Mike Brunt has posted an excellent article called Good Practices For Scaling ColdFusion Applications, in which he speaks about the importance of memory management within ColdFusion applications, and calls out certain avenues of scope usage that every CF developer should pay attention too.
Great post Mike! Another 'scope' issue that I've often seen (and abused myself) is the misuse of the VARIABLES scope. Back in the early days of CF, every example you saw showed an unscoped variable, placing it into the VARIABLES scope by default. This was fine, back then, as your view layers primarily consisted of Custom Tags. Use of the VARIABLES scope, within a custom tag, was permissable as those variables were then only available within that tag (or within a nested tag, if the nested tag used the CALLER scope). Once execution of the tag was completed, the memory space of those variables was cleared.
But, since custom tags were made from .cfm templates, there is no constraint within CFML from placing VARIABLES scoped variables within any .cfm template. This, in itself, is bad, as the VARIABLES scope (in the case of a base template, not a custom tag) is not properly cleared from memory space.
What I find people doing (and I've done it myself) is using the VARIABLES scope in place of the REQUEST scope. Memory allocated to REQUEST variables is properly recovered at the end of a request. Although variables in the VARIABLES scope are no longer available (except within a persistent CFC), their memory isn't properly released at the end of request execution. (For more information on this, see Jason Sheedy's post on the ColdFusion Memory Leak, and the accompanying comment thread and other referenced posts.)
A technique that some used, to get around this issue, was to have a StructClear(VARIABLES) statement as the last line of their onRequestEnd.cfm. This would explicitly clear out the VARIABLES scope at the end of a request. But, with the advent of Application.cfc, this no longer became an option (if you use Application.cfc.) If you tried to use that statement within the onRequestEnd() method of Application.cfc, you would only be clearing the VARIABLES scope of the CFC itself, and not the request.
That's actually a good pointer at the true underlying issue: proper scope usage. Use of the VARIABLES scope within a CFC makes variables within that scope only available directly by all methods of that CFC. Use of the VARIABLES scope within a custom tag makes variables within that scope only available within that tag (or a nested tag using the CALLER scope to access them.) Within a custom tag, you can not access variables of the calling template's VARIABLES scope, unless you use the CALLER scope to do so (which should only be in the case of a nested custom tag). You can, however, access REQUEST variables through out the request, from within a custom tag, an include file, or even within a CFC (though I wouldn't suggest doing that one.) By confining the usage of the VARIABLES scope to within custom tags and CFC's, and properly setting request level variables within the REQUEST scope, you could better manage your memory usage and application performance.
I've personally seen major differences in application performance, both through Mike's advice on persistent scope usage, as well as what I've outlined above. Unlike Jason Sheedy though, I'm not very familiar with Java profiling tools, so I'll have to leave it up to someone else to test all of this for the scientific proof. I only know that it appears my applications are running better as I migrate them to this paradigm. What are your thoughts?
*** Post Revision 080609: 2230 hrs *** More revision to my comments than the post itself, but Ray, the ColdFusion Jedi himself, corrected me. According to Ray, the VARIABLES scope wasn't added until CFMX, so my supposition on the 'intention' of the VARIABLES scope was incorrect. You really have to read the comment thread to see the fun we're having;) Hopefully we'll find an answer. Cutter