Welcome!

AJAX & REA Authors: Scott Hirsch, David H Deans, Carmen Gonzalez, Pat Romanski, Aria Blog

Related Topics: Web 2.0, AJAX & REA

Web 2.0: Article

Alex Iskold's "AJAX Patterns" Series: Concurrent Document Loader Pattern

Solving the need to load multiple documents

Pattern: Concurrent Document Loader
Problem: Need to load multiple documents and can't proceed until all of them are loaded
Example: Load configuration files for an AJAX application

Introduction

AJAX stands for Asynchronous JavaScript processing with XML. It is the asynchronous part that gets everyone excited. Fetching the data without reloading the page allows AJAX applications to mimic the desktop-like behavior.

Since AJAX applications are getting quite complex these days, they tend use XML files to store the initial configuration settings and bootstrap information. When the application starts, it needs to load the configuration files in order to setup the application screens.


 
Loading each configuration time sequentially would take a long time. Clearly, we would like to leverage the power of AJAX and load the files concurrently. To do that we need to create a separate XMLHttpRequest for each configuration file and then dispatch the requests one of another.

We can set the callback on each request, which would be invoked once the configuration file is loaded. But the problem is that all these documents can come at different times, so how do we know when all of them are loaded? To solve this problem, we need to have a loader that keeps track of what has been loaded and invokes a callback once all the documents have been fetched.

Concurrent Document Loader Interface

We start by thinking how we want to interact with this document loader. We want to issue multiple requests and then get a callback when all of them are done.



The callback function will have the array of XMLHttpRequests as its parameter:



So the consumer of this API, would be able to write the following code:



Note how the callback function is declared in place in the example above. This is called closure and looks similar to anonymous functions in Java. Actually it is much more complex than it looks, so if you have not seen this before,  you will find complete explanation in this article.

Concurrent Document Loader Implementation

The following implementation has been tested in Firefox 1.5.0.4. We implement the ConcurrentDocumentLoader as a singleton, declaring it like this:



Lets focus on a few things in the code above. The first thing to note is that we keep a count of the number of the outstanding requests using a variable. Every time when a request comes back, we decrement the count. Once the count reaches zero, we know that we are done and we invoke the callback function. This is thread-safe, since there is no one polling on the outstanding requests variable.

The second thing to note is the call to the asyncLoadDocument function. This function is going to do the actual loading using XMLHttpRequest. You can write this function using either your favorite AJAX library like prototype or MochiKit or you can do it by directly manipulating the object as shown in the listing below.

   

And finally, we list the function that creates XMLHttpRequest in different browsers.

 
Conclusion

The Concurrent Document Loader pattern is useful in cases when you need to load multiple XML documents concurrently and get notified when all requests are completed.

Download the Source code for this article.


More Stories By Alex Iskold

Alex Iskold is the Founder and CEO of adaptiveblue (http://www.adaptiveblue.com), where he is developing browser personalization technology. His previous startup, Information Laboratory, created innovative software analysis and visualization tool called Small Worlds. After Information Laboratory was acquired by IBM, Alex worked as the architect of IBM Rational Software Analysis tools. Before starting adaptiveblue, Alex was the Chief Architect at DataSynapse, where he developed GridServer and FabricServer virtualization platforms. He holds M.S. in Computer Science from New York University, where he taught an award-winning software engineering class for undergraduate students. He can be reached at [email protected]

Comments (1)

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.


