|By Charlie Arehart||
|July 11, 2001 12:00 AM EDT||
You've probably seen the use of a variable called "application.dsn" (or "application.datasource") in code. Perhaps you've even been taught to use the method in a class.
Maybe you've even been doing it in your own code. I'm talking about setting this variable in the application.cfm file to hold the name of your data source for a given application.
It seems so innocuous, and it seems to provide ways to make your code easier to maintain (change the dsn variable once in the application.cfm, and all the templates that use it under control of that application.cfm get the benefit of the change).
However, there's a problem and it can be a nasty one. This issue has to do with the locking of (or failure to lock) shared-scope variables such as this one, and the fact that rarely does any discussion of this approach include the consequences of using shared variable scopes. There have been other articles in CFDJ on the subject of shared variable locking, as well as Macromedia Knowledge Base articles.
Maybe you didn't make the connection between them and this issue. This article puts the problem in perspective and offers some explanations of how to understand and resolve it. The ultimate solution involves using a "request"-scoped variable instead of an "application"-scoped one.
If you're not familiar with shared variable scopes, or are fuzzy about locking issues, or perhaps have never understood what "request" scope variables are about, this article should help you.
If you do understand these things, change your references to application.dsn to request.dsn, and if you find any locks around code that was using such an application.dsn variable, you need to consider whether those should stay as well. I'll also offer insights into how to find and fix such locking references.
You may see code doing this in application.cfm:
<CFSET application.dsn = "whatever">
which declares the variable as "global," in essence, and can therefore be used later in all other templates as in:
<CFQUERY datasource="#application.dsn#" ... >
to refer to that data source name. The upside to this is that if the data source name needs to be changed (from "whatever" to "whatever_test", for instance), you can simply modify the application.cfm to point to the new name, and all templates under its control get the benefit of the change.
It's a good plan, but the use of an application-scoped variable is flawed. It opens you to potentially troublesome locking issues (for more on that, including a good explanation for why it's a problem, see "ColdFusion Locking Best Practices" at www.allaire.com/Handlers/index.cfm?ID=20370&Method=Full). More important, you can have the intended benefit with an equally useful and less troublesome way.
For reasons I'll explain in a moment if it's still unclear, I'm suggesting that you stop using the application scope to hold the data source name. However, instead of dropping "application." from the variable name, I'm suggesting that you use the "request" scope. In other words, do the following (in your application.cfm):
<CFSET request.dsn = "whatever">
and then in all your templates do:
<CFQUERY datasource="#request.dsn#" ...>
If it's not clear why this is useful, or how it works, then there may be confusion about:
- How the application.cfm works like a CFINCLUDE (and how we could, but won't, use a local variable called "dsn")
- What the request scope is about (and why it's better to use "request.dsn")
How Application.cfm Works Like a CFINCLUDE
And How We Could, but Won't, Use a Local Variable Called "dsn"
Nearly every CF developer knows that whenever a CF template is run, CF first tries to execute any application.cfm that exists in the same directory (or its parent directory, or its grandparent, and so on).
What may not be obvious is that CF actually runs the application.cfm like a CFINCLUDE, which means that any variables set there, including "local" variables (such as <CFSET firstname="bob">), are then available to the template that was originally being run.
So let's say we have a template that does the following:
If it did this and nothing else, you might expect it to fail since you can't refer to variables that don't exist, and it's only outputting the variable, not creating it. But if firstname was set in application.cfm (assuming this template is in a directory controlled by that application.cfm), it can indeed refer to the variable.
Knowing that, you may wonder why the folks who promoted this solution of setting the dsn variable to the application scope even bothered. They could just as easily have said:
<CFSET dsn = "whatever">
and then in all their templates do:
<CFQUERY datasource="#dsn#" ...>
It would work. In fact, there's no need to be using the application scope to pass the variable to all templates (make it global), because any variables set in the application.cfm "trickle down" to all templates, in effect making them global. There are certainly good uses of application scoped variables, but this isn't one of them.
The simple example of setting a variable called "dsn" (or what could be formally specified as variables.dsn, which is the same thing) to hold the name of the data source would work, and would trickle down to all the templates. It's effectively "global," at least for the life of that template, and it's reset at the execution of each template by being executed in the application.cfm each time.
I've recommended that you use "request.dsn" rather than "dsn" or "variables.dsn". Why? And what is the request scope, anyway?
What the Request Scope Is About
And Why It's Better to Use "Request.dsn"
If you understand that setting a local variable called "dsn" will work, can you think of any situation in which your code may expect to have access to that variable but won't? Think hard. Okay, custom tags. Yep, most know that custom tags have their own local variable scope. So a local variable set in the caller (or in the application.cfm) won't be available to the custom tag.
When we were setting the dsn variable to an application scope, it was available in the custom tag (as are all shared scopes and also form, url, cgi, and other variables that are passed to the calling template).
By changing the application.dsn variable to dsn (or variables.dsn, same thing) while it's available in all templates under control of the application.cfm, it's not available to any custom tags called by those templates. That's why we need to use the request scope instead.
