Welcome!

Weblogic Authors: Yeshim Deniz, Elizabeth White, Michael Meiner, Michael Bushong, Avi Rosenthal

Related Topics: Machine Learning , Java IoT, Microservices Expo

Machine Learning : Article

Using HTML5 Application Cache to Create Offline Web Applications

Provides an easy way to prefetch some or all of your web app’s assets

Application Cache Events
The window.applicationCache object fires several events related to the state of the cache. window.applicationCache.status is a numerical property that tells you the state of the cache:

0-UNCACHED
1-IDLE
2-CHECKING
3-DOWNLOADING
4-UPDATEREADY
5-OBSOLETE

The following table shows the window.applicationCache callback attributes you can use in your applications:

Callback Attribute

Event

onchecking

CHECKING

ondownloading

DOWNLOADING

onupdateready

UPDATEREADY

onobsolete

OBSOLETE

oncached

CACHED

onerror

ERROR

onnoupdateready

NOUPDATE

onprogress

PROGRESS

The following code snippet shows how you can use these event callback attributes in your code.

Note: you can programmatically call window.applicationCache.update() to check for updates and window.applicationCache.swapCache() to connect the browser to use the latest version of the updated cache. The browsers will do this under the covers when you refresh the page.

window.applicationCache.onchecking = function(e) {
log("Checking for application update");
}
window.applicationCache.onnoupdate = function(e) {
log("No application update found");
}
window.applicationCache.onupdateready = function(e) {
log("Application update ready");
//Now connect the browser to use the new cache
window.applicationCache.swapCache();
}
window.applicationCache.onobsolete = function(e) {
log("Application obsolete");
}
window.applicationCache.ondownloading = function(e) {
log("Downloading application update");
}
window.applicationCache.oncached = function(e) {
log("Application cached");
}
window.applicationCache.onerror = function(e) {
log("Application cache error");
}
window.applicationCache.onprogress = function(e) {
log("Application Cache progress");
}

You can use the application cache events to code up some cool user notification functionality. For example, while you're downloading an application (receiving progress events), you can show the progress of the download. And when the onupdateready event fires, you can swap the browser's active cache to the new cache and instruct the user that (1) a new update has been downloaded, and (2) they must refresh the page to see the changes (since the cache is now downloaded and ready for use, but the page was already loaded from the application cache before the download started and will not be automatically refreshed.

See Also: Check out Ben Nadel's blog post about handling application cache events: http://www.bennadel.com/blog/2029-Using-HTML5-Offline-Application-Cache-Events-In-Javascript.htm

Detecting Online and Offline Status
HTML5 also allows you to detect whether you're online or offline. The following code shows how you can add event listeners for online and offline events.

You can use this to stop communicating with a server and to instead store things in HTML5 Web Storage (window.localStorage or window.sessionStorage) or in an HTML5 Web SQL Database while you're offline.

window.addEventListener("online", function(e) {
log("Application is now online");
// Send app data to server
}, true);
window.addEventListener("offline", function(e) {
log("Application is now offline");
window.localStorage.myLocalKey = ‘Some Data';
}, true);

Since these events do not fire when the page loads, you can also detect the initial status of a page by calling window.applicationCache.status.

Accessing Application Cache Content
You might be wondering where all of those files are stored. The application cache files are actually somewhere on your device's hard disk. For example, if you open the page about:cache in Firefox it will show you the exact location. Internally, the files may be stored in a SQLite database, but how the files are stored is an implementation detail that is left up to the browsers to figure out.


The
about:cache page in Firefox

Security
Since it is possible to access other people's files from an application cache on a shared computer (by navigating to the same site), it is critical that you do not store personal or sensitive data in an application cache. It is also important to keep in mind that you cannot always write to the application cache even if your browser supports the feature. This is because most private browsing modes (For example, Safari's Private Browsing mode, shown in the following image, or Chrome's Incognito Mode) prevent you from writing to an application cache for security reasons. It is therefore important to check for errors and to never assume you can access the cache.


Safari's Private Browsing Mode

Disk Quota
If you try to store a lot of data in an application cache, you may run into quota errors. Firefox and Opera provide a handy way to increase storage size for specific offline web applications while Chrome and Safari support this only through the use of special startup parameters. In your applications you should listen for errors such as the following:

Application Cache Error event: Failed to commit new cache to storage as it would exceed the quota.

In the future, browsers will hopefully have graceful, on-the-fly quota upgrade mechanisms for Application caching, like those of Opera's Web Storage, which prompts you as you are about to exceed your quota.

