Re: [xml-dev] A schema defines a state space ... instances are sample st

[Michael Kay <mike@saxonica.com>]

I think the concept of "state space" can be quite helpful when doing database-style modelling (the schema of a database describes the set of permitted states of the database, and each state of the database corresponds to a state of the real world).

But somehow this kind of thinking seems to be more useful when we are talking about mutable state. With databases, change (from one valid state to another) is of the essence. That's not so true with XML.

Also, XML documents are often messages: "here is what you need to know", rather than models of the state of the world at a point in time. That doesn't make the terminology wrong, it just makes it, in my view, a bit less useful.

(Things also get complicated when a database records history. One state of the database then captures a sequence of states of the real world, and we can easily get confused whether we are talking about the state of the database or the state of the world.)

Michael Kay
Saxonica

> On 21 Aug 2016, at 14:20, Costello, Roger L. <costello@mitre.org> wrote:
> 
> Hi Folks,
> 
> Here is a simple schema:
> 
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";>
>    <xs:element name="color">
>        <xs:simpleType>
>            <xs:restriction base="xs:string">
>                <xs:enumeration value="red" />
>                <xs:enumeration value="green" />
>                <xs:enumeration value="blue" />
>            </xs:restriction>
>        </xs:simpleType>
>    </xs:element>
> </xs:schema>
> 
> Here is an instance: <color>red</color>
> 
> Here is another instance: <color>green</color>
> 
> Terminology:
> 
> 1. The XML Schema defines a state space.
> 
> 2. The instances shown are sample states.
> 
> Do you agree with this terminology?
> 
> I am reading a book [1] on modeling and "state space, sample states" is the terminology that the book uses. It dawned on me that the terminology also applies to the XML realm.
> 
> /Roger
> 
> [1] Software Abstractions: Logic, Language, and Analysis by Daniel Jackson
> 
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