Re: passive record/field to process when there is a CA read

[Andrew Johnson <anj@aps.anl.gov>]

Dehong Zhang wrote:
> What we have in mind is a simple case: if no CA client is "getting" data from 
> a PV, the PV can remain un-processed -- processor does nothing.  It can be
> waiting for events or PP etc.  Once a CA client tries to get/read data, the EPICS
> internal part checks to see whether there is a recent-enough copy, just return
> the data if yes, or process then return if the current data is too old.
>
> So we could have a scan option, which does "process upon CA request but not
> more than * Hz".  This way the IOC is protected.
>
> Without such an option, the user would need 2 actions like Dirk pointed out,
> or the processor has to waste some cycles to always make sure the data is
> current, either by listening to events or process periodically.

There is a significant difference between get and monitor behavior in 
Channel Access, and from your description you should be talking about 
monitors rather than gets.  Any long-running client program should be 
setting up monitors rather than polling for updates using ca_get(), and 
all the main EPICS extensions such as MEDM, EDM, ALH do that.  If that's 
true for your case you should be able to use the technique described on 
slide 33 of this lecture:

http://www.aps.anl.gov/epics/docs/USPAS2007/lectures/Database2.pdf

This technique is not guaranteed to be future-proof, but it works in all 
current releases and I'm not expecting it to go away soon. The important 
part is the subroutine record that can detect whether there are any CA 
monitors connected to itself; if you have control over the client 
programs or display screens that you want the database to detect, you 
only need to arrange that they monitor that subroutine record, and now 
the IOC can alter its behavior depending on whether anyone's listening 
or not.

HTH,

- Andrew
--
When a distinguished but elderly scientist states that something is
possible, he is almost certainly right.  When he states that something
is impossible, he is very probably wrong.  -- Arthur C. Clarke