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Re: [xsl] How to design XPath queries and XSLT code that can be readily repurposed?

Subject: Re: [xsl] How to design XPath queries and XSLT code that can be readily repurposed?
From: "G. Ken Holman g.ken.holman@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Fri, 2 Aug 2019 18:34:31 -0000
Regardless of what data techniques you decide to use, look into the use of <xsl:import> and the writing of "core" stylesheet libraries that are exploited by "shell" stylesheet contexts that deliver results based on the core.

Here is a case study of a stylesheet library created for the US intelligence community. How can multiple organizations (CENTCOM, STRATCOM, Coast Guard, etc.) get multiple renderings of one piece of XML without having to rewrite all of code? By putting the shared logic in a shared library and putting the specific logic in a shell (many, many, shells each adding more specialization to the core):

http://web.archive.org/web/20071113054804/http://www.idealliance.org/proceedings/xml05/ship/18/Holman-18.HTML

The (dreaded) XML namespaces is your friend here: build the library using namespaces on every possible global construct (keys, variables, template rules, etc.) to qualify the construct name in order to avoid accidental collision by someone writing a shell and not knowing the names of the constructs used in the library.

In fact I used two namespaces in the core: one for constructs that are used internally by the core and not expected to be touched by the shells, and one for constructs in the core available to be tweaked by a shell. All to prevent future inadvertent behaviours because the developers writing the shells are not always the developers writing the core. The core can be made an effective "black box" by using namespaces.

To document all this I created an XSLT documentation methodology named XSLStyle to enforce my stylesheet writing rules regarding the appropriate use of namespaces on global constructs. At the same time it produces summary documentation:

https://cranesoftwrights.github.io/resources/#xslstyle

And the documentation produced illuminates the import/include hierarchy and produces an alphabetized index of all of the global constructs.

I hope this is helpful.

. . . . . . Ken

At 2019-08-02 18:08 +0000, Costello, Roger L. costello@xxxxxxxxx wrote:
Hi Folks,

How to design XPath queries and XSLT code so that a complete rewrite isn't required every time there is a small change in requirements? Stated another way, how to design XPath queries and XSLT code that can be readily repurposed? Allow me to explain what I mean with a concrete example, please ...

I have three XML files:

1. routes.xml ... this file shows the routes that cars take to travel between two cities.
2. rest-stops.xml ... this file shows the rest stops on routes.
3. gas-stations.xml ... this file shows the location of gas stations. Some rest stops have gas stations, others don't.

My initial task was to display the gas stations at rest stops on the routes from Boston to NYC. I wrote some XPath queries and XSLT code. It worked - yea!

Then I was tasked to display, for each route, the gas stations at rest stops. For example, display the gas stations at the rest stops on route I-84, display the gas stations at the rest stops on route I-95, and so on.

I was shocked at the amount of re-coding that I had to do for the second task. After all, in the first task I had already figured out how to identify the rest stops along each route, which rest stops have gas stations, and so on. The second task should have been a simple matter of reusing the XPath query results from the first task to display the results in a slightly different way. But no, I had to do a complete rewrite.

And now I am getting tasked to display the data in still another way - show the data as triples (route, rest stop, gas station). Eek! I can already see that I will have to do a third rewrite.

Clearly my XPath queries and XSLT code are not suitable for repurposing. I remember reading long ago a book about this issue of writing code in a way that when a new requirement is introduced it doesn't require a complete rewrite. Wish I could remember what the book's solution to the problem is.

How can I design my XPath queries and XSLT code in a way that they can be readily repurposed?

Here is routes.xml

<Routes>
    <row>
        <IDENT>I-84</IDENT>
        <Start-End>BOS-NYC</Start-End>
    </row>
    <row>
        <IDENT>I-95</IDENT>
        <Start-End>BOS-NYC</Start-End>
    </row>
    <row>
        <IDENT>RTE-1A</IDENT>
        <Start-End>Bangor-Miami</Start-End>
    </row>
</Routes>

The IDENT element is a primary key and also a foreign key into the rest stop file. The rest stops for a route with IDENT i are those rows in rest-stops.xml with the Road element equal to i.

