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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

If you need to reuse a YAML anchor between multiple files, you can define the anchor in a separate file and include it in your main YAML file using the << (merge) operator.

For example, let’s say you have an anchor that you want to reuse in multiple YAML files, like this:

# Define an anchor for an object
&my_obj
  name: John
  age: 30
  hobbies:
    - reading
    - hiking

You can include this anchor in another YAML file using the « operator, like this:

# Include the anchor from the other file
<<: anchor.yaml

# Use the anchor in your YAML document
person:
  info: *my_obj

In this example, we include the anchor.yaml file using the « operator, which merges the contents of anchor.yaml into our main YAML file. We then reference the my_obj anchor within the person object using the *my_obj alias.

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.