XML

Last updated on 2025-04-29 | Edit this page

Overview

Questions

What is XML?
What are the elements of XML?
How do you annotate data in XML?

Objectives

After completing this episode, participants should be able to

describe the XML format for storing and processing metadata,
name syntax rules for a well-formed XML file,
investigate the structure of a simple XML file,
write extracted metadata in a simple XML format.

One of the most widely used file formats in the cultural sector is XML (eXtensible Markup Language). It is used to describe, structure, store and transport data. XML refers to the file format as well as to the syntax in which data is recorded: the markup language. By marking up the text with tags, data and metadata can be combined in a single document.

XML Elements

An XML document contains XML elements to structure the data. They are formed using tags in angle brackets and are divided into an opening tag and a closing tag, indicated by a leading slash. The content is placed in between.

XML

<tag>element</tag>

The tags are given a name that describes the content:

XML

<photographer>Walker Evans</photographer> 
<name>Walker Evans</name>
<artist>Walker Evans</artist>

You may be familiar with this structure. The HTML language, which is used to organise web pages, has a similar structure. It is also based on SGML. HTML makes it possible to mark up the typical elements of a text-based document - such as headings, paragraphs, lists or tables - as such on a web page and to structure the page semantically. Unlike HTML, where there are rules for naming tags - such as <h1></h1> for headings - XML tags can be named freely, subject to a few technical rules. This makes it particularly interesting for use in the development of metadata standards and ontologies.

Rules for naming tags¹

Names:

Must start with a letter or underscore.
Cannot start with the letters xml (or XML, or Xml, etc).
Can contain letters, digits, hyphens, underscores, and periods.
Cannot contain spaces.

Best Naming Practices and what to consider:

Create descriptive names.
Create short and simple names.
Avoid - / . / : in the names, e.g. <first-name> (: is reserved for namespaces which have a special function).
XML tags are case sensitive. The tag is different from the tag .
XML does not truncate multiple white-spaces.

XML also provides the ability to have comments that are not automatically read:

XML

<!-- Here is the content of the comment, there is no closing tag -->

In XML, all elements must be properly nested, which means that an element opened inside another element must also be closed inside it:

XML

<collection><name>MET</name><place>New York</place></collection>

Indentation can be used to make the structure more human-readable, especially if elements’ nesting is deep:

XML

<collection>
  <name>MET</name>
  <place>collection of the MET in New York</place>
  <artist>
    <name>Fullname</name>
    <dateOfBirth>
      <day>Day of Birth</day>
      <month>Month of Birth</month>
      <year>Year of Birth</year>
    </dateOfBirth>
  </artist>
</collection>

As this example shows, XML documents have a tree structure. They start with the root element and then branch out deeper and deeper. The tag <artist> is called a parent element and the subordinated elements <name> and <dateOfBirth> are child elements. In the deeper level, <dateOfBirth> is the parent element for the tags day, month and year as child elements.

Characters used in the structure of the XML syntax must comply with certain rules. Certain characters, such as “<” meaning “is less than”, must be replaced by a special string so that they won’t cause problems. If you just use “<”, XML understands it as an opening tag and expects it to be closed at some point. To avoid errors in this and similar cases, the character is replaced with an entity reference:

string	character	meaning
<	<	less than
>	>	greater than
&	&	ampersand
'	’	apostrophe
"	”	quotation mark

XML

<photographer>Bernd &amp; Hilla Becher</photographer>

Attributes

In XML, as in HTML, tags can also have attributes, which define the content of the tags in more detail. These attributes provide metadata for the element that they refer to. They are named, the content is assigned to them with a =, and they are enclosed in quotation marks:

XML

<title lang="author's original language">Always give the title in the author's language</title>

<commonTitle lang="title as commonly known">Always give the title as it is commonly known</commonTitle>

Attributes are often used in a hierarchy to record information that applies to all the underlying data:

XML

<collection name="Modern Art" place="New York">
  <item1/>
  <item2/>
</collection>

The slash here at the end of the tag (self-closing tag) means that the element is empty and self-contained. It is not necessary to write an opening and closing tag if there are no other elements between them. You often see this in XML output when the metadata field is empty.