Here is rest-stops.xml

<Rest-Stops>
    <row>
        <Name>Willington</Name>
        <Road>I-84</Road>
        <Direction>south</Direction>
        <Gas>BP-Willington-South</Gas>
    </row>
    <row>
        <Name>Willington</Name>
        <Road>I-84</Road>
        <Direction>north</Direction>
        <Gas>BP-Willington-North</Gas>
    </row>
    <row>
        <Name>Stormville</Name>
        <Road>I-84</Road>
        <Direction>north</Direction>
    </row>
    <row>
        <Name>Southington</Name>
        <Road>I-84</Road>
        <Direction>south</Direction>
        <Gas>Exxon-Southington</Gas>
    </row>
    <row>
        <Name>Branford</Name>
        <Road>I-95</Road>
        <Direction>south</Direction>
        <Gas>Mobil-Branford-South</Gas>
    </row>
    <row>
        <Name>Branford</Name>
        <Road>I-95</Road>
        <Direction>north</Direction>
        <Gas>Mobil-Branford-North</Gas>
    </row>
    <row>
        <Name>Darien</Name>
        <Road>I-95</Road>
        <Direction>north</Direction>
    </row>
</Rest-Stops>

The Gas element is a foreign key into the gas station file. The gas station at rest stop r is the row in gas-stations.xml with the IDENT element equal to r.

Here is gas-stations.xml

<Gas-Stations>
    <row>
        <IDENT>BP-Miami</IDENT>
        <Gas>Shell</Gas>
        <Location>Miami</Location>
    </row>
    <row>
        <IDENT>BP-Washington</IDENT>
        <Gas>BO</Gas>
        <Location>Washington</Location>
    </row>
    <row>
        <IDENT>BP-Willington-North</IDENT>
        <Gas>BP</Gas>
        <Location>Willington</Location>
    </row>
    <row>
        <IDENT>BP-Willington-South</IDENT>
        <Gas>BP</Gas>
        <Location>Willington</Location>
    </row>
    <row>
        <IDENT>Shell-Willington</IDENT>
        <Gas>Shell</Gas>
        <Location>Willington</Location>
    </row>
    <row>
        <IDENT>Exxon-Southington</IDENT>
        <Gas>Exxon</Gas>
        <Location>Southington</Location>
    </row>
    <row>
        <IDENT>Mobil-Branford-North</IDENT>
        <Gas>Mobil</Gas>
        <Location>Branford</Location>
    </row>
    <row>
        <IDENT>Mobil-Branford-South</IDENT>
        <Gas>Mobil</Gas>
        <Location>Branford</Location>
    </row>
</Gas-Stations>

Here are the results for the first task (show the gas stations at rest stops on the routes from Boston to NYC):

<Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>
    <row>
        <IDENT>BP-Willington-South</IDENT>
        <Gas>BP</Gas>
        <Location>Willington</Location>
    </row>
    <row>
        <IDENT>BP-Willington-North</IDENT>
        <Gas>BP</Gas>
        <Location>Willington</Location>
    </row>
    <row>
        <IDENT>Exxon-Southington</IDENT>
        <Gas>Exxon</Gas>
        <Location>Southington</Location>
    </row>
    <row>
        <IDENT>Mobil-Branford-South</IDENT>
        <Gas>Mobil</Gas>
        <Location>Branford</Location>
    </row>
    <row>
        <IDENT>Mobil-Branford-North</IDENT>
        <Gas>Mobil</Gas>
        <Location>Branford</Location>
    </row>
</Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>

In my XSLT program, I stored the three files in three variables:

<xsl:variable name="routes" select="doc('routes.xml')" />
<xsl:variable name="rest-stops" select="doc('rest-stops.xml')" />
<xsl:variable name="gas-stations" select="doc('gas-stations.xml')" />

I selected the rows in $routes that are for BOS-NYC:

<xsl:variable name="BOS-NYC-routes"
        select="$routes//row[Start-End eq 'BOS-NYC']"
        as="element(row)*"/>

I queried for the rest stops on those routes:

<xsl:variable name="rest-stops-on-BOS-NYC-routes" as="element(row)*">
    <xsl:for-each select="$BOS-NYC-routes">
        <xsl:variable name="route" select="." as="element(row)" />
        <xsl:sequence select="$rest-stops//row[Road eq $route/IDENT]" />
    </xsl:for-each>
</xsl:variable>

Not all the rest stops have gas stations; I queried for those with a Gas element:

