xsl-list
[Top] [All Lists]

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

2019-08-02 13:08:00
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>
--~----------------------------------------------------------------
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
EasyUnsubscribe: http://lists.mulberrytech.com/unsub/xsl-list/1167547
or by email: xsl-list-unsub(_at_)lists(_dot_)mulberrytech(_dot_)com
--~--

<Prev in Thread] Current Thread [Next in Thread>