Module 6 Lesson 4: Extension Fields and YAML Anchors

As your docker-compose.yml grows from 2 services to 20, you will notice a lot of repetition. Every service might have the same environment variables, the same logging config, and the same network.

To avoid "Copy-Paste Errors," we use YAML Anchors and Extension Fields.

1. Extension Fields (x-)

You can define custom blocks at the top level of your file that Docker Compose ignores, but you can "Reference" later. These must start with x-.

x-common-config: &common  # The '&' creates the anchor
  restart: always
  networks:
    - backend
  environment:
    DEBUG: "true"

services:
  web:
    <<: *common           # The '<<: *' merges the anchor here
    image: my-web

  api:
    <<: *common           # The same config, without typing it again!
    image: my-api

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2. Overriding the Anchor

If a service needs mostly the same things as the anchor but one difference, you just define the difference below the merge:

  db:
    <<: *common
    environment:
      DEBUG: "false"      # This overwrites the "true" from the anchor

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3. Why Use This? (The DRY Principle)

DRY stands for "Don't Repeat Yourself."

• If you need to change the LOG_LEVEL for all 20 services, you only have to change it in one place (the extension field).
• This reduces the chance of someone forgetting to update one specific service during a deployment.

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4. Cleaning Up the File

Notice how much shorter your file becomes!

• It is easier to read.
• It is easier to audit.
• It is easier to maintain.

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Exercise: The Anchor Architect

1. Create a Compose file with 3 services: worker-a, worker-b, and worker-c.
2. Define an extension field x-worker-base that includes:
   • An image (busybox).
   • A command (sleep 3600).
   • A restart policy (on-failure).
3. Use the <<: * syntax to apply this base to all three workers.
4. For worker-c, override the command to sleep 5000.
5. Run docker-compose config. Look at the output. Did Docker correctly "Expand" your anchors into a full file?

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Summary

YAML anchors and extension fields are the "Variables" and "Functions" of the DevOps world. They keep your configuration clean, predictable, and scalable as your architecture grows in complexity.

Next Lesson: Environment splitting: Multi-file configuration (base + override).