Re: [xml-dev] What is this principle called: "I' send data in my UOM and

["W. E. Perry" <wperry@fiduciary.com>]

Hi Roger.

I'm here, reading xml-dev as I have pretty regularly these 13+ years.

I'm not sure that there is an established or accepted name for the principle which you describe. I refer to it as "In markup, you are what you produce, not what you consume." In the example which you give, the system producing an output of "1/3 meter" cannot know what other systems might consume that data, nor for what purposes, and certainly cannot be responsible for unknown systems getting data in a form or to a precision which they require. The 'contract' in markup is that each system produce--and in the usual case, publish--output in a form which is knowable and identifiable with that producing system, however idiosyncratic. Other systems which need to consume that data, for whatever purpose, are responsible for fetching it and then instantiating it in whatever form they require internally. This is the converse of the object-oriented interface, where the
'contract' is "if you pass me the precise data structure which I require, I promise to execute my particular operations upon it". The principle in markup is that the consumer is responsible for the instantiation of the particular data which it requires (and in markup there is *always* a parse and a particular instantiation required--no non-trivial system consumes markup directly). It is not the input interface which is public, but rather the output. Thinking this way can be difficult, and particularly so if you hope to find interoperability inhering in the object, or in whatever is the text or data exchanged between systems. In markup the particular instantiation of data is a local and a private matter, precisely because such interoperability as we might achieve is based on parsing a general form into a specific one. The particular, fully-specified form is local
to each system; what systems exchange is a more general form which may be locally instantiated into widely differing particulars.

Respectfully,

Walter Perry


"Costello, Roger L." wrote:

> Hi Folks,
>
> Suppose that every member in a community is required to exchange data using one unit of measure. For example, suppose length is required to be expressed as a decimal value in centimeters. Consider a system that internally deals with length measurements in meters. Suppose the system wishes to exchange the value "1/3 meter." Recall that the system is required to exchange values in decimal centimeters. However, there is no exact conversion of "1/3 meter" to centimeters. Should the system send the value 33 cm, 33.3 cm, 33.33333 cm? In the general case, the sending system has no insight into the precision required by recipients. Exchanging the wrong amount of precision could be catastrophic. There is a principle that a sender should transmit its original data (e.g., 1/3 meter); it is up to recipients to convert the data, if necessary, to the precision they require.
>
> What's the name of the principle? I remember someone on this list talking about it long ago. Perhaps Len? Perhaps Walter Perry? (BTW, anyone know the whereabouts of Walter?)
>
> /Roger