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<== Date ==> <== Thread ==>
Subject: Re: evaluation of OMG DDS as a controls protocol for EPICS
From: Claude Saunders <[email protected]>
To: "Lawrence T. Hoff" <[email protected]>
Cc: [email protected]
Date: Thu, 15 Feb 2007 09:46:40 -0600
I've been watching DDS from the sidelines as well, and only recently decided to take the plunge. It really has not achieved widespread support, and I'm not sure if that's because of limitations like those I discovered, or because it didn't reallly follow what I would consider a normal progression towards being a standard. It started out as proprietary, then became an odd API-only "standard" through OMG for some time. Clearly not interoperable. Only later was a wire protocol released. Even so, there are so many options and extensible areas of the specification that it seems unlikely initial implementations will play well together. The DDS spec is really quite recent (2004), so if CORBA is any guide, we should see this mature in about 10 years :)

I only looked at a few of the QoS capabilities implemented by ORTE, but the RTPS specification is an interesting read in this area, and can give lots of ideas. For example, they consider timestamping of messages a QoS option. Also, you can have multiple publishers of the same message type on the same topic, each with a relative "strength". Subscribers can indicate that they want a message from strength "2" publishers first, unless some time window passes without a message, in which case the subscriber will accept a message from strength "1" publishers. So in this manner one could implement a form of failover. The specification also has something called "Content-Filtered Topics", which is much like the "beam flavor subscriptions" that Jeff has been talking about for CA. The filtering can be implemented on the subscriber side, or on the publisher side (preferred, but more complex). There is also something called "Coherent Sets", which seems to be similar to the channel access "sync groups". At any rate, it's interesting to find the parallels. I think DDS makes many of these concepts more explicit.

Just out of curiosity, what other RTI S/W have you used? Other than NDDS, I'm only aware of their SkyBoard product (which is very odd and interesting at the same time).

regards, Claude

Lawrence T. Hoff wrote:

Thanks for sharing, Claude!

I have been watching DDS "from the sidelines" for some time now.

Ignoring the technical details, my chief reservation is that DDS does not
seem to be achieving widespread support, despite active marketing by RTI.
Disclaimer: I am actually a very satisfied user of other RTI S/W, but trying
to use a communication standard that is not widely adopted gives me visions
of trying to make hotel reservations in Esperanto. :-)

   On the plus side, DDS does provide "connection management" (built
on top of UDP/IP), and (more importantly, perhaps) includes some level
of support for user-defined Quality of Service (QoS) parameters when
establishing connections.

   Did you learn anything about DDS's QoS capabilities that could
(theoretically) be applied to CA?

-- Larry


Replies:
Re: evaluation of OMG DDS as a controls protocol for EPICS Kay-Uwe Kasemir
Re: evaluation of OMG DDS as a controls protocol for EPICS Lawrence T. Hoff
References:
evaluation of OMG DDS as a controls protocol for EPICS Claude Saunders
Re: evaluation of OMG DDS as a controls protocol for EPICS Lawrence T. Hoff
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