I think it is because a 'config element' is processed once when the test plan 
is loaded, but a 'pre-processor' is processed before each sample is run. There 
is a similar 'config element' called "User Defined Variables" that are 
processed at load-time for the test case. But bear in mind, if you use these - 
only the last one in the whole test plan is valid for a given variable. 
Whereas, if you use "User Parameters" those are valid for a specific sample 
based on the how they are defined as the test case is processing.

For example, say you have a test plan that looks like this:

User Defined Variables: username = foobar
Thread Group 1: (uses '${username}' variable)
User Defined Variables: username = notfoobar

In this case, when you run the test plan, the ${username} variable will be set 
to "notfoobar" - because the "User Defined Variables" are all processed 
_before_ the thread group runs - and the value is set to the last one that 
defined it.

In example 2, we see User Parameters at work:

Thread Group 1:
                User Parameter: username = foobar${__threadNum}
                Sampler A: (uses ${username} variable)
Thread Group 2:
                User Parameter: username = notfoobar${__threadNum}
                Sampler B: (uses ${username} variable)

In this example, Thread Group 1 will increment the 'username' value through 
"foobar1", "foobar2", etc. for as many different threads as you run through it. 
Likewise, thread group 2 will increment the username variable through 
"notfoobar1", "notfoobar2", etc. because the User Parameters are evaluated at 
the sample run time.    

--
Robin D. Wilson
Sr. Director of Web Development
KingsIsle Entertainment, Inc.
VOICE: 512-777-1861
http://www.kingsisle.com

-----Original Message-----
From: Shmuel Krakower [mailto:shmul...@gmail.com] 
Sent: Monday, March 24, 2014 3:03 AM
To: JMeter Users List
Subject: User Parameters Pre Processor

Hi,
I was wondering why the User Parameters is a pre-processor rather than a Config 
Element?
I just spent few minutes looking for it in the wrong place... ;)

Best,

Shmuel Krakower.
www.Beatsoo.org - re-use your jmeter scripts for application performance 
monitoring from worldwide locations for free.


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