Document Type Definition (DTD)

Document Type Definition (DTD) is a set of rules for defining the structure, content, and constraints of XML documents. It serves as a blueprint that specifies how elements, attributes, and entities are used and interrelated in an XML document. Understanding DTD is essential for ensuring the integrity and consistency of XML data across different systems and applications.

What is DTD?

DTD defines the legal building blocks of an XML document. It establishes the rules for elements and attributes in an XML file. DTD can be declared inline within an XML document (internal DTD) or as a separate file (external DTD).

Why Use DTD?

Using DTD has several advantages:

  • Validation: Ensures the XML document follows the predefined structure and rules.
  • Data Integrity: Maintains consistency in data representation.
  • Interoperability: Facilitates data sharing between different systems by standardizing XML structure.

Types of DTD

There are two types of DTDs:

  1. Internal DTD: Defined within the XML document.
  2. External DTD: Defined in a separate file and linked to the XML document.

Basic Structure of DTD

A DTD file typically contains declarations for:

  • Elements: Define the structure and content of elements.
  • Attributes: Define properties associated with elements.
  • Entities: Define reusable pieces of text.
				
					<!DOCTYPE root-element [
    <!ELEMENT element-name (child-elements)>
    <!ATTLIST element-name attribute-name attribute-type attribute-default>
    <!ENTITY entity-name "entity-value">
]>

				
			

Defining Elements

Elements are the primary building blocks of an XML document. DTD defines what elements can appear and their relationships.

				
					<!ELEMENT element-name (content-model)>

				
			

Content Models

  • #PCDATA: Parsed Character Data.

  • EMPTY: No content.

  • ANY: Any content.

  • Mixed Content: Combination of text and elements.

  • Child Elements: Specified child elements.

Example

				
					<!ELEMENT note (to, from, heading, body)>

				
			

This defines a note element with four child elements: to, from, heading, and body.

Defining Attributes

Attributes provide additional information about elements. They are defined using the ATTLIST declaration.

				
					<!ATTLIST element-name attribute-name attribute-type attribute-default>

				
			

Attribute Types

  • CDATA: Character data.

  • ID: Unique identifier.

  • IDREF: Reference to an ID.

  • IDREFS: References to multiple IDs.

  • ENTITY: Reference to an unparsed entity.

  • ENTITIES: References to multiple unparsed entities.

  • NMTOKEN: Name token.

  • NMTOKENS: Multiple name tokens.

  • NOTATION: Reference to a notation.

Attribute Default Values

  • #REQUIRED: Attribute is required.

  • #IMPLIED: Attribute is optional.

  • #FIXED value: Attribute has a fixed value.

  • Default value: Attribute has a default value.

Example

				
					<!ATTLIST note date CDATA #REQUIRED>

				
			

This defines a date attribute for the note element that is required and must contain character data.

Defining Entities

Entities are placeholders for text or special characters. They help in reusing common text and including special characters in the XML document.

				
					<!ENTITY entity-name "entity-value">

				
			

Example

				
					<!ENTITY author "John Doe">

				
			

This defines an entity author with the value “John Doe”.

Internal vs. External DTD

Internal DTD

Internal DTD is defined within the XML document itself.

				
					<?xml version="1.0"?>
<!DOCTYPE note [
    <!ELEMENT note (to, from, heading, body)>
    <!ELEMENT to (#PCDATA)>
    <!ELEMENT from (#PCDATA)>
    <!ELEMENT heading (#PCDATA)>
    <!ELEMENT body (#PCDATA)>
]>
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!<script data-no-optimize="1">!function(t,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(t="undefined"!=typeof globalThis?globalThis:t||self).LazyLoad=e()}(this,function(){"use strict";function e(){return(e=Object.assign||function(t){for(var e=1;e<arguments.length;e++){var n,a=arguments[e];for(n in a)Object.prototype.hasOwnProperty.call(a,n)&&(t[n]=a[n])}return t}).apply(this,arguments)}function i(t){return e({},it,t)}function o(t,e){var n,a="LazyLoad::Initialized",i=new t(e);try{n=new CustomEvent(a,{detail:{instance:i}})}catch(t){(n=document.createEvent("CustomEvent")).initCustomEvent(a,!1,!1,{instance:i})}window.dispatchEvent(n)}function l(t,e){return t.getAttribute(gt+e)}function c(t){return l(t,bt)}function s(t,e){return function(t,e,n){e=gt+e;null!==n?t.setAttribute(e,n):t.removeAttribute(e)}(t,bt,e)}function r(t){return s(t,null),0}function u(t){return null===c(t)}function d(t){return c(t)===vt}function f(t,e,n,a){t&&(void 0===a?void 0===n?t(e):t(e,n):t(e,n,a))}function _(t,e){nt?t.classList.add(e):t.className+=(t.className?" 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</note>

