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Subject: Re: EPICS communication paradigm
From: Mark Rivers <rivers@cars.uchicago.edu>
To: renato sanhueza <renato.sanhueza@alumnos.usm.cl>
Cc: "tech-talk@aps.anl.gov" <tech-talk@aps.anl.gov>
Date: Sat, 11 Nov 2017 23:36:03 +0000
I'm not a computer scientist by any means.  But I think it is fair to say that Channel Access operates at a fairly low level of abstraction.  It deliberately hides the "object" nature of EPICS records.  It only provides access to specific fields in those records.  So there is no capability of introspection of the record structure, etc.


There is a new EPICS protocol called pvAccess, commonly known as EPICS V4.  It provides a much higher level of abstraction, and is designed with middleware services as one of its target applications.  It allows the transmission of arbitrary structured data with full introspection.


Mark



________________________________
From: renato sanhueza <renato.sanhueza@alumnos.usm.cl>
Sent: Saturday, November 11, 2017 4:28 PM
To: Mark Rivers
Cc: tech-talk@aps.anl.gov
Subject: Re: EPICS communication paradigm

Hi Mark,

Thanks for your answer. You give a good summary of the whole communication process. I was asking about the communication paradigm as an attempt to determine in which level of abstraction EPICS works.

For example the Message Passing Paradigm is associated with the lowest level of abstraction in distributed inter-process communications. Message Systems Paradigm (aka Message Oriented Middleware) works in a higher level of abstraction than Message Passing, but lower than Distributed Object Paradigm.

EPICS use the Channel Access Protocol to communicate clients and servers, but I couldn't intuitively map this protocol to any of the "common" communication paradigms I know (besides the Distributed Objects Paradigm).

On Sat, Nov 11, 2017 at 6:59 PM, Mark Rivers <rivers@cars.uchicago.edu<mailto:rivers@cars.uchicago.edu>> wrote:
Hi Renato,


What you said is basically correct.   An EPICS control system typically consists of multiple channel access servers located on multiple computers.  There are also multiple channel access clients located on multiple computers.  The clients normally locate EPICS process variables (PVs) on the servers using a UDP broadcast with the PV name, and then connect to them with TCP.  They can then subscribe to callbacks from the servers whenever the PV changes (publish/subscribe model), and can write new values to the PVs.

I'm not sure if this answers your question?

Mark


________________________________
From: tech-talk-bounces@aps.anl.gov<mailto:tech-talk-bounces@aps.anl.gov> <tech-talk-bounces@aps.anl.gov<mailto:tech-talk-bounces@aps.anl.gov>> on behalf of renato sanhueza <renato.sanhueza@alumnos.usm.cl<mailto:renato.sanhueza@alumnos.usm.cl>>
Sent: Saturday, November 11, 2017 1:05 PM
To: tech-talk@aps.anl.gov<mailto:tech-talk@aps.anl.gov>
Subject: EPICS communication paradigm

Hi, I am learning about EPICS and I think I understand the basic concepts about it but I am still unable to determine in which communication paradigm is EPICS based on.

My best guess is that EPICS implements the Distributed Object Paradigm because Records are object distributed across different Channel Access Servers. Other technologies which implement this paradigm are CORBA and JavaRMI for example.

I would really appreciate if someone can point me in the right direction.

Thanks in advance
--
Renato Sanhueza Ulsen
Ing Civil Informática.



--
Renato Sanhueza Ulsen
Ing Civil Informática.

Replies:
Re: EPICS communication paradigm renato sanhueza
References:
EPICS communication paradigm renato sanhueza
Re: EPICS communication paradigm Mark Rivers
Re: EPICS communication paradigm renato sanhueza

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