Welcome!

You will be redirected in 30 seconds or close now.

ColdFusion Authors: Yakov Fain, Maureen O'Gara, Nancy Y. Nee, Tad Anderson, Daniel Kaar

Related Topics: ColdFusion

ColdFusion: Article

Why It's Wrong to Use Application.dsn in Your Templates

Why It's Wrong to Use Application.dsn in Your Templates

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).

The Problem
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.

Some Background
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.

The Solution
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")
Let me explain both. Though it would seem that many understand the first point, I find they often don't and it's fundamental to the rest of the discussion. The use of request scoped variables is also poorly understood by many. The end result is that you'll no longer have to deal with locking issues with regard to this variable.

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:

<CFOUTPUT>Hello #firstname#

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.
Each of these cases requires a little more care to resolve. I provide further insights into solving these problems on my Web site, www.systemanage.com/cff/application_dsn_bad.cfm.

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.

More Stories By Charlie Arehart

A veteran ColdFusion developer since 1997, Charlie Arehart is a long-time contributor to the community and a recognized Adobe Community Expert. He's a certified Advanced CF Developer and Instructor for CF 4/5/6/7 and served as tech editor of CFDJ until 2003. Now an independent contractor (carehart.org) living in Alpharetta, GA, Charlie provides high-level troubleshooting/tuning assistance and training/mentoring for CF teams. He helps run the Online ColdFusion Meetup (coldfusionmeetup.com, an online CF user group), is a contributor to the CF8 WACK books by Ben Forta, and is frequently invited to speak at developer conferences and user groups worldwide.

Comments (3) View Comments

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.


Most Recent Comments
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.

@ThingsExpo Stories
The Internet of Things is not new. Historically, smart businesses have used its basic concept of leveraging data to drive better decision making and have capitalized on those insights to realize additional revenue opportunities. So, what has changed to make the Internet of Things one of the hottest topics in tech? In his session at @ThingsExpo, Chris Gray, Director, Embedded and Internet of Things, discussed the underlying factors that are driving the economics of intelligent systems. Discover how hardware commoditization, the ubiquitous nature of connectivity, and the emergence of Big Data a...
"BSQUARE is in the business of selling software solutions for smart connected devices. It's obvious that IoT has moved from being a technology to being a fundamental part of business, and in the last 18 months people have said let's figure out how to do it and let's put some focus on it, " explained Dave Wagstaff, VP & Chief Architect, at BSQUARE Corporation, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
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 (http://www.CloudComputingExpo.com), moderated by Ashar Baig, Research Director, Cloud, at Gigaom Research, Nate Gordon, Director of T...
SYS-CON Events announced today that IDenticard will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. IDenticard™ is the security division of Brady Corp (NYSE: BRC), a $1.5 billion manufacturer of identification products. We have small-company values with the strength and stability of a major corporation. IDenticard offers local sales, support and service to our customers across the United States and Canada. Our partner network encompasses some 300 of the world's leading systems integrators and security s...
SYS-CON Events announced today that Windstream, a leading provider of advanced network and cloud communications, has been named “Silver Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. Windstream (Nasdaq: WIN), a FORTUNE 500 and S&P 500 company, is a leading provider of advanced network communications, including cloud computing and managed services, to businesses nationwide. The company also offers broadband, phone and digital TV services to consumers primarily in rural areas.

ARMONK, N.Y., Nov. 20, 2014 /PRNewswire/ --  IBM (NYSE: IBM) today announced that it is bringing a greater level of control, security and flexibility to cloud-based application development and delivery with a single-tenant version of Bluemix, IBM's platform-as-a-service. The new platform enables developers to build ap...

