Re: question about pvAccess monitor

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

Hi Renato,

Since EPICS hasn't had native RPC functionality at all before it was
added in pvAccess, we don't claim to know what the best way might be to
implement the introspection of RPC interfaces. We could have created a
standard while writing pvAccess, but we have found in the past that if
you try to make something generic before you have at least two and
preferably three or more different examples you often don't make the
result flexible enough to meet later needs.

I hope Diamond's "experiment" as Tom called it will help the community
eventually come up with an "official" approach, assuming of course that
there is enough demand for a standard that someone is willing to fund
the development work with either effort or money. If you have specific
requirements we would be interested in hearing about them.

Regards,

- Andrew


On 04/09/2018 02:13 AM, tom.cobb@diamond.ac.uk wrote:
> Hi Renato,
> 
> That is correct, I would class this as "experiments in how you might do this" rather than "the official way to do this".
> 
> Thanks,
> Tom
> 
>> -----Original Message-----
>> From: renato sanhueza <renato.sanhueza@alumnos.usm.cl>
>> Sent: 08 April 2018 04:47
>> To: Cobb, Tom (DLSLtd,RAL,TEC) <tom.cobb@diamond.ac.uk>
>> Cc: tech-talk@aps.anl.gov
>> Subject: Re: question about pvAccess monitor
>>
>> Hi Tom,
>>
>>
>> Your implementation of dynamic RPC using EPICS is very impressive.
>> Nonetheless, if I understand correctly, it is not part of the "official" version of
>> EPICS which I am studying right now.
>>
>>
>> Thanks for sharing this information,
>>
>> Renato
>>
>>
>> On Thu, Apr 5, 2018 at 9:45 AM, tom.cobb@diamond.ac.uk
>> <mailto:tom.cobb@diamond.ac.uk>  <tom.cobb@diamond.ac.uk
>> <mailto:tom.cobb@diamond.ac.uk> > wrote:
>>
>>
>> 	>       2. Can I use this introspection interface to retrieve the name of a
>> 	> service and its parameter list to compose a RPC during runtime by
>> following
>> 	> Marty's approach?
>> 	>
>> 	> I suspect that the answer for pvAccess itself is no.
>> 	>
>> 	> But even if the answer is no a channelRPC service can implement
>> it's own
>> 	> query service that describes itself.
>> 	> But it must describe itself via pvData.
>>
>> 	That is the approach that Diamond has taken in its continuous
>> scanning middleware:
>>
>> 	http://pymalcolm.readthedocs.io/en/latest/reference/structure.ht
>> ml <http://pymalcolm.readthedocs.io/en/latest/reference/structure.html>
>>
>> 	We define a Block to be a container for a number of Methods and
>> Attributes. Attributes are represented by NTScalars, NTScalarArrays and
>> NTTables, but with their metadata abstracted to separate Meta objects.
>> These same Meta objects are used to describe the arguments that a Method
>> should be passed when doing an RPC on it, and the structure it will return.
>> We implement this in Python, exposing the structures either over pvAccess
>> (currently with a Diamond specific fork of an old version of pvaPy, but we are
>> working on this) or as JSON over websockets.
>>
>> 	Thanks,
>> 	Tom
>>
>>
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>>
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>>
>> --
>>
>> Renato Sanhueza Ulsen
>> Ing Civil Informática.
> 
> 

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