Clearing the Cache
In order to debug and test web apps that use application cache, it is a good idea to start with a clean slate to avoid false positives. To do this you must first blow away any existing application caches. Typically you do this by clearing the cache. Ensure that you are not doing this while a page from an offline web application is still open in the browser; that may cause problems with clearing the cache for that site.

Note: An application cache is created using the manifest's complete URL. You can have multiple manifest files in a site, which then allows you to split up loading of files. Each of these manifests will create a separate application cache. Here is how you can clear the cache and application cache in the various browsers:

Browser

Steps to Clear a Cache

Chrome

Settings Menu > Tools >
Clear Browsing Data

Firefox

Tools > Clear Recent History

&
Tools > Options (Preferences on Mac OS X) > Advanced > Network > Select specific application cache > Remove

 

Safari

Settings Menu > Reset Safari

Opera

Tools > Preferences > Storage

(+ Tools > Clear Private Data)

 

Internet Explorer

N/A

Best Practices
To recap, here are some of the best practices when it comes to using HTML5 offline web applications:

  • Manifest errors are often fatal, so it is best to use a predeployment script that programmatically creates a list of files to cache so you avoid typos and missing files.
  • The manifest file must be updated to trigger any changed files to be downloaded. Use a version Comment in the manifest file to track the changes and force application cache busting.
  • If you're testing on a web server on your local machine, you can avoid having to clear the cache non-stop by hosting your site on different domain names. You can accomplish this by hacking your hosts file, which is located at \WINDOWS\system32\drivers\etc\hosts on Windows and at /etc/hosts on UNIX. For example, the following entries allow a locally running web server to host files on both offline0.example.com and offline1.example.com, thus creating two separate application caches:

127.0.0.1 localhost
127.0.0.1 offline0.example.com
127.0.0.1 offline1.example.com

  • Check to see if files are really requested from the server or pulled from the application cache by monitoring the web server logs, as shown in the following screen-shot

Checking which files are requested and served up in Python's SimpleHTTPServer server log

Summary
In this article you learnt about the ins and outs of HTML5 Application Cache, a new feature that can be used to create offline web applications. This article explained how application caching worked and added some clarification about common misconceptions while providing a few best practices along the way.

More Stories By Peter Lubbers

Peter Lubbers is the Director of Documentation and Training at Kaazing where he oversees all aspects of documentation and training. He is the co-author of the Apress book Pro HTML5 Programming and teaches HTML5 training courses. An HTML5 and WebSocket enthusiast, Peter frequently speaks at international events.

Prior to joining Kaazing, Peter worked as an information architect at Oracle, where he wrote many books. He also develops documentation automation solutions and two of his inventions are patented.

A native of the Netherlands, Peter served as a Special Forces commando in the Royal Dutch Green Berets. In his spare time (ha!) Peter likes to run ultra-marathons. He is the 2007 and 2009 ultrarunner.net series champion and three-time winner of the Tahoe Super Triple marathon. Peter lives on the edge of the Tahoe National Forest and loves to run in the Sierra Nevada foothills and around Lake Tahoe (preferably in one go!).

