0129     break; //because the manifest has been updated, the folders don't have to be checked anymore for files with more recent modification times

0130    }

0131   }

0133   //if there are any folders listed in folders.txt that don't exist anymore, delete them from this file:

0134   if (count($delete_folders)) {

0135    $this->_folders = array_diff($this->_folders, $delete_folders);

0136    file_put_contents($this->_folder_index, serialize($this->_folders));

0137   }

0138  }

0141  // only if an update of the manifest has not been neccessary until now, check the shylesheets for modification. If a stylesheet exists that is more recent than the manifest, update the manifest:

0142  if (!$this->_manifest_updated) {

0143   foreach ($this->_css_folders as $folder) {

0144    $folder_to_check = $this->_root_folder . substr($folder, 0, -1);

0146    $most_recent_page = $this->_get_most_recent_file_in_folder($folder_to_check, Array("extension" => "css"));

0148    if ($most_recent_page > $this->_manifest_filemtime) {

0149     $this->_manifest_update();

0151     break; //because the manifest has been updated, the other folders don't have to be checked anymore for files with more recent modification times

0152    }

0153   }

0154  }

0156  //send some headers for the manifest. Specifically needed for FireFox, which browser has a tendency to cache the manifest and therefor not to update the AppCache. With these headers the manifest will be marked as always expired, even for Firefox:

0158  header("Content-Type: text/cache-manifest", true);

0159  header("Cache-Control: must-revalidate, proxy-revalidate, max-age = 0", true);

0160  header("Expires: ".gmdate("D, d M Y H:i:s")." GMT", true); // Always expired

0161  header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT", true);// always modified

0164  //echo the contents of the sourcefile manifest.appcache:

0165  readfile($this->_manifest);

0167 } //end of _echo_manifest()

0170/**

0171 * Update the manifest, to force an update of the AppCache in the browser

0172 *

0173 * @return    void

0174 */

0175 private function _manifest_update () {

0177  $this->_manifest_contents = file_get_contents($this->_manifest);

0179  $this->_manifest_contents = preg_replace_callback("/Version:\s*(\d+)/", create_function(

0180    '$var',

0181    '

0182    $var[1] += 1;

0183    return "Version: "  . $var[1];

0184    '

0185  ), $this->_manifest_contents);

0187  $this->_manifest_contents = preg_replace("/# Updated: .+/", "# Updated: " . date("Y-m-d, H:i:s"), $this->_manifest_contents);

0189  file_put_contents($this->_manifest, $this->_manifest_contents);

0191  //log the modified-status of the manifest; this way further checks for the need to update the manifest will be skipped

0192  $this->_manifest_updated = true;

0194 } //end of _manifest_update()

0196/**

0197 * If an update thereof is required, update the list of folders with pages in folders.txt

0198 *

0199 * @return   void

0200 */

0201 private function _update_folders () {

0203  //determine the path to files relative to the document root of the site:

0204  $folder = preg_replace("/[^\/]+$/", "", $_SERVER['REQUEST_URI']);

0206  //if "http" has been used in the request_uri, a hacker probably is trying things. So we won't continue with the method in that case.

0207  if (stristr($folder, "http"))return;

0209  //you have to make sure the folder index file folders.txt exists AND it has to be writable.

0210  // if a new folder has been detected that isn't in this list, it has to be added to the index file:

0211  if (!in_array($folder, $this->_folders)) {

0212   $this->_folders[] = $folder;

0213   file_put_contents($this->_folder_index, serialize($this->_folders));

0214  }

0215 }

0217/**

0218 * Returns the most recent file in a folder

0219 *

0220 * @param string $folder The folder that is being searched

0221 * @param array $options Options for the method

0222 *

0223 * @return    numeric file modification time

0224 */

0225 private function _get_most_recent_file_in_folder ($folder, $options = Array()) {

0227  if(!file_exists($folder) || !is_dir($folder))return "";

0229  extract($options);

0231  if (!isset($extension))$extension = "php";

0233  $list = scandir ($folder); //scandir() returns a list of files (only the filenames) in the folder

0234  if (count($list) == 2)return ""; // then only .. and . found, which are no regular files

0236  $most_recent_filemtime = 0;

0238  foreach ($list as $file) {

0240   $full_path = $folder . "/" . $file;

