Learning ColdFusion 8: CFThread's VARIABLES Scope Update (Thanks Dan G. Switzer, II)
The other day, I posted a tutorial on ColdFusion 8's new CFThread tag, specifically, how to get data into and out of the threads. I had misunderstood the documentation when it said that all threads on the page share the same VARIABLES scope. I had taken this to mean each CHILD thread. Thankfully, Dan G. Switzer, II pointed out my misunderstanding and I was able to change the previous CFThread blog post. What Dan pointed out to me was that all the threads do indeed share the same VARIABLES scope, but that the primary page (or parent page) is also included in that pool of threads. Meaning, all threads on a given page request share the same VARIABLES scope.
To demonstrate this concept, take a look at this:
<!--- Set a message into the VARIABLES scope. --->
<cfset VARIABLES.Message = "From Parent Page" />
<!--- Launch a child thread. --->
<cfthread
action="run"
name="ThreadOne">
<!---
Store a message into this thread's VARIABLES
scope. This is NOT the page's VARIABLES scope;
threads have their own version.
--->
<cfset VARIABLES.Message = "From Thread One!" />
</cfthread>
<!---
Let's wait for thread one to finish so that we know
exactly where all of our values are coming from.
Remember, these threads are asyncronous. In fact, they
might not even execute in the same order in which they
were defined.
--->
<cfthread
action="join"
name="ThreadOne"
/>
<!--- Launch a child thread. --->
<cfthread
action="run"
name="ThreadTwo">
<!---
Get the message from the VARIABLES scope
and store it into this thread's publically
accessible THREAD scope.
--->
<cfset THREAD.Message = VARIABLES.Message />
</cfthread>
<!---
Join the second thread to the current page process.
After this, all our child threads will have finished
executing (and in the same order as they were declared).
--->
<cfthread
action="join"
name="ThreadTwo"
/>
<!--- Output the message stored into thread two. --->
Thread Two: #ThreadTwo.Message#<br />
<!--- Output the message stored in the parent. --->
Parent: #VARIABLES.Message#
Notice that while I am launching two child threads, I am requesting that the parent page wait for each thread to finish processing before it executes the next. This effectively defeats the purpose of the CFThread tag, but it is necessary to show that each thread shares the same VARIABLES scope. Running the above code, we get:
Thread Two: From Thread One!
Parent: From Thread One!
What you can see by the output is that the first thread updates the message value. This change is reflected not only in the message available to the second thread, but also to the parent page.
Thanks Dan, you the man!
Want to use code from this post? Check out the license.
Reader Comments
Good notes about ColdFusion 8.
I enjoy reading your articles.
Keep it up.
Pinal Dave