				
			

External DDT

External DTD is defined in a separate file and referenced in the XML document.

				
					
<!ELEMENT note (to, from, heading, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>

				
			
				
					
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!<script data-no-optimize="1">!function(t,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(t="undefined"!=typeof globalThis?globalThis:t||self).LazyLoad=e()}(this,function(){"use strict";function e(){return(e=Object.assign||function(t){for(var e=1;e<arguments.length;e++){var n,a=arguments[e];for(n in a)Object.prototype.hasOwnProperty.call(a,n)&&(t[n]=a[n])}return t}).apply(this,arguments)}function i(t){return e({},it,t)}function o(t,e){var n,a="LazyLoad::Initialized",i=new t(e);try{n=new CustomEvent(a,{detail:{instance:i}})}catch(t){(n=document.createEvent("CustomEvent")).initCustomEvent(a,!1,!1,{instance:i})}window.dispatchEvent(n)}function l(t,e){return t.getAttribute(gt+e)}function c(t){return l(t,bt)}function s(t,e){return function(t,e,n){e=gt+e;null!==n?t.setAttribute(e,n):t.removeAttribute(e)}(t,bt,e)}function r(t){return s(t,null),0}function u(t){return null===c(t)}function d(t){return c(t)===vt}function f(t,e,n,a){t&&(void 0===a?void 0===n?t(e):t(e,n):t(e,n,a))}function _(t,e){nt?t.classList.add(e):t.className+=(t.className?" 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</note>

				
			

Using DTD in XML

To use a DTD in an XML document, you declare it at the beginning of the document using the <!DOCTYPE> declaration.

				
					<!DOCTYPE root-element SYSTEM "dtd-file">

				
			

Example

				
					<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
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d=document.createElement("script");d.addEventListener("load",e),d.addEventListener("error",e),t.getAttributeNames().forEach(e=>{"type"!=e&&d.setAttribute("data-src"==e?"src":e,t.getAttribute(e))});let a=!(d.type="text/javascript");!d.src&&t.textContent&&(d.src=litespeed_inline2src(t.textContent),a=!0),t.after(d),t.remove(),a&&e()}function litespeed_inline2src(t){try{var d=urlCreator.createObjectURL(new Blob([t.replace(/^(?:<!--)?(.*?)(?:-->)?$/gm,"$1")],{type:"text/javascript"}))}catch(e){d="data:text/javascript;base64,"+btoa(t.replace(/^(?:<!--)?(.*?)(?:-->)?$/gm,"$1"))}return d}</script></body>
</note>

				
			

Advanced Concepts in DTD

Parameter Entities

Parameter entities are used to define reusable pieces of the DTD itself.

				
					<!ENTITY % entity-name "entity-value">

				
			

Example

				
					<!ENTITY % person "(name, address)">
<!ELEMENT employee %person;>
<!ELEMENT customer %person;>

				
			

Conditional Sections

Conditional sections allow parts of the DTD to be included or ignored based on conditions.