At the top of an XML document you will often find something called a prolog or declaration:

XML

<?xml version="1.0" encoding="UTF-8"?>

It is optional, but provides information about the version and encoding² used. If included, it must be placed at the top of the document above the root element. The XML prolog or declaration does not have a closing tag.

Callout

XML documents that conform to all these rules are considered “Well-Formed” XML documents.

Exercise

Open the moma_artworks.csv file. Choose the data of an artwork and write it in the XML format.

Give me a hint

Is there a structure for the data? Can data be collected under a category?
You can use indentation to create a hierarchy.

Show me the solution

XML

<?xml version="1.0" encoding="UTF-8"?>
<artworks>
  <artwork>
    <title>Green-Blue-Red (for Parkett no. 35)</title>
    <artist>
      <name>Gerhard Richter</name>
    </artist>
    <constituentID>4907</constituentID>
    <artistBio>
      <bio>German, born 1932</bio>
    </artistBio>
    <nationality>German</nationality>
    <beginDate>1932</beginDate>
    <endDate>0</endDate>
    <gender>male</gender>
    <date>1993</date>
    <medium>Multiple of oil on canvas</medium>
    <dimensions>composition: 11 7/16 × 15 3/4&quot; (29 × 40 cm); sheet: 11 3/4 × 15 3/4&quot; (29.9 × 40 cm)</dimensions>
    <creditLine>Riva Castleman Endowment Fund, Lily Auchincloss Fund, and Gift of Parkett</creditLine>
    <accessionNumber>110.1998.1</accessionNumber>
    <classification>Multiple</classification>
    <department>Drawings &amp; Prints</department>
    <dateAcquired>1998-03-05</dateAcquired>
    <cataloged>Y</cataloged>
    <objectID>61953</objectID>
    <url>https://www.moma.org/collection/works/61953</url>
    <imageURL></imageURL>
    <onView></onView>
    <height>29.0</height>
    <width>40.0</width>
  </artwork>  

  <artwork>
    <title>Untitled</title>
    <artist>
      <name>Blinky Palermo</name>
    </artist>
    <constituentID>4474</constituentID>
    <artistBio>
      <bio>German, 1943–1977</bio>
    </artistBio>
    <nationality>German</nationality>
    <beginDate>1943</beginDate>
    <endDate>1977</endDate>
    <gender>male</gender>
    <date>1970</date>
    <medium>Dyed cotton mounted on muslin</medium>
    <dimensions>6' 6 3/4\" x 6' 6 3/4\" (200 x 200 cm)</dimensions>
    <creditLine>Gift of Jo Carole and Ronald S. Lauder</creditLine>
    <accessionNumber>650.1997</accessionNumber>
    <classification>Painting</classification>
    <department>Painting &amp; Sculpture</department>
    <dateAcquired>1997-06-02</dateAcquired>
    <cataloged>Y</cataloged>
    <objectID>78283</objectID>
    <url>https://www.moma.org/collection/works/78283</url>
    <imageURL>https://www.moma.org/media/W1siZiIsIjE1MTQ0MCJdLFsicCIsImNvbnZlcnQiLCItcmVzaXplIDEwMjR4MTAyNFx1MDAzZSJdXQ.jpg?sha=c54d00f27ed284be</imageURL>
    <onView></onView>
    <height>200.0</height>
    <width>200.0</width>
  </artwork>  

  <artwork>
    <title>Daylight Savings Time</title>
    <artist>
      <name>Pierre Roy</name>
    </artist>
    <constituentID>5065</constituentID>
    <artistBio>
      <bio>French, 1880–1950</bio>
    </artistBio>
    <nationality>French</nationality>
    <beginDate>1880</beginDate>
    <endDate>1950</endDate>
    <gender>male</gender>
    <date>1929</date>
    <medium>Oil on canvas</medium>
    <dimensions>21 1/2 x 15&quot; (54.6 x 38.1 cm)</dimensions>
    <creditLine>Gift of Mrs. Ray Slater Murphy</creditLine>
    <accessionNumber>1.1931</accessionNumber>
    <classification>Painting</classification>
    <department>Painting &amp; Sculpture</department>
    <dateAcquired>1931-01-19</dateAcquired>
    <cataloged>Y</cataloged>
    <objectID>78294</objectID>
    <url>https://www.moma.org/collection/works/78294</url>
    <imageURL>https://www.moma.org/media/W1siZiIsIjIzMzkzNyJdLFsicCIsImNvbnZlcnQiLCItcmVzaXplIDEwMjR4MTAyNFx1MDAzZSJdXQ.jpg?sha=74825d4c62cd5a8a</imageURL>
    <onView></onView>
    <height>54.6</height>
    <width>38.1</width>
  </artwork>  