@CloudExpo Stories
The major cloud platforms defy a simple, side-by-side analysis. Each of the major IaaS public-cloud platforms offers their own unique strengths and functionality. Options for on-site private cloud are diverse as well, and must be designed and deployed while taking existing legacy architecture and infrastructure into account. Then the reality is that most enterprises are embarking on a hybrid cloud strategy and programs. In this Power Panel at 15th Cloud Expo, moderated by Ashar Baig, Research ...
In her General Session at 15th Cloud Expo, Anne Plese, Senior Consultant, Cloud Product Marketing, at Verizon Enterprise, will focus on finding the right mix of renting vs. buying Oracle capacity to scale to meet business demands, and offer validated Oracle database TCO models for Oracle development and testing environments. Anne Plese is a marketing and technology enthusiast/realist with over 19+ years in high tech. At Verizon Enterprise, she focuses on driving growth for the Verizon Cloud pla...
StackIQ offers a comprehensive software suite that automates the deployment, provisioning, and management of Big Infrastructure. With StackIQ’s software, you can spin up fully configured big data clusters, quickly and consistently — from bare-metal up to the applications layer — and manage them efficiently. Our software’s modular architecture allows customers to integrate nearly any application with the StackIQ software stack.
As Platform as a Service (PaaS) matures as a category, developers should have the ability to use the programming language of their choice to build applications and have access to a wide array of services. Bluemix is IBM's open cloud development platform that enables users to easily build cloud-based, creative mobile and web applications without having to spend large amounts of time and resources on configuring infrastructure and multiple software licenses. In this track, you will learn about the...
The Internet of Things will greatly expand the opportunities for data collection and new business models driven off of that data. In her session at Internet of @ThingsExpo, Esmeralda Swartz, CMO of MetraTech, will discuss how for this to be effective you not only need to have infrastructure and operational models capable of utilizing this new phenomenon, but increasingly service providers will need to convince a skeptical public to participate. Get ready to show them the money! Speaker Bio: ...
Compute virtualization has been transformational, yet security policy implementation and enforcement has lagged behind in agility and automation. There are a number of key considerations when implementing policy in private and hybrid clouds. In his session at 15th Cloud Expo, Holland Barry, VP of Technology at Catbird, will discuss the impact of this new paradigm and what organizations can do today to safely move to software-defined network and compute architectures, including: How normal ope...
Samsung VP Jacopo Lenzi, who headed the company's recent SmartThings acquisition under the auspices of Samsung's Open Innovaction Center (OIC), answered a few questions we had about the deal. This interview was in conjunction with our interview with SmartThings CEO Alex Hawkinson. IoT Journal: SmartThings was developed in an open, standards-agnostic platform, and will now be part of Samsung's Open Innovation Center. Can you elaborate on your commitment to keep the platform open? Jacopo Lenzi: S...
How do APIs and IoT relate? The answer is not as simple as merely adding an API on top of a dumb device, but rather about understanding the architectural patterns for implementing an IoT fabric. There are typically two or three trends: Exposing the device to a management framework Exposing that management framework to a business centric logic • Exposing that business layer and data to end users. This last trend is the IoT stack, which involves a new shift in the separation of what stuff hap...
SYS-CON Events announced today that SOA Software, an API management leader, will exhibit at SYS-CON's 15th International Cloud Expo®, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. SOA Software is a leading provider of API Management and SOA Governance products that equip business to deliver APIs and SOA together to drive their company to meet its business strategy quickly and effectively. SOA Software’s technology helps businesses to accel...
SYS-CON Events announced today that Utimaco will exhibit at SYS-CON's 15th International Cloud Expo®, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Utimaco is a leading manufacturer of hardware based security solutions that provide the root of trust to keep cryptographic keys safe, secure critical digital infrastructures and protect high value data assets. Only Utimaco delivers a general-purpose hardware security module (HSM) as a customiz...
Almost everyone sees the potential of Internet of Things but how can businesses truly unlock that potential. The key will be in the ability to discover business insight in the midst of an ocean of Big Data generated from billions of embedded devices via Systems of Discover. Businesses will also need to ensure that they can sustain that insight by leveraging the cloud for global reach, scale and elasticity.
SYS-CON Events announced today that ElasticBox is holding a Hackathon at DevOps Summit, November 6 from 12 pm -4 pm at the Santa Clara Convention Center in Santa Clara, CA. You can enter as an individual or team of up to 10 developers. A New Star Is Born Every Month! All completed ElasticBoxes will then be sent to a judging panel - 12 winners will be featured on the ElasticBox website in 2015. All entrants will receive five full enterprise licenses for one year + ElasticBox headphones + Elasti...
Once the decision has been made to move part or all of a workload to the cloud, a methodology for selecting that workload needs to be established. How do you move to the cloud? What does the discovery, assessment and planning look like? What workloads make sense? Which cloud model makes sense for each workload? What are the considerations for how to select the right cloud model? And how does that fit in with the overall IT tranformation? In his session at 15th Cloud Expo, John Hatem, head of V...
Cloud services are the newest tool in the arsenal of IT products in the market today. These cloud services integrate process and tools. In order to use these products effectively, organizations must have a good understanding of themselves and their business requirements. In his session at 15th Cloud Expo, Brian Lewis, Principal Architect at Verizon Cloud, will outline key areas of organizational focus, and how to formalize an actionable plan when migrating applications and internal services to...
SAP is delivering break-through innovation combined with fantastic user experience powered by the market-leading in-memory technology, SAP HANA. In his General Session at 15th Cloud Expo, Thorsten Leiduck, VP ISVs & Digital Commerce, SAP, will discuss how SAP and partners provide cloud and hybrid cloud solutions as well as real-time Big Data offerings that help companies of all sizes and industries run better. SAP launched an application challenge to award the most innovative SAP HANA and SAP ...
Ixia develops amazing products so its customers can connect the world. Ixia helps its customers provide an always-on user experience through fast, secure delivery of dynamic connected technologies and services. Through actionable insights that accelerate and secure application and service delivery, Ixia's customers benefit from faster time to market, optimized application performance and higher-quality deployments.
SYS-CON Events announced today that Calm.io has been named “Bronze Sponsor” of DevOps Summit Silicon Valley, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Calm.io is a cloud orchestration platform for AWS, vCenter, OpenStack, or bare metal, that runs your CL tools puppet, Chef, shell, git, Jenkins, nagios, and will soon support New Relic and Docker. It can run hosted, or on premise and provides VM automation / expiry, self-service portals,...
SYS-CON Events announced today that Aria Systems, the recurring revenue expert, has been named "Bronze Sponsor" of SYS-CON's 15th International Cloud Expo®, which will take place on November 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Aria Systems helps leading businesses connect their customers with the products and services they love. Industry leaders like Pitney Bowes, Experian, AAA NCNU, VMware, HootSuite and many others choose Aria to power their recurring revenue bu...
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce t...
Blue Box has closed a $10 million Series B financing. The round was led by a strategic investor and included participation from prior investors including Voyager Capital and Founders Collective, as well as the Blue Box executive team. This round follows a $4.3 million Series A closed in December of 2012 and led by Voyager Capital. In May of this year, the company announced general availability of its private cloud as a service offering, Blue Box Cloud. Since that release, the company has dem...