Ask Ben: ColdFusion Variable Caching And How To Use It (Simple Demo)
There is no particular question here. I have been asked about variable caching in ColdFusion and I felt a demo would explain what a back-and-forth email conversation could not. This is a demonstration of very simple variable caching in a small ColdFusion application. Here, we are caching a value (a Struct in this case) in the APPLICATION scope. We are then referencing it on the index.cfm page.
The demo has two pages. The first is our Application.cfm page. I used the Application.cfm rather than the Application.cfc as I feel for demonstration purposes, there is less "noise" in the Application.cfm page. Then, we have our index.cfm page. The index page merely references the cached variable and displays the data.
<!--- Kill extra output. ---> <cfsilent> <!--- Define the application. Here, I am setting a very small Application timeout (since I am on the work server) - In reality, this would either be left out (using CFAdmin default) or 2 days or something. ---> <cfapplication name="SimpleCacheDemo" applicationtimeout="#CreateTimeSpan( 0, 0, 5, 0 )#" sessionmanagement="false" setclientcookies="true" /> <!--- Set request settings. ---> <cfsetting showdebugoutput="false" /> <!--- We need to check to see if our application is initialized. For convenience, I am going to store an Applcation variable that flags for initialization (DateInitialized). If this value does NOT yet exists OR the user has flagged a reset via the URl, we are going to initialize the application. NOTE: By initializing the application using the Application.cfm (or Application.cfc) we ensure that the application get initialized no matter what page in our application gets requested. ---> <cfif ( (NOT StructKeyExists( APPLICATION, "DateInitialized" )) OR StructKeyExists( URL, "resetapp" ) )> <!--- Either a re-init of the application was requested, or the app has not yet been requested. Since we are potentially updating live, shared memory (APPLICATION), throw a Lock on it to force a single thread. ---> <cflock scope="APPLICATION" type="EXCLUSIVE" timeout="20"> <!--- Since we are in a multi-threaded environment, it is possible that why we were locking, someone else swooped in an performed the initialization before we could do it. Therefore, we need to perform a double-check lock (pattern) to see if initialization is still required. However, if we have requested a manual re-init, then we are going to proceed no matter what. ---> <cfif ( (NOT StructKeyExists( APPLICATION, "DateInitialized" )) OR StructKeyExists( URL, "resetapp" ) )> <!--- We see that, indeed, the application still is not initialized (DateInitialization flag has not been set). We can now safely proceed with application initialization. ---> <!--- Clear the existing Application to make sure we don't have any old data. ---> <cfset StructClear( APPLICATION ) /> <!--- Flag initialization. ---> <cfset APPLICATION.DateInitialized = Now() /> <!--- Now, we are going to cache some objects. For now, we are simply caching built in CF data types for demonstration. However, these could easily be ColdFusion components (or even Java objects) created using the CreateObject() function. ---> <!--- Cache this struct. By declaring the struct directly in the APPLICATION scope, we are caching in the APPLICATION scope. This struct will exist for as long as the Application does (or until the scope is cleared or the variable is deleted). ---> <cfset APPLICATION.Actress = StructNew() /> <!--- Set values in the struct. ---> <cfset APPLICATION.Actress[ "Christina" ] = "Cox" /> <cfset APPLICATION.Actress[ "Lori" ] = "Petty" /> <cfset APPLICATION.Actress[ "Linda" ] = "Hamilton" /> <cfset APPLICATION.Actress[ "Angella" ] = "Bassette" /> <!--- You could perform as much cacheing as you want at this point. I am only caching one object for demonstration purposes. ---> </cfif> </cflock> </cfif> </cfsilent>
<html> <head> <title>Simple ColdFusion Cache Demo</title> </head> <body> <h1> Here Are Your Cached Actresses (Lucky You!) </h1> <cfoutput> <!--- Loop over the actresses in the struct. We can reference the cached struct directly in the APPLICATION. Technically we are referencing shared memory, and there are those that would argue this requires a CFLock to be used... however, I am of the mind set that even the "worst case" scenario in this case will not be bad - therefore, no lock. ---> <cfloop item="FirstName" collection="#APPLICATION.Actress#"> <p> #FirstName# #APPLICATION.Actress[ FirstName ]# </p> </cfloop> </cfoutput> </body> </html>
I am sure this is pure review for many of you, but it may be new to even more of you. Please let me know if any further explanation is needed.
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Nicely done, Ben. One clarification: the cflock and double-check for if NOT StructKeyExists( APPLICATION, "DateInitialized" ) is *only* necessary because the StructKeyExists( URL, "resetapp" ) is included, correct? If we didn't want to allow re-initialization of the Application-scoped variables via the URL (for security reasons), then the cflock and double-check are unnecessary in CF 6.1+.
I actually made one small mistake. In the inner CFIF statement, I forgot to check for the manual reset. I have updated the code (see the CFIF inside the CFLock). If someone requests a manual reset, the DateInitialized IS going to be present which is why the original code wouldn't work.
And to answer your question, Yes, the double-check locking is required ONLY because of the manual reset. If this code was fired because the application was just starting up, theoretically, this would be a single-threaded environment. But actually, as I am even typing this, I am not so sure. Because these are based on page requests (the Application.cfm does not get processed until a page is requested) then I actually don't see why two simultaneous requests couldn't cause conflicts - see, checking for DateInitialized existence and then setting the value has some time between it (nanoseconds??). Is it possible for two threads to overlap in this action???? I was have to say Yes to be cautious.
So, long story short, when using Application.cfm, I would recommend always doing the double-check locking. But, if you are using the Application.CFC with OnApplicationStart(), ColdFusion takes care of single-threading that on application start. Of course, if you put a manual reset in that, I would again, perform a double-check lock.
Why not perform the CFLOCK first and then do the check to make sure the app is not already initialized. If it is then bail on the lock otherwise complete the initialization action?
If you do the CFLock out side of the CFIF statements that you force a portion of the Application.cfm to become single-threaded regardless of whether or not any app-initialization is going to take place. CFLock comes with slight overhead; for small sites, this might not matter, however, for a heavily trafficked site, this slight overhead on every single page request might start to slow things down.
Would people notice the slow down? Not sure, can't say. But, as a generally good practice, I would say you want to avoid locking whenever possible.
Hi, slightly off topic but i noticed you use StructKeyExists alot instead of isdefined(), is there really much difference. Ie ,y friend told me not to use isdefined() anymore as it checks all the scopes and used more resources but is there really much difference?
IsDefined() works. But, personally, I think it is a "weak" statement. I like my ColdFusion code to be as assertive and descriptive as possible. If I know that a key should or should not exist in a given Struct, then I use StructKeyExists() because it is a very bold statement. There's something about IsDefined() that seems almost like a cop-out; it's like you're not even sure if it will be in a given struct or not.
It's a personal choice, mostly. But, I have heard that StructKeyExists() is faster.
Thanks for writing this.
I needed a way to Clear all persistent Application variables this worked great.
<cfset StructClear( APPLICATION ) />
However can I have this in a non application.cfm file for it to work?
You should be able to put that anywhere in your application (since APPLICATION is a globally accessible scope). However, you're probably gonna want to do it right before you initialize the Application scope, otherwise you might get "undefined" variable errors in the rest of the page request.