0242   if (in_array($file, Array(".", "..")) || is_dir($full_path) || ($extension != "" && !preg_match("/\." . $extension . "$/", $file)))continue;

0244   $time = filemtime($full_path);

0245   if ($time > $most_recent_filemtime) {

0246    $most_recent_filemtime = $time;

0247   }

0248  } //end of the loop through all files

0250  return $most_recent_filemtime;

0252 } //end of get_most_recent_file_in_folder()

0254} //end of class AppCache

0256//instantiate the class, so that the manifest will be echoed or updated:

0257new AppCache();

Sample files

Below you find examples of three regular pages (two in the root folder of your site and one in a subfolder “subdir”) and of the stylesheet for these pages.

Notice how the manifest loader is being loaded in the start section of your pages. You only have to add some extra PHP-code above the doctype and in the HTML element of your pages. Of course your webpages must have the extension .php for this to work.

In the root folder: page.php (in the manifest)

0001<?php

0002 define("CALLER", __FILE__);

0003 require_once($_SERVER['DOCUMENT_ROOT'] . "/appcache/manifest-loader.php");

0004?><!DOCTYPE HTML>

0006<!--the manifest seemingly resides in the root folder of your site, when actually it is located in the subfolder /appcache -->

0007<html manifest = "/manifest.appcache">

0008<head>

0009   <meta http-equiv = "Content-Type" content = "text/html; charset = UTF-8" />

0010   <title>AppCache demo page 1</title>

0012   <link href = "/css/css.css" rel = "stylesheet" />

0013</head>

0015<body>

0016<h1>Page 1</h1>

0017<p>This is the only page that is explicitly listed in the manifest. The other pages will be added to the AppCache when the visitor loads them in his browser.</p>

0018<p>To <a href = "page2.php">page 2</a></p>

0019<p>To <a href = "subdir/page3.php">page 3</a></p>

0020</body>

0021</html>

In the root folder: page2.php (not in the manifest)

0001<?php

0002 define("CALLER", __FILE__);

0003 require_once($_SERVER['DOCUMENT_ROOT'] . "/appcache/manifest-loader.php");

0004?><!DOCTYPE HTML>

0006<!--the manifest seemingly resides in the root folder of your site, when actually it is located in the subfolder /appcache -->

0007<html manifest = "/manifest.appcache">

0008<head>

0009   <meta http-equiv = "Content-Type" content = "text/html; charset = UTF-8" />

0010   <title>AppCache demo page 2</title>

0012   <link href = "/css/css.css" rel = "stylesheet" />

0013</head>

0015<body>

0016<h1>Page 2</h1>

0017<p>This page is not listed in the manifest; so it will only be added to the AppCache if and when the visitor has visited this page in his browser.</p>

0018<p>To <a href = "page.php">page 1</a></p>

0019<p>To <a href = "subdir/page3.php">page 3</a></p>

0020</body>

0021</html>

In a subfolder: page3.php (not in the manifest)

0001<?php

0002 define("CALLER", __FILE__);

0003 require_once($_SERVER['DOCUMENT_ROOT'] . "/appcache/manifest-loader.php");

0004?><!DOCTYPE HTML>

0006<!--the manifest seemingly resides in the root folder of your site, when actually it is located in the subfolder /appcache -->

0007<html manifest = "/manifest.appcache">

0008<head>

0009   <meta http-equiv = "Content-Type" content = "text/html; charset = UTF-8" />

0010   <title>AppCache demo page 3</title>

0012   <link href = "/css/css.css" rel = "stylesheet" />

0013</head>

0015<body>

0016<h1>Page 3 (in subfolder)</h1>

0017<p>This page is not listed in the manifest; so it will only be added to the AppCache if and when the visitor has visited this page in his browser.</p>

0018<p>To <a href = "../page.php">page 1</a></p>

0019<p>To <a href = "../page2.php">page 2</a></p>

0020</body>

0021</html>

The stylesheet: css/css.css (in the manifest)

The contents of this file are not very relevant. I’ve only added it here because this sheets also will be checked for modifications by manifest-loader.php.

