Skip to main content

Appcache manifest file issues/caveats


Application cache (appcache) is a powerful feature in HTML5. However, it does come with baggage. Many (see links below) advocated ferociously against it due to tricky issues it comes with. For someone who is just testing waters, these issues may throw them off grid. Knowing them before hand helps reduce some unpredictable effects.

Most important coding caveats about the manifest file and appcache.

  • The main html file that declares the manifest is ALSO cached in app cache, even if it is not defined in CACHE section. So, if you update the html itself (master),  in the manifest must change (a update-time-stamp) to see the changes (also will require user refresh or swapCache call)
  • If the master is rendered from server and it has varying (seeded or dynamic) parameters, a separate copy of the html is stored in appcache per canonical url (index.php?q=search1, index.php?q=search2 will have separate cached html). This is generally unnecessary, but if you do want to have parameters to pass to javascript, ensure that it is passed via location.hash(e.g: index.php#!q=search1)
  • Manifest file must begin with the line "CACHE MANIFEST" instruction. Empty lines are treated just like comments in some browsers, so ensure it is the first one
  • Manifest file must  be served with a text/cache-manifest MIME type. Chrome seems to be fine otherwise, but you dont write for chrome alone, do you?
  • Manifest file must  be encoded with 8-bit Unicode Transformation Format (UTF-8) character encoding. This is as per W3C stature, may be easy to do, before you find yourself in trouble.
  • The path URLs in CACHE section cannot have wildcards. (qualified - relative, canonical and external urls for each resource must be defined separately). URLs in  NETWORK and FALLBACK sections can have wildcards (/api will apply for api directory tree; /api/1.html and /api/2.html but not /api2 or /api2/1.html) .
  • URLs defined in CACHE are all or nothing. This is probably the biggest issue when you use external URLs, your app is now at the mercy of the availability and correct-ness (at the time of caching) of the external providers.
  • Preferred extension for file is .appcache, however, browsers seem to be fine with anything.
  • more in gotchas in get-offline, and here.

Popular posts from this blog

javascript maxlength for textarea with \r\n breaks in java (esp Firefox)

Textareas allow new lines to enter. These are represented by \n (1) or \r\n (2) characters. But when you save to DB you have a limit to certain length of chars. There is no maxlength attribute in HTML that will stop you from entering data. This is generally acomplished by Javascript. You do a onkeyup hook and stop event or trim after textarea.value.length > maxlength. There are many other solutions out there.. But.. Here is the problem that most of those solutions overlook, How do you deal with the count on \n and \r\n representations. Lets first see how it matters. If the text entered has new lines, the length is calculated differently in Firefox and IE. When you enter a Text like 01234 567890 You expect the textarea.value.length to be 11. (10 chars + new line).On the backend, however, java would recieve it as 12 chars (10 chars + \r\n) (this is irrespective of FF or IE). So you are effectively saving 12 chars to DB. Worse yet, IE seems to figure textarea.value.length as 12 (

How to Make a Local (Offline) Repository in Ubuntu / Debian

If you are in a place where you dont have internet (or have a bad one) You want to download .deb packages and install them offline. Each deb file is packaged as a seperate unit but may contain dependencies (recursively). apt-get automagically solves all the dependencies and installs all that are necessary. Manually install deb files one by one resolving each dependency would be tedious. A better approach is to make your own local repository. Before you actually make a repo, You need *all* deb files. You dont practically have to mirror all of the packages from the internet, but enough to resolve all dependencies. Also, You have to make sure, you are getting debs of the correct architecture of your system (i386 etc) # 1. make a dir accessible (atleast by root) sudo mkdir /var/my-local-repo # 2. copy all the deb files to this directory. # 3. make the directory as a sudo dpkg-scanpackages /var/my-local-repo /dev/null > \ /var/my-local-repo/Packages # 4. add the local repo to sour

MySql Copying Table Structures.

Some times you need to copy only table structures across databases. This article describes two ways of doing it. If the whole database schema need to be exported, mysqldump is very effective. A --nodata flag will dump all tables' schema. Like this. mysqldump --nodata -p -u username databaseName But if you want to copy a specific table, individually, you could use "create table like" feature. You could create it even from a different database. However it must be on the same mysqld instance. Like this. create table newtable like oldtable; --Or from a table in other database create table mytable like otherdatabase.tablename;