RE: Re: xAP - The Proposed Architecture Explained - the first extensio

["Mark Harrison" <Mark.Harrison@xxxxxxx>]

Patrick wrote (inter alia):

I also think you need to review the addressing mechanism. In your 
previous mail you spoke of the input plug-in using specifying the 
output-plugin address. I don't know if this is a typo, or what you 
intenteded, but the input device should simply identify the unique 
data source, nothing more. The output device is then configured to 
listen to a given data source. This is an important and subtle 
distinction.

--- It was a typo. The input device simply identifies the unique data
source. The output device is configured as to which data sources it's
listening out for.

Imagine that you have a caller id plugin. This plugin is dumb and 
broadcasts just the phone number. You want to display the name of the 
caller too. This requires cascading of plug-ins. The caller-id output 
is picked up by a name/address lookup device and then re-broadcast 
with additional information appended for display.

--- Yup - makes a lot of sense.

It is also common-sense to establish some standard HA data types, so 
that all devices represent certain attributes (e.g. temperature) in 
the same way. I would recommend a policy which includes a text device 
status field (for display use etc) and also includes value fields.

e.g. "Outside temp. 31'c"; out_temp_C=31.2

--- This is why I'm suggesting an XML-style model. What I've called
"value" is better as "display value", but other fields
can be broadcast.

This suggests that messages should have a header + body, with the 
body being hierarchial. It also suggests it might be useful to embed 
some kind of hop-count might be useful. Conveniently this meets most 
of the requirements of a bridging protocol too.

Finally, some thought needs to be given to group addressing or the 
concept that a device might belong to more than one address space. 
Eg. it may be desirable to send messages to all 
displays "downstairs". Where is this mapping maintained? Probably
as 
a service running fault-tolerant on one or more PC's with an LDAP 
backend.

- The messages are, in my view, simply broadcast. The displays downstairs
have to work out whether they want to do anything with the messages
received. This mapping, therefore is maintained in the config of the
display devices. (Which may, of course, have a central management tool /
console / database, but that's within the scope of the set of display
devices possible.)

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