XML

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XML (Extensible Markup Language) is a markup language used to store, organize, and exchange data. Information is placed inside tagged elements, creating a structure that can be read by both people and software.

XML is not a programming language or a database. It does not define how content should look when displayed. Its purpose is to describe data and the relationships between individual pieces of information in a structured format.

What does an XML document look like?

A simple XML document may look like this:

<?xml version="1.0" encoding="UTF-8"?>
<product id="123">
    <name>Safety shoes</name>
    <price currency="PLN">249.00</price>
    <available>true</available>
</product>

The <product> element is the document’s root element. It contains additional elements describing the product’s name, price, and availability. Data can also be provided through attributes such as id and currency.

The declaration beginning with <?xml can identify the XML version and character encoding. It is not required in every document, but it commonly appears at the beginning of an XML file.

What are the basic XML syntax rules?

A well-formed XML document must follow rules that include:

having one root element that contains the remaining content;
closing every opened tag;
nesting elements correctly;
treating names as case-sensitive, which means <Product> and <product> are different;
placing attribute values inside quotation marks;
encoding special characters correctly, for example as < and &.

Unlike HTML, XML does not provide a single predefined collection of tags. Element names can be created for the needs of a particular application as long as the document follows XML syntax.

What is XML used for?

Common uses of XML include:

exchanging data between applications and systems;
creating XML sitemaps;
publishing RSS and Atom feeds;
storing application settings;
supporting web services based on SOAP;
representing data in formats such as SVG;
integrating ecommerce platforms with wholesalers, ERP systems, and marketplaces;
transferring product catalogs, orders, invoices, and inventory data.

The meaning of each element depends on the system that creates or processes the document. Two XML files may therefore use completely different tags and describe unrelated types of data.

How is XML different from HTML?

HTML is primarily used to structure web pages. It provides predefined elements for headings, paragraphs, links, forms, images, and other content that browsers understand.

XML is used to describe data. A tag such as <price> does not tell a browser how the value should be displayed. An application, stylesheet, or another processing mechanism must determine how the data will be used.

How is XML different from JSON?

Both XML and JSON can be used to exchange structured data. JSON is generally more concise, so it has become common in modern web applications and APIs.

XML supports features such as attributes, namespaces, comments, and extensive validation mechanisms. It remains widely used in systems that rely on established standards, require schema validation, or integrate data from several sources.

What makes an XML document correct?

A document is well-formed when it follows the fundamental XML syntax rules. An error such as a missing closing tag may prevent the file from being processed.

An XML document can also be validated against a DTD or an XSD schema. A schema can define permitted elements, their order, required attributes, and data types. A document may therefore be well-formed while still failing the requirements of a particular system.

XML namespaces help distinguish elements from different vocabularies, even when those elements use the same local name.

How does XML relate to SEO?

The most common SEO application of XML is an XML sitemap. It lists website URLs and can provide search engines with additional information about updates, alternate language versions, images, videos, and news content.

A properly maintained sitemap helps search engines discover important URLs, especially on large websites and ecommerce stores. It should contain current, accessible pages that are intended for indexing. It does not replace a clear website structure, internal links, or correct technical configuration.

Is XML still used?

XML is no longer the default format for every new integration because simpler JSON documents are now used in many applications. However, XML remains important in business systems, documents, sitemaps, data feeds, and integrations based on established specifications.

The choice between XML and another format should depend on the system’s requirements, available libraries, validation needs, and the standard supported by every party involved in the integration.