Re: Link arrays / syntax

[Tim Mooney <mooney@aps.anl.gov>]

Steve Lewis wrote:

> At 7:44 AM -0500 2005/10/19, Marty Kraimer wrote:
>
> > Thus we can define
> >
> > block - wait until all previous record processing activity has completed
> > process - request the linked record be processed
> > wait - do not complete this link until the linked record has completed 
> > processing.
>
>
> I like that; think of
>   block  => a pre-condition
>   process => an attribute of the link
>   wait => a post-condition
>
> Also, whatever scheme is chosen, it should map nicely into a visual 
> representation in VDCT.
>
> Could we called the "linked to" record the "target" record?


I don't think viewing "block" as a precondition to link processing
captures enough of its capability, because it misses the code between
link invocations.  This is most clear for input links:

     read something (process target first; notify me on completion)
     do some calculation using the data we will eventually have read
     write something (block for outstanding callbacks before writing)

Block occurs too late to save the calculation from stale data, but
it would have been useful attached to the read link.  If block is
implemented as a precondition, I would expect the following almost
immediately:

     read something (process target first; notify me on completion)
 --> link to nothing (block before not linking to anything)
     do some calculation using the data we've now received
     write something ...



I think attaching "block" to links is just expedient; it doesn't
fit perfectly, but putting it there gets a useful capability into
*all* record types.

I just wonder how an end user is going to specify that a link
should wait or block.  Right now, I have a custom seq record that
allows the user to specify wait/nowait for each output link, but
of course this requires an extra field associated with each link.


--
Tim Mooney (mooney@aps.anl.gov) (630)963-1860
Beamline Controls & Data Acquisition Group
Advanced Photon Source, Argonne National Lab.