<xsl:variable name="rest-stops-on-BOS-NYC-routes-with-gas-station"
        select="$rest-stops-on-BOS-NYC-routes/self::row[Gas]"
        as="element(row)*"/>

I then used the Gas element as a foreign key into gas-stations.xml:

<xsl:variable name="gas-stations-on-BOS-NYC-routes" as="element(row)*">
    <xsl:for-each select="$rest-stops-on-BOS-NYC-routes-with-gas-station">
        <xsl:variable name="rest-stop" select="." as="element(row)" />
        <xsl:sequence select="$gas-stations//row[IDENT eq $rest-stop/Gas]" />
    </xsl:for-each>
</xsl:variable>

Lastly, I displayed the results (gas stations at rest stops on routes from Boston to NYC):

<Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>
    <xsl:sequence select="$gas-stations-on-BOS-NYC-routes" />
</Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>

The complete XSLT program is shown below. I also show the code to implement the second task (display, for each route, the gas stations at rest stops); notice that the code is essentially a complete rewrite of the first task. How can I design the XPath queries and XSLT code so that I don't have to do a complete rewrite every time there is a requirement change? /Roger

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
    xmlns:xs="http://www.w3.org/2001/XMLSchema";
    exclude-result-prefixes="xs"
    version="3.1">

<xsl:output method="xml" />

    <xsl:variable name="routes" select="doc('routes.xml')" />
    <xsl:variable name="rest-stops" select="doc('rest-stops.xml')" />
    <xsl:variable name="gas-stations" select="doc('gas-stations.xml')" />

<xsl:template match="/">
<xsl:variable name="BOS-NYC-routes" select="$routes//row[Start-End eq 'BOS-NYC']" as="element(row)*"/>
<xsl:variable name="rest-stops-on-BOS-NYC-routes" as="element(row)*">
<xsl:for-each select="$BOS-NYC-routes">
<xsl:variable name="route" select="." as="element(row)" />
<xsl:sequence select="$rest-stops//row[Road eq $route/IDENT]" />
</xsl:for-each>
</xsl:variable>
<xsl:variable name="rest-stops-on-BOS-NYC-routes-with-gas-station" select="$rest-stops-on-BOS-NYC-routes/self::row[Gas]" as="element(row)*"/>
<xsl:variable name="gas-stations-on-BOS-NYC-routes" as="element(row)*">
<xsl:for-each select="$rest-stops-on-BOS-NYC-routes-with-gas-station">
<xsl:variable name="rest-stop" select="." as="element(row)" />
<xsl:sequence select="$gas-stations//row[IDENT eq $rest-stop/Gas]" />
</xsl:for-each>
</xsl:variable>
<Results>
<Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>
<xsl:sequence select="$gas-stations-on-BOS-NYC-routes" />
</Gas-stations-at-rest-stops-on-routes-from-BOS-to-NYC>
<!-- Eek! Creating a new display of the data involves essentially a complete rewrite! -->
<Gas-stations-on-each-BOS-NYC-route>
<xsl:for-each select="$BOS-NYC-routes">
<xsl:variable name="route" select="." as="element(row)" />
<xsl:variable name="rest-stops-on-BOS-NYC-route" select="$rest-stops//row[Road eq $route/IDENT]" as="element(row)*" />
<xsl:variable name="rest-stops-on-BOS-NYC-route-with-gas-station" select="$rest-stops-on-BOS-NYC-route/self::row[Gas]" as="element(row)*"/>
<xsl:variable name="gas-stations-on-BOS-NYC-route" as="element(row)*">
<xsl:for-each select="$rest-stops-on-BOS-NYC-route-with-gas-station">
<xsl:variable name="rest-stop" select="." as="element(row)" />
<xsl:sequence select="$gas-stations//row[IDENT eq $rest-stop/Gas]" />
</xsl:for-each>
</xsl:variable>
<Gas-stations-on-one-route>
<Route>
<xsl:sequence select="$route" />
</Route>
<Gas-stations>
<xsl:sequence select="$gas-stations-on-BOS-NYC-route" />
</Gas-stations>
</Gas-stations-on-one-route>
</xsl:for-each>
</Gas-stations-on-each-BOS-NYC-route>
</Results>
</xsl:template>

</xsl:stylesheet>



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