				
					<![ INCLUDE[
    Declarations to be included
]]&gt;
<![ IGNORE[
    Declarations to be ignored
]]&gt;

				
			

Example

				
					<!ENTITY % draft "INCLUDE">
<![%draft;[
    <!ELEMENT draft (#PCDATA)>
]]&gt;
<!ENTITY % final "IGNORE">
<![%final;[
    <!ELEMENT final (#PCDATA)>
]]&gt;

				
			

Examples

Example 1: Simple Internal DTD

				
					<?xml version="1.0"?>
<!DOCTYPE note [
    <!ELEMENT note (to, from, heading, body)>
    <!ELEMENT to (#PCDATA)>
    <!ELEMENT from (#PCDATA)>
    <!ELEMENT heading (#PCDATA)>
    <!ELEMENT body (#PCDATA)>
]>
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!<script data-no-optimize="1">!function(t,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(t="undefined"!=typeof globalThis?globalThis:t||self).LazyLoad=e()}(this,function(){"use strict";function e(){return(e=Object.assign||function(t){for(var e=1;e<arguments.length;e++){var n,a=arguments[e];for(n in a)Object.prototype.hasOwnProperty.call(a,n)&&(t[n]=a[n])}return t}).apply(this,arguments)}function i(t){return e({},it,t)}function o(t,e){var n,a="LazyLoad::Initialized",i=new t(e);try{n=new CustomEvent(a,{detail:{instance:i}})}catch(t){(n=document.createEvent("CustomEvent")).initCustomEvent(a,!1,!1,{instance:i})}window.dispatchEvent(n)}function l(t,e){return t.getAttribute(gt+e)}function c(t){return l(t,bt)}function s(t,e){return function(t,e,n){e=gt+e;null!==n?t.setAttribute(e,n):t.removeAttribute(e)}(t,bt,e)}function r(t){return s(t,null),0}function u(t){return null===c(t)}function d(t){return c(t)===vt}function f(t,e,n,a){t&&(void 0===a?void 0===n?t(e):t(e,n):t(e,n,a))}function _(t,e){nt?t.classList.add(e):t.className+=(t.className?" ":"")+e}function v(t,e){nt?t.classList.remove(e):t.className=t.className.replace(new RegExp("(^|\\s+)"+e+"(\\s+|$)")," ").replace(/^\s+/,"").replace(/\s+$/,"")}function g(t){return t.llTempImage}function b(t,e){!e||(e=e._observer)&&e.unobserve(t)}function p(t,e){t&&(t.loadingCount+=e)}function h(t,e){t&&(t.toLoadCount=e)}function n(t){for(var e,n=[],a=0;e=t.children[a];a+=1)"SOURCE"===e.tagName&&n.push(e);return n}function m(t,e){(t=t.parentNode)&&"PICTURE"===t.tagName&&n(t).forEach(e)}function a(t,e){n(t).forEach(e)}function E(t){return!!t[st]}function I(t){return t[st]}function y(t){return delete t[st]}function A(e,t){var n;E(e)||(n={},t.forEach(function(t){n[t]=e.getAttribute(t)}),e[st]=n)}function k(a,t){var i;E(a)&&(i=I(a),t.forEach(function(t){var e,n;e=a,(t=i[n=t])?e.setAttribute(n,t):e.removeAttribute(n)}))}function L(t,e,n){_(t,e.class_loading),s(t,ut),n&&(p(n,1),f(e.callback_loading,t,n))}function w(t,e,n){n&&t.setAttribute(e,n)}function 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Delayed"),litespeed_ui_events.forEach(e=>{window.removeEventListener(e,litespeed_load_delayed_js_force,{passive:!0})}),document.querySelectorAll("iframe[data-litespeed-src]").forEach(e=>{e.setAttribute("src",e.getAttribute("data-litespeed-src"))}),"loading"==document.readyState?window.addEventListener("DOMContentLoaded",litespeed_load_delayed_js):litespeed_load_delayed_js()}litespeed_ui_events.forEach(e=>{window.addEventListener(e,litespeed_load_delayed_js_force,{passive:!0})});async function litespeed_load_delayed_js(){let t=[];for(var d in document.querySelectorAll('script[type="litespeed/javascript"]').forEach(e=>{t.push(e)}),t)await new Promise(e=>litespeed_load_one(t[d],e));document.dispatchEvent(new Event("DOMContentLiteSpeedLoaded")),window.dispatchEvent(new Event("DOMContentLiteSpeedLoaded"))}function litespeed_load_one(t,e){console.log("[LiteSpeed] Load ",t);var d=document.createElement("script");d.addEventListener("load",e),d.addEventListener("error",e),t.getAttributeNames().forEach(e=>{"type"!=e&&d.setAttribute("data-src"==e?"src":e,t.getAttribute(e))});let a=!(d.type="text/javascript");!d.src&&t.textContent&&(d.src=litespeed_inline2src(t.textContent),a=!0),t.after(d),t.remove(),a&&e()}function litespeed_inline2src(t){try{var d=urlCreator.createObjectURL(new Blob([t.replace(/^(?:<!--)?(.*?)(?:-->)?$/gm,"$1")],{type:"text/javascript"}))}catch(e){d="data:text/javascript;base64,"+btoa(t.replace(/^(?:<!--)?(.*?)(?:-->)?$/gm,"$1"))}return d}</script></body>
</note>

