How can I communicat​e betwen parallel asynchrono​us VIs?

Question

Howdy all,
Here's my situation:
- I have some task that needs to be executed in parallel to the currently running VI. In fact, this task needs to executed in parallel to a number of VIs,
so I've decided to build a sub-vi to handle this task.
- This task involves performing some processes every X number of milliseconds (usually in multiples of 1000). This task will be performed until it receives a stop-command (the user generates this signal - so I have no way of knowing when it will be)
- I still need to be able to handle user interaction with this main VI while the task is executing, so my task must execute asynchronously.
- I want to be able to send a termination command to my asynchronously executing task, and have it respond IMMEDIATELY (polling a global variable is unnacceptable).
The only way that I am aware of running a VI asynchronously is to start it with the VI Server (either the Call by Reference Node, or the Invoke Node).
Originally I planned on using notifiers to send commands to the asynchronous VI. Notifiers would have been perfect, because I could break out of the waiting portion of the task as soon as I received the command (this isn't really possible when polling a global). Later I found out that notifiers are not supposed to work with VIs loaded with the VI Server. My notifier approach worked fine during testing, but wouldn't work when I compiled the program as an executable (which is what the end product will be, so I need a new solution).
I'm about to change my implementation to use Queues instead of notifiers, because I think I can achieve the same "Wait on Notification" functionality with a queue, except I don't think there is a restriction on using a Queue across the VI Server.
My Questions are:
Is there another way to get VIs to execute asynchronously OTHER than the VI Server?
Is there a better way to communicate between them? (I considered using a User Event control, and generating the commands using that, however I can't poll the status of the User Event without an Event structure, preventing me from terminating my task once the user generates the "end-task" request.
Does anybody else find the LabVIEW documentation to be frustratingly lacking at times?
Any help, comments, or stories of similar situations would be appreciated.

Answer 1

tmh,
No, I'm pretty sure you understand perfectly, as that's exactly what I wanted to do. I knew that if you had two loops on a block diagram, they would execute in parallel, but I assumed that if you had a loop and a SubVI on a block diagram, that whichever one started executing first would have to terminate first. I was making the problem much more difficult then it needs to be, as that method would allow me to use notifiers to achieve the intended effect.
As far as notifiers 'not working with the VI Server' as I must have put it, I got that straight from the LabVIEW help files. Under the topic "Notifier Operation Functions" I read that "You cannot use notifiers to communicate with VIs on other computers. For example, you cannot use notifiers to communicate across a network or the VI Server."
Originally, I was using a named notifier. So, my "master" vi would create a notifier to send commands with, then it would launch the "slave" vi asynchronously with an invoke node, and send commands when necessary. The Slave obtained the notifier by using the "Obtain Notifier" vi, supplying the same name as the Master did for the name parameter. This worked fine when I was testing - however once I built my program to be an executable, the Slave created a new notifier, rather than obtaining the same one being used by the Master. (I'm not 100% sure that's the exact reason - it may just not have received commands [it was a while ago that I was playing with that approach]. But I do know that that method works fine in an uncompiled program, but once it's compiled, it stops working. Who knows - there may exist actual ways to pass the notifier, like a global variable or something like that).
According to the *fabulous* LabVIEW documentation, Queues don't have this limitation, so I rewrote my VI to use a queue of length one, rather than the notifier.
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(does anyone find that the LabVIEW documentation can be frustratingly lacking at times? Like, why can't I use notifiers across the VI server? And why can I use queues? I'm not asking for them to show me the guts of LabVIEW, I just think that it would give developpers more control and confidence when coding that they know what's going on, and have at least a general idea of how things work)
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However, somewhere in my switch between Queues and Notifiers, I must have messed up something, as after that worked fine during the testing phase, after a compilation, I received an error (1013 I believe) whenever I tried to load my slave saying "Front Panel Heap Dump" - which is strange because the front panel of my slave should not have been loading, and was supposed to have been removed from the executable.
Not that that last bit is super relevent to the original topic of discussion, but it might be useful to someone later on.
In any event, thank you for your help. I suspect that that will be the perfect solution for my needs, and when I get back to that chunk I was working on and get it working, I'll let you know.
Thanks again.