XML syntax

XML is a markup language that structurally organizes data and enables two different systems or applications to exchange and understand the content. It provides a standard format that is both human- and machine-readable. XML accomplishes this with the use of descriptive tags to identify specific information, much like HTML. This markup language also lets administrators define their own tags to meet their needs. Programming languages that recognize the XML standard can interpret its data.

XML defines the data’s format in documents, with .xml as the file extension. An example of this is Microsoft 365 applications, like Word and Excel. These applications use XML to define the structure of a file’s contents. Microsoft 365 document files have an x character at the end of the extension, like .docx, which reflects XML format use.

XML documents consist of a tree structure that begin with a root element and branch into one or more child elements. The file relies on a document type declaration (DTD) that defines the file’s structure, elements and attributes. The DTD tag begins with <!DOCTYPE> and contains the rules for the document’s structure. Tags or an external document reference define the structure. Comments in XML documents reside within the <–! comment-text –> tag.

Linux developers use XML to pass structured information between applications. It’s less critical to data center administrators as a configuration tool, although they may use it to connect to data sources. Infrastructure as code (IaC) tools, like Ansible, can also read XML files using the Ansible XML module.

YAML syntax

YAML is a data serialization language in IaC configuration files that declares settings. YAML uses a different structure and syntax from XML, which makes it human-readable, not machine-readable. The primary function of YAML is data transmission rather than data formatting.

Its syntax is strict but simple. It uses white spaces — not tabs — to define a hierarchical structure. Sections nest using these spaces. YAML doesn’t rely on formatting symbols like XML, which makes it easier to read and understand.

From a data center perspective, YAML is a more useful tool. IaC tools, like Ansible, Docker Compose, Terraform and AWS CloudFormation, use YAML to define and manage configurations for various bare-metal and virtual systems.

YAML is the more common configuration file format to achieve automation benefits, like consistency, repeatability and easy modification, especially with tools like Ansible. For example, Docker Compose uses YAML files to define multiple services or applications. Kubernetes container management also relies on YAML. Administrators often employ YAML-based IaC tools to manage Linux-based technologies. YAML’s simplicity also makes it simpler to review files and envision data center layout.

The sample playbook in Figure 1 comes from the original Ansible documentation and depicts the YAML structure that defines the file’s functionality. This functionality enables on-premises and cloud automation and scalability.

YAML code begins with three dashes (—) and concludes with three dots (…). Sections of YAML documents are called maps. Maps are hierarchical lists of values.

Lists begin with a key that links to one or more values. For example, I might have a key named birds with three values (eagle, chicken, hawk):

# YAML bird list
---
birds:
- eagle
- chicken
- hawk
...

Work with XML and YAML files

Powerful text editors, such as Vim and Emacs, are extensible to support XML and YAML documents. In addition, strong integrated development environments (IDEs) make managing these files easier. File authors may use any editor, but they should check utilities for XML and YAML extensions that provide additional functionality. Tools such as Visual Studio Code, Eclipse, Notepad++ and PyCharm offer standard development features, like error checking, autocompletion and syntax highlighting.

XML validators enable authors to verify the syntax and structure of their documents before using them in production. For example, validators identify one of the most common XML mistakes: missing closing tags. Validation may be built into an editor or IDE, or it may be an external service. Errors in XML documents cause an immediate failure, so catching these early is crucial.

As with any configuration file today, version control measures are essential. Use a repository system, like Git, to manage files.

XML and YAML file security

Security is not an inherent part of text files. Taking other precautions can protect the integrity and privacy of XML and YAML resources in data centers, however. Here are a few basic ideas:

• Use a version control system to ensure the use of current files.
• Run XML or YAML files from trusted sources.
• Test configuration files on VMs or devices.
• Control access using permissions.
• Consider file encryption to protect content.

Damon Garn owns Cogspinner Coaction and provides freelance IT writing and editing services. He has written multiple CompTIA study guides, including the Linux+, Cloud Essentials+ and Server+ guides, and contributes extensively to TechTarget Editorial and CompTIA Blogs.