Its sole purpose, poorly understood though it is, is to create local variables in a program that are available within custom tags called by that program (and vice versa). That's it. Nothing more.
Many confuse the request scope with some sort of persistence or shared nature (and the fact that there's a different kind of "request" scope in other languages like ASP and JSP only confuses matters further). It's best to think of it as nothing more than a scope that allows local variables to be seen in a custom tag called by the template setting the request scope variable. And since our request.dsn variable is set in the application.cfm, it trickles down to the template being executed and is therefore also available to any custom tags we call as well.
That's why you should use the request scope rather than a local scope.
Remediation of Application.dsn Misuse
One last thought: you may not be able to blindly do a global search and replace application.dsn with request.dsn wherever it occurs. Depending on the savvy of the coder, the use of application.dsn may have at least two wrinkles that require more care than just replacing application.dsn with request.dsn:
- You may be testing for the existence of application.dsn before setting it in the application.cfm, in which case you need to set the request.dsn outside that test since it will never "already exist."
- You may have CFLOCKs surrounding CFQUERYs, or CFLOCKs that are moving the application.dsn to a local variable. You wouldn't want to just rename the scope in those instances.
I hope this article not only helps prevent problems with application.dsn, specifically, but also increases your understanding of request scope variables and application.cfm processing in general.
|charlie arehart 04/15/08 02:16:10 PM EDT|
Thanks for that, Brian. I should add that while this article was from 2001, since CF7 we now have application.cfc and its onapplicationstart method. That does change things a bit, in that one could safely create an application variable there and it really would be created only once, at the start of the application. For that reason, I suppose it may no longer be so "wrong" to see application.dsn--but again, that's only if you're using this feature. Otherwise, it may be better to stick with the request scope as discussed in the article.
|Brian 04/04/08 03:17:23 PM EDT|
Very informative! I've been wondering what the request scope is for such a long time and no other resource did such a great job of explaining it.
|Charlie Arehart 08/31/01 10:36:00 AM EDT|
A read asked by email: "don't you also need to lock request vars?" The answer is no. They're not shared by any other user. More particularly, they're not shared by any other "thread".
The request scope, like the variables scope, is local to the running program and belongs only to it. It's a single thread of execution, so there's no risk of conflict with other threads, as with the "shared scope" variables, server, application, and session.
"We work in the area of Big Data analytics and Big Data analytics is a very crowded space - you have Hadoop, ETL, warehousing, visualization and there's a lot of effort trying to get these tools to talk to each other," explained Mukund Deshpande, head of the Analytics practice at Accelerite, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Jul. 1, 2016 12:30 AM EDT Reads: 658
Cloud Expo, Inc. has announced today that Andi Mann returns to 'DevOps at Cloud Expo 2016' as Conference Chair The @DevOpsSummit at Cloud Expo will take place on November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. "DevOps is set to be one of the most profound disruptions to hit IT in decades," said Andi Mann. "It is a natural extension of cloud computing, and I have seen both firsthand and in independent research the fantastic results DevOps delivers. So I am excited t...
Jul. 1, 2016 12:00 AM EDT Reads: 572
IoT offers a value of almost $4 trillion to the manufacturing industry through platforms that can improve margins, optimize operations & drive high performance work teams. By using IoT technologies as a foundation, manufacturing customers are integrating worker safety with manufacturing systems, driving deep collaboration and utilizing analytics to exponentially increased per-unit margins. However, as Benoit Lheureux, the VP for Research at Gartner points out, “IoT project implementers often ...
Jun. 30, 2016 07:30 PM EDT Reads: 711
Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, discussed how research has demonstrated the value of Machine Learning in delivering next generation analytics to imp...
Jun. 30, 2016 04:00 PM EDT Reads: 1,027
The cloud promises new levels of agility and cost-savings for Big Data, data warehousing and analytics. But it’s challenging to understand all the options – from IaaS and PaaS to newer services like HaaS (Hadoop as a Service) and BDaaS (Big Data as a Service). In her session at @BigDataExpo at @ThingsExpo, Hannah Smalltree, a director at Cazena, provided an educational overview of emerging “as-a-service” options for Big Data in the cloud. This is critical background for IT and data profession...
Jun. 30, 2016 04:00 PM EDT Reads: 490
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, wh...
Jun. 30, 2016 03:00 PM EDT Reads: 1,282
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...
Jun. 30, 2016 02:45 PM EDT Reads: 369
When people aren’t talking about VMs and containers, they’re talking about serverless architecture. Serverless is about no maintenance. It means you are not worried about low-level infrastructural and operational details. An event-driven serverless platform is a great use case for IoT. In his session at @ThingsExpo, Animesh Singh, an STSM and Lead for IBM Cloud Platform and Infrastructure, will detail how to build a distributed serverless, polyglot, microservices framework using open source tec...