  <artwork>
    <title>The Bather</title>
    <artist>
      <name>Paul Cézanne</name>
    </artist>
    <constituentID>1053</constituentID>
    <artistBio>
      <bio>French, 1839–1906</bio>
    </artistBio>
    <nationality>French</nationality>
    <beginDate>1839</beginDate>
    <endDate>1906</endDate>
    <gender>male</gender>
    <date>c. 1885</date>
    <medium>Oil on canvas</medium>
    <dimensions>50 x 38 1/8&quot; (127 x 96.8 cm)</dimensions>
    <creditLine>Lillie P. Bliss Collection</creditLine>
    <accessionNumber>1.1934</accessionNumber>
    <classification>Painting</classification>
    <department>Painting &amp; Sculpture</department>
    <dateAcquired>1934-09-23</dateAcquired>
    <cataloged>Y</cataloged>
    <objectID>78296</objectID>
    <url>https://www.moma.org/collection/works/78296</url>
    <imageURL>https://www.moma.org/media/W1siZiIsIjQ0NjA2NyJdLFsicCIsImNvbnZlcnQiLCItcmVzaXplIDEwMjR4MTAyNFx1MDAzZSJdXQ.jpg?sha=c6bd692fa0fe0685</imageURL>
    <onView>&quot;MoMA, Floor 2, 2 South&quot;</onView>
    <height>127.0</height>
    <width>96.8</width>
  </artwork>  

  <artwork>
    <title>Syntheses of Naples</title>
    <artist>
      <name>Enrico Prampolini</name>
    </artist>
    <constituentID>4720</constituentID>
    <artistBio>
      <bio>Italian, 1894–1956</bio>
    </artistBio>
    <nationality>Italian</nationality>
    <beginDate>1894</beginDate>
    <endDate>1956</endDate>
    <gender>male</gender>
    <date>before 1930</date>
    <medium>Oil on canvas</medium>
    <dimensions>39 3/8 x 39 1/2&quot; (100 x 100.3 cm)</dimensions>
    <creditLine>Gift of Dr. Julius Spitzer</creditLine>
    <accessionNumber>1.1942</accessionNumber>
    <classification>Painting</classification>
    <department>Painting &amp; Sculpture</department>
    <dateAcquired>1941-12-10</dateAcquired>
    <cataloged>Y</cataloged>
    <objectID>78299</objectID>
    <url>https://www.moma.org/collection/works/78299</url>
    <imageURL>https://www.moma.org/media/W1siZiIsIjE4NjgzNSJdLFsicCIsImNvbnZlcnQiLCItcmVzaXplIDEwMjR4MTAyNFx1MDAzZSJdXQ.jpg?sha=9740d0c731c867c2</imageURL>
    <onView></onView>
    <height>100.0</height>
    <width>100.3</width>
  </artwork>
</artworks>

This is one possible solution. You can choose other ways to structure the data by nesting elements like “artistBio” or “gender” within a parent element like “artist”:

XML

<artist>
  <artistBio/>
  <gender/>
</artist>

Discussion

Compare your XML file with the other participants’ documents. What do you notice?

Key Points

XML is one of the most widely used metadata file formats.
An XML document contains XML elements to structure the data.
Attributes provide additional information about elements or groups of elements.

[1]: w3school XML elements
[2]: For further information see the glossary.

1↩︎
2↩︎

XML

Overview

Questions

Objectives

XML Elements

XML

XML

Rules for naming tags1

XML

XML

XML

XML

Attributes

XML

XML

XML

Callout

Exercise

Give me a hint

Show me the solution

XML

XML

Discussion

Key Points

Rules for naming tags¹