XML Validation and Schema
An XML element is a complete unit consisting of an opening tag, content, and a closing tag; it can also be represented as a self-closing tag if empty.
Why XML Structure Matters
XML organizes data so that both humans and machines can understand what the data means. Unlike HTML, which focuses on how information looks on a screen, XML focuses on the meaning and organization of information. XML documents achieve this clarity through a strict structure built from elements, attributes, and nesting.
When you read an XML document, look for three building blocks: complete elements, attributes that describe elements, and nested relationships between elements.
Anatomy of One Element
An XML element is a complete unit of data. In its standard form, it has an opening tag, content, and a closing tag. The opening tag begins with a less-than symbol, contains the element name, and ends with a greater-than symbol. The closing tag uses the same element name, but places a forward slash before that name. Everything between the two tags is the element's content.
Reading a Name Element
Identify the three parts of the XML element <name>Chuck</name>.
Opening tag: The first part is <name>. It begins the element and identifies the element name as name.
Content: The text between the tags is Chuck. This is the element's content.
Closing tag: The final part is </name>. The forward slash indicates that the name element is closing.
The complete element is made from the opening tag <name>, the content Chuck, and the closing tag </name>.
Metadata Inside Opening Tags
Attributes are optional pieces of metadata attached to an opening tag. Each attribute has a name, an equals sign, and a quoted value in the form name="value". Multiple attributes can appear in one opening tag, separated by spaces. An element does not need to have any attributes.
Separating Metadata from Content
Interpret the phone element represented by <phone type="intl">main phone number</phone>.
Find the attribute: type="intl" is inside the opening tag. It describes what kind of phone number the element contains.
Find the content: The phone number appears between the opening and closing tags. It is the primary data of the element.
Apply the distinction: The type classification belongs as metadata, while the actual phone number belongs as content.
The attribute describes the phone element; it is not part of the phone number content.
Use attributes for metadata, properties, or classifications such as type, id, status, or version. Use nested elements for primary content or for data that might have its own structure or attributes.
Empty Elements and Self-Closing Tags
An element with no text and no child elements is empty. XML provides two valid ways to represent it. The traditional form uses an opening tag followed immediately by a closing tag. The concise form combines both into one self-closing tag that ends with a forward slash before the closing bracket. Both forms mean the same thing.
Parent and Child Relationships
XML elements can be placed inside other elements. An element that contains other elements is a parent, and the elements inside it are children. Repeated nesting creates a tree-like hierarchy. The outermost element that contains all other elements is the root element, so an XML document has a single root element containing the rest of its structure.
Tracing the Person Hierarchy
Explain the relationships in an XML structure containing person, name, phone, and email elements.
Locate the root: person is the outermost element, so it is the root element.
Identify the children: name, phone, and email are contained by person, so they are its child elements.
Inspect each child: name contains the text Chuck. phone contains a phone number and has the type="intl" attribute. email is empty but remains a valid child.
Read the relationship: Because phone and email are inside person, the structure shows that those pieces of contact information belong to that person.
The hierarchy is person as the root, with name, phone, and email as its children.
Mistakes When Reading XML
Treating an attribute as the element's content
The attribute is attached to the opening tag and provides metadata about the element. The primary phone data belongs between the opening and closing tags.
Fix:
Separate descriptive metadata such as type from the element's primary content.Assuming an empty element is invalid
An empty element is valid when it has no text and no child elements.
Fix:
Represent it with paired empty tags or a self-closing tag such as <email />.Missing the parent-child relationship
Nested elements form a hierarchy, and contained elements are children of the element that contains them.
Fix:
Start at the root element and trace which elements are nested inside each parent.Confusing a closing tag with another opening tag
The forward slash before the element name marks the closing tag.
Fix:
Match each standard opening tag with its corresponding closing tag, or identify a self-closing tag when the element is empty.
Examine the element <phone type="intl">main phone number</phone>. Identify its opening tag, attribute, content, and closing tag. Then decide whether type="intl" is primary content or metadata.
Hints
- The opening tag comes before the text content.
- Look inside the opening tag for the name-equals-quoted-value pattern.
- The closing tag contains a forward slash before the element name.
Key Takeaways
- A standard XML element contains an opening tag, content, and a closing tag.
- Attributes are optional metadata attached to opening tags using a name, equals sign, and quoted value.
- An empty element can use paired empty tags or a self-closing tag, and both forms have the same meaning.
- Nested elements create parent-child relationships and a tree-like hierarchy with one root element.
- Use attributes for metadata and classifications; use nested elements for primary content or child data that may have its own structure.
Key Takeaways
- An XML element is a complete unit made from an opening tag, content, and a closing tag.
- Attributes describe an element and belong inside its opening tag, while primary data belongs in content or nested elements.
- Self-closing tags provide a compact representation for elements with no text or child elements.
- Nesting creates a hierarchy of parents and children, with a single root element containing the document's other elements.