Jun. 30, 2016 02:00 PM EDT Reads: 704
Connected devices and the industrial internet are growing exponentially every year with Cisco expecting 50 billion devices to be in operation by 2020. In this period of growth, location-based insights are becoming invaluable to many businesses as they adopt new connected technologies. Knowing when and where these devices connect from is critical for a number of scenarios in supply chain management, disaster management, emergency response, M2M, location marketing and more. In his session at @Th...
Jun. 30, 2016 01:30 PM EDT Reads: 1,366
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, provided an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life sett...
Jun. 30, 2016 01:00 PM EDT Reads: 1,526
"delaPlex is a software development company. We do team-based outsourcing development," explained Mark Rivers, COO and Co-founder of delaPlex Software, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Jun. 30, 2016 11:45 AM EDT Reads: 624
IoT is rapidly changing the way enterprises are using data to improve business decision-making. In order to derive business value, organizations must unlock insights from the data gathered and then act on these. In their session at @ThingsExpo, Eric Hoffman, Vice President at EastBanc Technologies, and Peter Shashkin, Head of Development Department at EastBanc Technologies, discussed how one organization leveraged IoT, cloud technology and data analysis to improve customer experiences and effi...
Jun. 30, 2016 11:30 AM EDT Reads: 656
Basho Technologies has announced the latest release of Basho Riak TS, version 1.3. Riak TS is an enterprise-grade NoSQL database optimized for Internet of Things (IoT). The open source version enables developers to download the software for free and use it in production as well as make contributions to the code and develop applications around Riak TS. Enhancements to Riak TS make it quick, easy and cost-effective to spin up an instance to test new ideas and build IoT applications. In addition to...
Jun. 30, 2016 11:15 AM EDT Reads: 751
The cloud market growth today is largely in public clouds. While there is a lot of spend in IT departments in virtualization, these aren’t yet translating into a true “cloud” experience within the enterprise. What is stopping the growth of the “private cloud” market? In his general session at 18th Cloud Expo, Nara Rajagopalan, CEO of Accelerite, explored the challenges in deploying, managing, and getting adoption for a private cloud within an enterprise. What are the key differences between wh...
Jun. 30, 2016 11:00 AM EDT Reads: 1,130
The idea of comparing data in motion (at the sensor level) to data at rest (in a Big Data server warehouse) with predictive analytics in the cloud is very appealing to the industrial IoT sector. The problem Big Data vendors have, however, is access to that data in motion at the sensor location. In his session at @ThingsExpo, Scott Allen, CMO of FreeWave, discussed how as IoT is increasingly adopted by industrial markets, there is going to be an increased demand for sensor data from the outermos...
Jun. 30, 2016 11:00 AM EDT Reads: 485
CenturyLink has announced that application server solutions from GENBAND are now available as part of CenturyLink’s Networx contracts. The General Services Administration (GSA)’s Networx program includes the largest telecommunications contract vehicles ever awarded by the federal government. CenturyLink recently secured an extension through spring 2020 of its offerings available to federal government agencies via GSA’s Networx Universal and Enterprise contracts. GENBAND’s EXPERiUS™ Application...
Jun. 30, 2016 11:00 AM EDT Reads: 503
Presidio has received the 2015 EMC Partner Services Quality Award from EMC Corporation for achieving outstanding service excellence and customer satisfaction as measured by the EMC Partner Services Quality (PSQ) program. Presidio was also honored as the 2015 EMC Americas Marketing Excellence Partner of the Year and 2015 Mid-Market East Partner of the Year. The EMC PSQ program is a project-specific survey program designed for partners with Service Partner designations to solicit customer feedbac...
Jun. 30, 2016 10:45 AM EDT Reads: 710
The IoT is changing the way enterprises conduct business. In his session at @ThingsExpo, Eric Hoffman, Vice President at EastBanc Technologies, discussed how businesses can gain an edge over competitors by empowering consumers to take control through IoT. He cited examples such as a Washington, D.C.-based sports club that leveraged IoT and the cloud to develop a comprehensive booking system. He also highlighted how IoT can revitalize and restore outdated business models, making them profitable ...
Jun. 30, 2016 10:30 AM EDT Reads: 595
There are several IoTs: the Industrial Internet, Consumer Wearables, Wearables and Healthcare, Supply Chains, and the movement toward Smart Grids, Cities, Regions, and Nations. There are competing communications standards every step of the way, a bewildering array of sensors and devices, and an entire world of competing data analytics platforms. To some this appears to be chaos. In this power panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, Bradley Holt, Developer Advocate a...
Jun. 30, 2016 10:15 AM EDT Reads: 1,001
SYS-CON Events has announced today that Roger Strukhoff has been named conference chair of Cloud Expo and @ThingsExpo 2016 Silicon Valley. The 19th Cloud Expo and 6th @ThingsExpo will take place on November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. "The Internet of Things brings trillions of dollars of opportunity to developers and enterprise IT, no matter how you measure it," stated Roger Strukhoff. "More importantly, it leverages the power of devices and the Interne...
Jun. 30, 2016 10:00 AM EDT Reads: 584