Providing policy around messages (was Re: Feedback from UHKA 2003)

[Patrick Lidstone]

> The other thing that came out from the developer side (for
example
> David Buckley) is that the flexibility of xAP (which he loves)
creates a
> problem in quite what 'xAP support' means and the complexity of
supporting
> 'devices' within an application like HomeBrain or Autom8it for
example. Over
> the discussion I think we saw a need to define for a 'base level'
device a
> few parameters that are mandatory in every device type message
including an
> ON/OFF status and a level status expressed as a percentage
probably - this
> maps well to most (almost every) type of controllable device. In
this way
> every xAP device could be represented within an application
assuming it
> included a
>
> Mode=ON
> Level=20%
>
> Or something similar - perhaps raising the issue yet again of class
> inheritance within the way we structure class.type section.sub etc
>
> Dwelling on my conversation with David B (and Patricks' below)
there is some
> validity to the end to end integrity worry - not so much an issue on
> ethernet but certainly on lower speed (RS485/232) or bridged
networks.
> Sequencing is obviously an option but as P says below this raises a
whole
> range of other issues to do with the broadcast/targeted model. We
should
> talk this through though.

I think this is one area where XPL have actually done things quite
well (*). There is scope for providing a policy layer on top of the
existing infrastructure which bridges the gap between the
"formless"
nature of xAP messages at the transport layer and strong typing of
schema. This could be as simple as defining a policy for inheritance
within xAP, and then defining one or two base schema along the lines
that Kevin is suggesting which allow simple actions to be performed
on a class device. The risk of this approach is that we end up losing
the elegant abstraction and interoperability that you get with schema
because it encourages schema to be created on demand.

I'd also like to extend the xAP software frameworks to allow pre-
defined schema to be used as templates, pinching David Buckleys ideas
again. Integrating XSLT support would also be nice (with a mapping
between xml and xAP messages by rule) - this might make the whole xAP
framework more accessible?

Tell me what you think!

Patrick

(*) It is pretty clear that xAP and XPL are not going to manage to co-
operate any time in the near future, which is a great pity, so we
just have to get on and do our own thing...