Week 02: XML, schema and validation, DOM
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Agenda
Markup languages
XML basics
XML schema
DOM
Short demo
Hands on: Iteration 01
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Let's dive into it!
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Markup languages (recap)
natural language + special constructs ("marks")
for instance HTML, Markdown, TeX
easily readable for both computers as well as humans
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Example: Markdown
bold text
Gitlab FI MUNI
Heading level 5
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XML
eXtensible Markup Language
data exchange format
translations
web scraping
.xml file extension
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<!-- example.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
<book category="children">
<title lang="en">Harry Potter</title>
<author>J. K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="web" cover="paperback">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
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XML document structure
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comment
processing instruction
root element
child/nested elements
start/end tags
text node
attribute
Note: Elements are also nodes.
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Basic rules
all elements must have an end tag OR be empty and self-closing
all elements must be properly nested (overlapping is not allowed)
all attribute values must be enclosed in quotes
each document must have a unique root element
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Naming conventions and names of elements are free to choose.
But remember,
"with great power comes great responsibility."
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To avoid
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Element or attribute?
<book>
<category>children</category>
<title>
<lang>en</lang>
Harry Potter
</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
vs.
<book category="children">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
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Element or attribute?
Attribute value:
cannot be further structured
"atomic" value
keeps an additional info of another element
for instance, lang
Note: do not use too many attributes on a single element. It can get hard to read for humans very quickly.
Let's say that 3-4 attributes are a maximum.
Element:
even if nested, it is a meaningful structure itself
usually structured
contains any number of nodes
for instance, person
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XML Schema
the structure of an XML document
what elements and attributes are allowed to appear, in which order, how many times...
data types of elements and attributes
default or fixed values for elements/attributes
namespaces
follows XML syntax itself
.xsd file extension
Note: we validate an XML document against a schema. So a document
is valid if it conforms to a given schema.
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<!-- example.xsd -->
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
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Definition Header
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
...
...
</xs:schema>
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Element
This is a simple element. It can contain only text.
<xs:element name="element_name" type="element_type" />
Limiting occurences
<xs:element name="element_name" minOccurs="1" maxOccurs="unbounded" />
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Types: simple types
Basic types
xs:string
xs:decimal
xs:integer
xs:boolean
xs:date
xs:time
...
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Types: simple types
definition inside of a xs:simpleType element
user defined types
restrictions, unions, enumerations
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<!-- Example 01: base restriction -->
<xs:simpleType name="typeName">
<xs:restriction base="baseTypeName"> ... </xs:restriction>
</xs:simpleType>
<!-- Example 02: content length restriction -->
<xs:simpleType name="typeName">
<xs:restriction base="xs:string">
<xs:maxLength value="32"/>
</xs:restriction>
</xs:simpleType>
<!-- Example 03: regex restriction -->
<xs:simpleType name="isbnType">
<xs:restriction base="xs:string">
<xs:pattern value="[0-9]{10}"/>
</xs:restriction>
</xs:simpleType>
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<!-- Example 04: enumeration restriction with usage -->
<xs:simpleType name="grade">
<xs:restriction base="xs:NMTOKEN">
<xs:enumeration value="A" />
<xs:enumeration value="B" />
<xs:enumeration value="C" />
</xs:restriction>
</xs:simpleType>
<xs:element name="Course">
<xs:complexType>
<xs:sequence>
<xs:element name="Grade" type="grade" />
</xs:sequence>
</xs:complexType>
</xs:element>
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Attributes
<!-- Example 01: with default value -->
<xs:attribute name="lang" type="xs:string" default="EN" />
<!-- Example 02: with required flag -->
<xs:attribute name="lang" type="xs:string" use="required" />
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Complex types
user defined
definition inside of a xs:complexType element
xs:sequence => all child elements, the order is specified
xs:all => all child elements, the order is not important
xs:choice => only one child element
...
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<!-- Sequence -->
<xs:element name="elementName">
<xs:complexType>
<xs:sequence>
<xs:element name="elem1" type="xs:string"/>
<xs:element name="elem2" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<!-- All -->
<xs:element name="elementName">
<xs:complexType>
<xs:all>
<xs:element name="elem1" type="xs:string"/>
<xs:element name="elem2" type="xs:string"/>
</xs:all>
</xs:complexType>
</xs:element>
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Is the following XML element valid? For xs:sequence? For xs:all?
<elementName>
<elem2>World</elem2>
<elem1>Hello</elem1>
</elementName>
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Relational vs. Non-relational data model
Relational (ERD model)
atomic, flat
general view
no data duplication => usage of unique keys as reference
data relations => entity relations, foreign keys
usage of ids
Non-relational (XML document)
structured, nested
specific view
data duplication (sometimes only partially)
data relations => elements nested one in another
ids not necessary
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Bonus topic
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DOM
Document Object Model
interface (cross-platform, language-
independent)
represents a document as a tree structure
each node contains an object
nodes can have event handlers
used by browsers to represent an HTML page
applicable to XML as well
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Demo
Modelling Discord using XML
1. data modelling
2. schema definition
3. document validation
Note: Demo code is available in the Interactive
syllabus for seminars.
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Starting point
Discord ERD model (slightly extended)
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How to run a validation?
online, for instance: freeformatter or utilities-online
VS Code extension, for instance: XML extension from Red Hat (does also formatting and other)
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Now, it's your turn :)
The assignment for Iteration 01 can be found in Gitlab issues.
Now, you can continue as described in How to download new iteration on Gitlab Wiki.
If you struggle, don't hesitate to ask for help :)
Note: even though the nature of XML does not require indentation and proper formatting,
please format it anyway. It makes the document much more readable.
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