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<== Date ==> <== Thread ==>

Subject: Re: Question about CA event (i.e. monitor) time order
From: Andrew Johnson <anj@aps.anl.gov>
To: tech-talk@aps.anl.gov
Date: Thu, 5 Jul 2012 12:00:14 -0500
Hi Dirk,

On 2012-07-05 Dirk Zimoch wrote:
> 
> Consider a client that subscribes for two PVs (here in particular two
> fields of the same record). Now the IOC (the record support) posts
> events on both fields. Is the time order of these two events preserved?
> 
> In other words, are the monitor callbacks of the client called in the
> same order as the IOC calls db_post_events()?
> 
> In my particular case, I have a client (with unknown source code) that
> sometimes sees value changes on two fields of the motor record, DMOV and
> RRBV, in the wrong order. Thus in the DMOV callback, RRBV still has the
> old value. I checked in the motor record that DMOV is always posted
> after RRBV.
> 
> Is there any mechanism in EPICS that could swap the execution order or
> should the problem be searched in the client?

Yes, the IOC server's event queue is not strictly ordered before R3.15.  This 
issue is described in the tech-talk email thread pointed to by the "full 
specification" link on this page:

https://blueprints.launchpad.net/epics-base/+spec/strict-ordered-event-queue

- Andrew
-- 
Never interrupt your enemy when he is making a mistake.
-- Napoleon Bonaparte

References:
Question about CA event (i.e. monitor) time order Dirk Zimoch

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