Re: question about pvAccess monitor

[renato sanhueza <renato.sanhueza@alumnos.usm.cl>]

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 <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.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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Renato Sanhueza Ulsen
Ing Civil Informática.