YAML Anchors and Aliases: Reusable and Maintainable YAML Documents

If you’ve ever worked with YAML files, you know how quickly they can become long and unwieldy. When you have repeated sections of YAML code or complex structures, it can be difficult to maintain your YAML files and keep them organized. YAML anchors and aliases offer a solution to this problem, allowing you to create reusable and maintainable YAML documents.

What are YAML anchors and aliases?

YAML anchors and aliases are a way to define a value once and reference it multiple times within a YAML document. An anchor is a value that is defined with an ampersand “&” followed by a name. The value can be anything, including an object, array, or scalar. An alias is a reference to an anchor that is defined with an asterisk “*” followed by the anchor name.

Using YAML anchors and aliases to simplify your YAML files
Let’s say you have a YAML file with several similar objects, like this:

person1:
  name: John Smith
  age: 35
  hobbies:
    - reading
    - hiking

person2:
  name: Jane Doe
  age: 28
  hobbies:
    - reading
    - yoga

You can simplify this YAML file by using anchors and aliases, like this:

# Define an anchor for the hobbies array
&hobbies
  - reading

person1:
  name: John Smith
  age: 35
  hobbies: *hobbies

person2:
  name: Jane Doe
  age: 28
  hobbies:
    *hobbies
    - yoga

In this example, we define an anchor called hobbies that contains the reading value. We then reference this anchor using the *hobbies alias within the person1 and person2 objects. This allows us to define the hobbies value once and reuse it multiple times, simplifying our YAML document and making it easier to maintain.

Reusing YAML anchors between multiple files

In YAML, anchors (defined with &) and aliases (referenced with *) are powerful tools for reusing repeated data within a single file. However, reusing anchors across multiple files is not supported by the YAML specification. Attempting to reference an anchor defined in another file will result in an error, as standard YAML parsers do not recognize such cross-file references.

Workarounds and Extensions

While YAML itself does not support cross-file anchors, several tools and extensions can help achieve similar functionality:

1. Custom Processing Tools: Tools like yq can preprocess or manipulate YAML files to include content from different files before passing them to a YAML parser. This preprocessing step allows for combining multiple files into a single YAML document where anchors can be referenced correctly.
2. Extended Parsers: Some YAML parsers, such as ruamel.yaml, offer more advanced features that might support complex scenarios, including custom implementations for cross-file references. For example, the ccorp_yaml_include plugin extends ruamel.yaml to implement the !include tag, allowing the use of YAML anchors across included files by processing !include tags at composition time. You can find more details about this plugin here.

For a pure YAML approach without custom tooling, handle inclusions or merges within your application logic or use preprocessing scripts to combine the files into a single YAML document.

Conclusion

YAML anchors and aliases are a powerful way to simplify and maintain your YAML documents. By defining values once and referencing them multiple times, you can create more readable and reusable YAML files. Whether you’re working with complex YAML structures or just looking to streamline your YAML code, anchors and aliases are a useful tool for any software engineer.