0001body {

0002 background: red;

0003 color: white;

0004}

0006a {

0007 color: white;

0008 text-decoration: underline;

0009}

0011a:hover {

0012 text-decoration: none;

0013}

Posted in AppCache, Browsers, HTML5, PHP | Leave a reply

Know your AppCache

Posted on

August 28, 2011

by SmartScripter

AppCache can significantly speed up the loading of the pages of your site and lets visitors surf your site even when they are offline and haven’t downloaded all pages yet.

In this post I’ll present a solution to get AppCache functioning correctly (which can be quite problematic). I’ll conclude with some interesting facts about AppCache and how different browsers cope with it.

How does AppCache work?

AppCache is a new feature of HTML5. In the html-tag of web pages must be a link present to the “manifest” of the AppCache (preferably with the extension .appcache):

<html manifest="/manifest.appcache">

Now this manifest is used by the browser as central point of reference: in this manifest the webmaster of a site can sum up all the pages, images and documents that have to be downloaded and cached in the background. Instead of checking online for each and every page if a new version thereof exists, the browser consults the manifest to check if that has been changed. If it has not and the requested page/file is present in the AppCache, the local copy will be loaded instead of the online one. Since the manifest is only one small file, checking this for updates instead of the numerous online pages and files is much, much faster.

For more detailed information and instructions how to write a manifest see the links at the end of this page.

The main subject of this post are the problems you can encounter when trying to use AppCache and how to solve them. As easy as the use of AppCache may seem in theory, in practice it soon will surprise you with its seemingly unreliable behavior. It can prove quite hard to force your browser to update it’s AppCache with modified online content.

The problem: AppCache conflicts with normal cache-control headers

The AppCache of any browser will only be renewed when the contents of the manifest have been changed. A simple way to make this change of content is to add a comment in the manifest in which you simply write down a version number. Whenever this version number changes, meaning the content of the manifest has changed, the browser will update the AppCache.However, any cache-control with Cache-Control and Expires headers set in a .htaccess-file in the root of a site, can and most probably will conflict with the cache-management by the AppCache. I noticed that whenever I used this standard method of cache-control, AppCache wouldn’t function correctly anymore: it would never update its cache afterwards with newer versions of the same pages. This meant that the visitor would only and forever get to see the out-of-date version of any page once it has been stored in the AppCache. That is, until the visitor deletes the AppCache of his browser.

Problems with AppCache and HTML forms

AppCache can also be problematic on pages that contain a form. When a visitor posts the form, you have make adaptations to ensure that the post will indeed get through. I’ll write about this subject in another post.

The solution to the problem

I have found a solution to this problem. The central point of it is that the Cache-Control headers are never to be set in .htaccess, but always to be sent dynamically by a scripting language. An example how to accomplish this with PHP can be found elsewhere in this blog. I have tested this solution thoroughly after including it in Site Optimizer, a system I wrote to optimize sites.

In the example script I’ve set the refresh time for the AppCache to 2 minutes. I found that if changed pages were present, the AppCache would indeed be updated after this interval (or a little longer). So the visitor will see changed content after a relatively short delay. When visiting the same page some hours later, the visitor will surely see the changed content of the AppCache, since the max-age of two minutes has by then expired long since. And as long as, in between visits, the manifest hasn’t changed, the pages will be shown from the browser’s AppCache.

The result is very interesting:

Despite the low max-age you still have avoided unnecessary downloading of pages that were already downloaded before and that haven’t been changed in the meantime.
What’s more: your visitor can see the contents of pages listed in the manifest even when he is offline and hadn’t visited this pages while online.
Only the files that actually have changed will be re-downloaded to the AppCache.

You may get penalty points in PageSpeed for the missing expires-headers. If so then PageSpeed obviously is not (yet) smart enough to detect that they are compensated with the advantages of AppCache. The most recent test in YSlow though does indeed compensate for this.

AppCache in various browsers

Note: I’ve only tested AppCache on a Windows computer with the browsers available to me on that platform

Chrome

Chrome has good support for AppCache.
Very handy feature: when the user deletes the conventional cache of his browser, the AppCache will be cleared also.
If you want, you can get a list of all the AppCaches managed by Chrome by visiting chrome://appcache-internals/ in this browser. In this list you can delete AppCaches on a per site basis.
On the visitor’s harddisk the AppCache is being stored in the directoryC:\Users\[User Name]\AppData\Local\Google\Chrome\User Data\Default\Application Cache, but in a non readable format.
Luckily you can download the extension Service Pages for Google Chrome. One of the utilities in this suite is “View HTTP Cache”. In this utility you can also see which pages Chrome has stored in it’s AppCache. If you click on a page, you see the contents thereof and the headers that were issued by the server when the page was downloaded. This led me to believe that the AppCache not only delivers the contents of pages to the browser, but also reconstructs and sends the original headers.