The BPM world is going through some evolution or changes where traditional business process management solutions really have nowhere to go in terms of development of the road map. In this demo at 15th Cloud Expo, Kyle Hansen, Director of Professional Services at AgilePoint, shows AgilePoint’s unique approach to dealing with this market circumstance by developing a rapid application composition or development framework.
“In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
DevOps Summit 2015 New York, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that it is now accepting Keynote Proposals. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce software that is obsolete at launch. DevOps may be disruptive, but it is essential.
"People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Nigeria has the largest economy in Africa, at more than US$500 billion, and ranks 23rd in the world. A recent re-evaluation of Nigeria's true economic size doubled the previous estimate, and brought it well ahead of South Africa, which is a member (unlike Nigeria) of the G20 club for political as well as economic reasons. Nigeria's economy can be said to be quite diverse from one point of view, but heavily dependent on oil and gas at the same time. Oil and natural gas account for about 15% of Nigera's overall economy, but traditionally represent more than 90% of the country's exports and as...
The Internet of Things is a misnomer. That implies that everything is on the Internet, and that simply should not be - especially for things that are blurring the line between medical devices that stimulate like a pacemaker and quantified self-sensors like a pedometer or pulse tracker. The mesh of things that we manage must be segmented into zones of trust for sensing data, transmitting data, receiving command and control administrative changes, and peer-to-peer mesh messaging. In his session at @ThingsExpo, Ryan Bagnulo, Solution Architect / Software Engineer at SOA Software, focused on desi...
"At our booth we are showing how to provide trust in the Internet of Things. Trust is where everything starts to become secure and trustworthy. Now with the scaling of the Internet of Things it becomes an interesting question – I've heard numbers from 200 billion devices next year up to a trillion in the next 10 to 15 years," explained Johannes Lintzen, Vice President of Sales at Utimaco, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
"For over 25 years we have been working with a lot of enterprise customers and we have seen how companies create applications. And now that we have moved to cloud computing, mobile, social and the Internet of Things, we see that the market needs a new way of creating applications," stated Jesse Shiah, CEO, President and Co-Founder of AgilePoint Inc., in this SYS-CON.tv interview at 15th Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Gridstore™, the leader in hyper-converged infrastructure purpose-built to optimize Microsoft workloads, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Gridstore™ is the leader in hyper-converged infrastructure purpose-built for Microsoft workloads and designed to accelerate applications in virtualized environments. Gridstore’s hyper-converged infrastructure is the industry’s first all flash version of HyperConverged Appliances that include both compute and storag...
Today’s enterprise is being driven by disruptive competitive and human capital requirements to provide enterprise application access through not only desktops, but also mobile devices. To retrofit existing programs across all these devices using traditional programming methods is very costly and time consuming – often prohibitively so. In his session at @ThingsExpo, Jesse Shiah, CEO, President, and Co-Founder of AgilePoint Inc., discussed how you can create applications that run on all mobile devices as well as laptops and desktops using a visual drag-and-drop application – and eForms-buildi...
We certainly live in interesting technological times. And no more interesting than the current competing IoT standards for connectivity. Various standards bodies, approaches, and ecosystems are vying for mindshare and positioning for a competitive edge. It is clear that when the dust settles, we will have new protocols, evolved protocols, that will change the way we interact with devices and infrastructure. We will also have evolved web protocols, like HTTP/2, that will be changing the very core of our infrastructures. At the same time, we have old approaches made new again like micro-services...
Code Halos - aka "digital fingerprints" - are the key organizing principle to understand a) how dumb things become smart and b) how to monetize this dynamic. In his session at @ThingsExpo, Robert Brown, AVP, Center for the Future of Work at Cognizant Technology Solutions, outlined research, analysis and recommendations from his recently published book on this phenomena on the way leading edge organizations like GE and Disney are unlocking the Internet of Things opportunity and what steps your organization should be taking to position itself for the next platform of digital competition.
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
As the Internet of Things unfolds, mobile and wearable devices are blurring the line between physical and digital, integrating ever more closely with our interests, our routines, our daily lives. Contextual computing and smart, sensor-equipped spaces bring the potential to walk through a world that recognizes us and responds accordingly. We become continuous transmitters and receivers of data. In his session at @ThingsExpo, Andrew Bolwell, Director of Innovation for HP's Printing and Personal Systems Group, discussed how key attributes of mobile technology – touch input, sensors, social, and ...