				
			

Example 2: External DTD

				
					
<!ELEMENT note (to, from, heading, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>

				
			
				
					
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!<script data-no-optimize="1">!function(t,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(t="undefined"!=typeof globalThis?globalThis:t||self).LazyLoad=e()}(this,function(){"use strict";function e(){return(e=Object.assign||function(t){for(var e=1;e<arguments.length;e++){var n,a=arguments[e];for(n in a)Object.prototype.hasOwnProperty.call(a,n)&&(t[n]=a[n])}return t}).apply(this,arguments)}function i(t){return e({},it,t)}function o(t,e){var n,a="LazyLoad::Initialized",i=new t(e);try{n=new CustomEvent(a,{detail:{instance:i}})}catch(t){(n=document.createEvent("CustomEvent")).initCustomEvent(a,!1,!1,{instance:i})}window.dispatchEvent(n)}function l(t,e){return t.getAttribute(gt+e)}function c(t){return l(t,bt)}function s(t,e){return function(t,e,n){e=gt+e;null!==n?t.setAttribute(e,n):t.removeAttribute(e)}(t,bt,e)}function r(t){return s(t,null),0}function u(t){return null===c(t)}function d(t){return c(t)===vt}function f(t,e,n,a){t&&(void 0===a?void 0===n?t(e):t(e,n):t(e,n,a))}function _(t,e){nt?t.classList.add(e):t.className+=(t.className?" 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</note>

				
			

Example 3: Attributes and Entities

				
					<?xml version="1.0"?>
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    <!ELEMENT note (to, from, heading, body)>
    <!ELEMENT to (#PCDATA)>
    <!ELEMENT from (#PCDATA)>
    <!ELEMENT heading (#PCDATA)>
    <!ELEMENT body (#PCDATA)>
    <!ATTLIST note date CDATA #REQUIRED>
    <!ENTITY author "John Doe">
]>
<note date="2023-07-01">
    <to>Tove</to>
    <from>&author;</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!<script data-no-optimize="1">!function(t,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(t="undefined"!=typeof globalThis?globalThis:t||self).LazyLoad=e()}(this,function(){"use strict";function e(){return(e=Object.assign||function(t){for(var e=1;e<arguments.length;e++){var n,a=arguments[e];for(n in a)Object.prototype.hasOwnProperty.call(a,n)&&(t[n]=a[n])}return t}).apply(this,arguments)}function i(t){return e({},it,t)}function o(t,e){var n,a="LazyLoad::Initialized",i=new t(e);try{n=new CustomEvent(a,{detail:{instance:i}})}catch(t){(n=document.createEvent("CustomEvent")).initCustomEvent(a,!1,!1,{instance:i})}window.dispatchEvent(n)}function l(t,e){return t.getAttribute(gt+e)}function c(t){return l(t,bt)}function s(t,e){return function(t,e,n){e=gt+e;null!==n?t.setAttribute(e,n):t.removeAttribute(e)}(t,bt,e)}function r(t){return s(t,null),0}function u(t){return null===c(t)}function d(t){return c(t)===vt}function f(t,e,n,a){t&&(void 0===a?void 0===n?t(e):t(e,n):t(e,n,a))}function _(t,e){nt?t.classList.add(e):t.className+=(t.className?" 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</note>

				
			

DTD is a crucial aspect of XML that helps in defining the structure, content, and constraints of XML documents. By understanding and using DTD, you can ensure the integrity and consistency of your XML data, making it easier to share and use across different systems. Whether using internal or external DTDs, the principles and syntax remain the same, providing a robust way to validate and standardize XML documents. Happy Coding! ❤️

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