Firefox

Firefox has the laudable but also annoying tendency to pop up a dialog that alerts the visitor that “this site is trying to store content for offline use”. In my mind this leaves the visitor with the impression that a site is engaged in shady activities.
Edit: in my current version of Firefox 6.0.2 I can disable this dialog by unticking a checkbox: Options > Options > “Tell me when a website asks to store data for offline use”. If I remember correctly this checkbox wasn’t available in previous versions of Firefox.
Firefox’s AppCache will not be deleted when you empty this browser’s conventional cache manually. Instead you have to follow another pathway in the menu: Options > Options > Advanced > Network. In the lower part of this dialog you’ll notice the AppCaches that Firefox manages; these can be erased manually in this dialog. Rather a pity that Firefox didn’t follow the same approach as Chrome i.e.: deleting the AppCaches simultaneously with the conventional cache.
Edit: in more recent versions of FireFox (6.0.2 and newer) the AppCache will also be deleted when the visitor empties the local cache of Firefox. Huzza!
Firefox stores it’s meta information about the AppCache in a SQLite database: C:\Users\[Username]\AppData\Local\Mozilla\Firefox\Profiles\[ProfileOfCurrentUser]\OfflineCache\index.sqlite. So this information can be accessed in a SQLite-viewer, for example in the Firefox-extension SQLite Manager. See here an example of the information FireFox stores:
When the browser is in the process of downloading the resources that are being listed in the manifest and the users goes off-line during this process, the browser crashes.

Internet Explorer, all versions

All versions of Internet Explorer up to version 9 don’t seem to support AppCache: none of the AppCache pages were shown to me when I went off-line. In Internet Explorer 9 only the pages I explicitly downloaded by actually visiting them where available to me when offline.

Opera

Although the latest version of Opera claims that it supports AppCache, in fact I noticed that it did not work. None of the pages in my manifest were shown to me when I went off-line and tried to surf the site.
Rather strange, because I did notice that Opera stores AppCache information. You can see what Opera has stored in it’s AppCache when you visit View > Developer Tools > Temporal Storage (maybe the exact wording differs, because it’s my translation of the menu-items in Dutch).
In C:\Users\[UserName]\AppData\Local\Opera\Opera\application_cache resides a file cache_groups.xml. In this file you can see which AppCaches the browser has stored.
The contents of the stored pages can be found in dcache4.url in the same folder as cache_groups.xml.

Safari for Windows

In first instance AppCache didn’t seem to work in this browser, because the pages in the manifest weren’t show to me when I went off-line. However, after adding the header “Pragma: cache” Safari handled the AppCache exactly like it should.

Using AppCache for mobile browsers

Since HTML5 is needed to leverage the advantages of AppCache, you have to determine if the browser supports HTML5 at all. For a mobile browser this can be done by checking in the database of TeraWurfl if it has support for the HTML5 canvas element. To have your website work properly in both desktop and mobile browsers with AppCache, you will need to automate this and more:

Have the correct http-headers send for using AppCache as mentioned above: the Cache-Control and Last-Modified headers
Adjustment of the version number inside the manifest
Detection of the desktop or mobile browser
Check if this browser support HTML5
Converting the pages to HTML5 (or back to XHTML, depending on your approach)

You can test for yourself the advantages of AppCache in mobile browsers by visiting www.smartscripts.nl in your mobile browser. The system I mentioned above, Site Optimizer, has automated the functions mentioned above, and more, like dynamically changing the content and the layout of pages for the mobile version of site.

403/forbidden-oddities

Sometimes when a visitor has been denied access to a page (403 error), the offline page defined in the manifest gets displayed instead of the “access denied” message. So I’m inclined to conclude that if a offline page has been defined in the manifest and a visitor tries to load a non existing page, this offline page wil be displayed. This is good to know, so you can adapt the text of the offline page, alerting the visitor to the fact that he either is offline or that he tried to load a private page.

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Category Archives: HTML5