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
SYS-CON Events announced today that Synametrics Technologies will exhibit at SYS-CON's 22nd International Cloud Expo®, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Synametrics Technologies is a privately held company based in Plainsboro, New Jersey that has been providing solutions for the developer community since 1997. Based on the success of its initial product offerings such as WinSQL, Xeams, SynaMan and Syncrify, Synametrics continues to create and hone inn...
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
"Evatronix provides design services to companies that need to integrate the IoT technology in their products but they don't necessarily have the expertise, knowledge and design team to do so," explained Adam Morawiec, VP of Business Development at Evatronix, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Digital Transformation (DX) is not a "one-size-fits all" strategy. Each organization needs to develop its own unique, long-term DX plan. It must do so by realizing that we now live in a data-driven age, and that technologies such as Cloud Computing, Big Data, the IoT, Cognitive Computing, and Blockchain are only tools. In her general session at 21st Cloud Expo, Rebecca Wanta explained how the strategy must focus on DX and include a commitment from top management to create great IT jobs, monitor ...
In his Opening Keynote at 21st Cloud Expo, John Considine, General Manager of IBM Cloud Infrastructure, led attendees through the exciting evolution of the cloud. He looked at this major disruption from the perspective of technology, business models, and what this means for enterprises of all sizes. John Considine is General Manager of Cloud Infrastructure Services at IBM. In that role he is responsible for leading IBM’s public cloud infrastructure including strategy, development, and offering m...
Recently, WebRTC has a lot of eyes from market. The use cases of WebRTC are expanding - video chat, online education, online health care etc. Not only for human-to-human communication, but also IoT use cases such as machine to human use cases can be seen recently. One of the typical use-case is remote camera monitoring. With WebRTC, people can have interoperability and flexibility for deploying monitoring service. However, the benefit of WebRTC for IoT is not only its convenience and interopera...
No hype cycles or predictions of a gazillion things here. IoT is here. You get it. You know your business and have great ideas for a business transformation strategy. What comes next? Time to make it happen. In his session at @ThingsExpo, Jay Mason, an Associate Partner of Analytics, IoT & Cybersecurity at M&S Consulting, presented a step-by-step plan to develop your technology implementation strategy. He also discussed the evaluation of communication standards and IoT messaging protocols, data...
Product connectivity goes hand and hand these days with increased use of personal data. New IoT devices are becoming more personalized than ever before. In his session at 22nd Cloud Expo | DXWorld Expo, Nicolas Fierro, CEO of MIMIR Blockchain Solutions, will discuss how in order to protect your data and privacy, IoT applications need to embrace Blockchain technology for a new level of product security never before seen - or needed.
Recently, REAN Cloud built a digital concierge for a North Carolina hospital that had observed that most patient call button questions were repetitive. In addition, the paper-based process used to measure patient health metrics was laborious, not in real-time and sometimes error-prone. In their session at 21st Cloud Expo, Sean Finnerty, Executive Director, Practice Lead, Health Care & Life Science at REAN Cloud, and Dr. S.P.T. Krishnan, Principal Architect at REAN Cloud, discussed how they built...
Nordstrom is transforming the way that they do business and the cloud is the key to enabling speed and hyper personalized customer experiences. In his session at 21st Cloud Expo, Ken Schow, VP of Engineering at Nordstrom, discussed some of the key learnings and common pitfalls of large enterprises moving to the cloud. This includes strategies around choosing a cloud provider(s), architecture, and lessons learned. In addition, he covered some of the best practices for structured team migration an...
The 22nd International Cloud Expo | 1st DXWorld Expo has announced that its Call for Papers is open. Cloud Expo | DXWorld Expo, to be held June 5-7, 2018, at the Javits Center in New York, NY, brings together Cloud Computing, Digital Transformation, Big Data, Internet of Things, DevOps, Machine Learning and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding busin...
"Digital transformation - what we knew about it in the past has been redefined. Automation is going to play such a huge role in that because the culture, the technology, and the business operations are being shifted now," stated Brian Boeggeman, VP of Alliances & Partnerships at Ayehu, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
In his session at 21st Cloud Expo, Raju Shreewastava, founder of Big Data Trunk, provided a fun and simple way to introduce Machine Leaning to anyone and everyone. He solved a machine learning problem and demonstrated an easy way to be able to do machine learning without even coding. Raju Shreewastava is the founder of Big Data Trunk (www.BigDataTrunk.com), a Big Data Training and consulting firm with offices in the United States. He previously led the data warehouse/business intelligence and B...
Cloud Expo | DXWorld Expo have announced the conference tracks for Cloud Expo 2018. Cloud Expo will be held June 5-7, 2018, at the Javits Center in New York City, and November 6-8, 2018, at the Santa Clara Convention Center, Santa Clara, CA. Digital Transformation (DX) is a major focus with the introduction of DX Expo within the program. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive ov...
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
Smart cities have the potential to change our lives at so many levels for citizens: less pollution, reduced parking obstacles, better health, education and more energy savings. Real-time data streaming and the Internet of Things (IoT) possess the power to turn this vision into a reality. However, most organizations today are building their data infrastructure to focus solely on addressing immediate business needs vs. a platform capable of quickly adapting emerging technologies to address future ...
With tough new regulations coming to Europe on data privacy in May 2018, Calligo will explain why in reality the effect is global and transforms how you consider critical data. EU GDPR fundamentally rewrites the rules for cloud, Big Data and IoT. In his session at 21st Cloud Expo, Adam Ryan, Vice President and General Manager EMEA at Calligo, examined the regulations and provided insight on how it affects technology, challenges the established rules and will usher in new levels of diligence arou...
SYS-CON Events announced today that Evatronix will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Evatronix SA offers comprehensive solutions in the design and implementation of electronic systems, in CAD / CAM deployment, and also is a designer and manufacturer of advanced 3D scanners for professional applications.
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...
22nd International Cloud Expo, taking place June 5-7, 2018, at the Javits Center in New York City, NY, and co-located with the 1st DXWorld Expo will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud ...