XML | XPath

[atc0005]

Background

I'm using (or trying to use) XPath with the atc0005/go-lockss project. That project processes a LOCKSS XML configuration file served up by a central network configuration server and then attempts to verify connectivity from the current node (where the app is running) to all peer nodes.

As part of this work, I've stumbled my way through using the https://github.com/antchfx/xmlquery package to process the XML file and retrieve the values I'm looking for. I thought I understood what was happening, but as atc0005/go-lockss#21 notes, I'm not getting the expected results.

In short, there is a bug with the application and I'm not (yet) knowledgeable enough to fix it properly. Thankfully the app's assumptions work as intended for two of the LOCKSS networks that I've tested against, but more work is needed to properly parse the XML file to work with other networks where group-based peers are configured.

Parsing that file will require a better understanding of XPath than I currently have.

What I've found

Guides

• https://dh.obdurodon.org/introduction-xpath.xhtml
• https://github.com/ebeshero/DHClass-Hub/wiki/XPath:-An-Intro-to-Functions
• https://dh.newtfire.org/explainXPath.html

Videos

• XML and XSD: How do they work?
  • 9. XPath examples and demonstrations in XML
  • plants.xml file used in video
• XPath Basics Tutorial HD
• XPath / XQuery Tutorial for SQL Pros
  • gives a great side-by-side comparison between XPath, XQuery and SQL queries that achieve (roughly) the same goal (helps with understanding)
  • https://github.com/altova/XPath-XQuery-SQL-Tutorial

Tools

• http://www.qutoric.com/xmlquire/

[atc0005]

Notes below were taken while watching the 9. XPath examples and demonstrations in XML video.

The plants.xml file was the sample XML document used by Brandon Jones (GitHub, YouTube) in the XPath examples shown in the video.

plants.xml file

<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="plants.xsl"?>
<plant xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:noNamespaceSchemaLocation="plants.xsd">
  <!-- All payload data belongs here -->
  <genus>Cercis</genus>
  <species>canadensis</species>
  <common_name>Eastern Redbud</common_name>
  <height>25</height>
  <begin_bloom_date>2017-03-15</begin_bloom_date>
  <end_bloom_date>2017-03-30</end_bloom_date>
  <edible>true</edible>
  <sun_tolerance>full_sun</sun_tolerance>
  <bloom_color>violet</bloom_color>
  <specimens>
    <specimen>
      <latitude>39.47</latitude>
      <longitude>-84.51</longitude>
      <planted_by>Brandan Jones</planted_by>
      <comments>Beautiful tree!</comments>
    </specimen>
    <specimen>
      <latitude>52.40</latitude>
      <longitude>1.00</longitude>
      <planted_by>Joe Bloggs</planted_by>
      <comments>RBG Kew</comments>
    </specimen>
    <specimen>
      <latitude>-22.50</latitude>
      <longitude>-68.25</longitude>
      <planted_by>Guillermo Sanchez</planted_by>
      <comments>Lovely Specimen!</comments>
    </specimen>
    <specimen>
      <latitude>-25.50</latitude>
      <longitude>38.25</longitude>
      <planted_by>Dave Jones</planted_by>
      <comments>wonderful Specimen!</comments>
    </specimen>
  </specimens>
</plant>

Other tools used in the video:

• https://codebeautify.org/xmlviewer
• https://freeformatter.com/xpath-tester.html

---

Examples pulled 1:1 from Notepad++ screenshot:

• All specimens
  • /plant/specimens
• The first specimen
  • /plant/specimens/specimen[1]
• The last specimen
  • /plant/specimens/specimen[last()]
• Specimens with longitude greater than 3
  • /plant/specimens/specimen[longitude>3]
• Comments from specimen with longitude greater than 3
  • /plant/specimens/specimen[longitude>3]/comments
• walk down... walk up
  • /plant/specimens/specimen[longitude>3]/../../genus

My scratch notes:

• ../../

  • go two levels "up" from a match
  • this seems to be useful when you want to be really specific with your "landing" or "starting" point (think of an anchor), then walk backwards from there until you're ready to drill down to what you're really after

• //species

  • will return all occurrences of species, no matter where it lives in the document

• //latitude

  • as with //species, will return all occurrences no matter where it lives in the document
  • "I don't know where this lives, but please give it to me"

• //specimen[latitude>0]/latitude or //..[latitude>0]/latitude

  • Find the latitudes greater than zero

• //..[latitude>0]/latitude

  • Find latitude, go to its parent, now only show me latitudes greater than zero
  • This feels "advanced" and I suspect will only feel natural with practice (i.e., I don't know that I would have naturally arrived at using this syntax on my own)

[atc0005]

I picked up this bit (watched this video before Brandon Jones' video series where he also covered this) from the XPath Basics Tutorial HD video.

This (incomplete) syntax:

food[@name='tacos']

indicates that you want to filter the food elements by the name=tacos attribute:

<food name="tacos">

The video is relatively short, but gave a great overview. The free XML Quire application is used in the examples.