YAML ‘Variables’ Are a Myth. Here’s How to *Actually* Reuse Code in 2026

Last updated on January 26, 2026

The term “YAML templates” causes some of the most common confusion in the automation and configuration world. If you’ve ever seen syntax like {{ variable }} and assumed it was a native YAML feature, this article is for you. As an IoT Engineer, I’ve wrangled thousands of config files, and today, in 2026, we’re going to demystify this concept, clarify the crucial difference between native YAML capabilities and external templating engines, and make you an expert at reusing code efficiently.

Throughout this definitive guide, we’ll explore how to reuse YAML code the native way and how to supercharge your configurations with external tools like Jinja2, all backed by practical, up-to-date examples.

The Core Misconception: What Are ‘YAML Templates,’ Really?

YAML (a recursive acronym for “YAML Ain’t Markup Language”) is a human-readable data serialization language. Its primary job is to structure data, not to process logic or dynamic variables. On its own, YAML has no idea what {{ some_variable }} means.

So, where does the idea of templates come from? It stems from two distinct concepts that often get jumbled together:

1. Native YAML Reusability: Through a brilliant and often underused feature called anchors and aliases. This lets you define a block of code once and reuse it in multiple places within the same file.
2. External Templating Engines: Tools like Jinja2, Go Templates, or Liquid process a text file (which happens to contain YAML syntax in our case) before it gets interpreted. These tools are what actually understand and replace variables, loops, and conditionals ({{ variable }}, {% for item in list %}, etc.).

To master YAML for configuration in 2026, it’s crucial to understand when and why to use each of these methods.

The Native Way: Reusing YAML with Anchors & Aliases

The cleanest, most straightforward way to avoid repetition in a YAML file is by using anchors and aliases. This mechanism is perfect for defining a common set of properties and applying them to different elements.

The syntax is simple:

• Anchor (&): Used to define a block of data and give it a name. For example, &my_anchor.
• Alias (*): Used to reference (reuse) the data block defined by an anchor. For example, *my_anchor.

Let’s look at a practical example in a docker-compose.yml file. Imagine we have several services that share the same logging and restart configuration:

version: "3.9"

# Define an anchor with the common configuration using a Docker Compose extension
x-common-config: &common-service-config
  restart: always
  logging:
    driver: "json-file"
    options:
      max-size: "10m"
      max-file: "3"

services:
  webserver:
    image: nginx:latest
    <<: *common-service-config # Use the alias to merge the configuration
    ports:
      - "80:80"

  database:
    image: postgres:16
    <<: *common-service-config # Reuse the same configuration
    environment:
      - POSTGRES_PASSWORD=supersecret

In this example, we define the restart and logging policy once under the &common-service-config anchor. We then inject it into both the webserver and database services using the *common-service-config alias. The <<: operator is a YAML merge key feature that lets you combine maps.

The External Engine: Supercharging YAML with Tools like Jinja2

When you need more than just static reuse—like inserting dynamic values, running loops, or applying conditional logic—you need a templating engine. The Jinja2 YAML combo is the most popular pairing, and you’ll find it everywhere in DevOps and home automation tools like Ansible and Home Assistant.

Here’s how the process works:

1. You write a .yaml.j2 file that contains a mix of YAML and Jinja2 syntax.
2. The program (e.g., Ansible) reads this file, processes it with a set of variables you provide, and generates a final, valid YAML file.
3. Finally, this pure YAML file is what’s used for the actual configuration.

Here’s a simple example in the context of Ansible:

# File: tasks/main.yml
- name: "Create a configuration file for the {{ environment }} environment"
  template:
    src: config.j2
    dest: /etc/app/config.yaml

# Template file: templates/config.j2
---
database:
  host: {{ db_host }}
  port: {{ db_port | default(5432) }}
  user: {{ db_user }}

api_keys:
  {% for key in api_keys_list %}
  - name: {{ key.name }}
    value: {{ key.value }}
  {% endfor %}

debug_mode: {% if environment == 'production' %}false{% else %}true{% endif %}

Here, Ansible will replace {{ environment }}, {{ db_host }}, etc., with values defined elsewhere (like in an inventory or role variables). It will also process the for loop and the if conditional to generate a valid config.yaml tailored to each specific environment.

Comparison Chart: YAML Anchors/Aliases vs. Jinja2

To make it crystal clear, this comparison table breaks down when and why you should use each method.

YAML Templating in Action: 2026 Real-World Examples

Theory is great, but let’s see how these concepts are applied in the tools we use every day in the IoT and DevOps space.

Home Assistant: Dynamic Automations

In Home Assistant, YAML configuration is still king for complex automations. Jinja2 is the engine that allows you to create truly intelligent notifications and actions.

# configuration.yaml or automations.yaml
alias: "High temperature notification for living room"
trigger:
  - platform: numeric_state
    entity_id: sensor.living_room_temperature
    above: 28
action:
  - service: notify.mobile_app_my_phone
    data:
      title: "Temperature Alert!"
      message: "The temperature in the living room just exceeded 28°C. It is now {{ states('sensor.living_room_temperature') }}°C."

Here, {{ states('sensor.living_room_temperature') }} is a Jinja2 template that Home Assistant evaluates in real-time to fetch the sensor’s current value and insert it into the notification.

Kubernetes with Helm: Packaging Applications

Helm is the de facto package manager for Kubernetes. It uses Go templates (which are very similar to Jinja2) to create reusable “charts.” A chart is a collection of YAML templates that describe an application.

Inside a Helm chart, a deployment.yaml file might look something like this:

# templates/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .Release.Name }}-webapp
spec:
  replicas: {{ .Values.replicaCount }}
  selector:
    matchLabels:
      app: {{ .Release.Name }}-webapp
  template:
    metadata:
      labels:
        app: {{ .Release.Name }}-webapp
    spec:
      containers:
        - name: web
          image: "{{ .Values.image.repository }}:{{ .Values.image.tag }}"
          ports:
            - containerPort: 80

When you install the chart, Helm replaces {{ .Values.replicaCount }} and the other variables with values defined in a values.yaml file, generating a Kubernetes manifest that’s ready to deploy.

Conclusion: When to Use Each Method

I hope this 2026 guide has finally cleared up the concept of YAML ‘templates’ for you. The key isn’t to pick one method over the other, but to understand which tool is right for the job.

My recommendation as an engineer is simple:

• Use YAML anchors and aliases whenever you need to reuse static blocks of code within a single file. It’s the cleanest, native, and most efficient solution for reducing duplication in configs like Docker Compose.
• Turn to templating engines like Jinja2 when your configurations need to be dynamic: injecting secrets, configuring different environments (dev, prod), iterating over lists of items, or applying conditional logic. It’s the backbone of large-scale automation with tools like Ansible, Helm, or Home Assistant.

Mastering both approaches will allow you to write cleaner, more maintainable, and powerful configuration files—an indispensable skill in today’s tech ecosystem.