Constructed & Composite Inventory

keyed_groups, compose, and merging multiple sources.

7 min readAnsible AutomationProvision

Constructed Inventory Plugin

The constructed inventory plugin transforms existing inventory data. It creates groups from host attributes, computes variables, and restructures inventory without querying external sources.

constructed.yml


---
plugin: ansible.builtin.constructed
strict: yes

groups:
  # Group by OS family
  debian: ansible_os_family == "Debian"
  redhat: ansible_os_family == "RedHat"

  # Group by environment tag
  production: environment == "production"
  staging: environment == "staging"

  # Group by role
  web: role == "webserver"
  db: role == "database"

compose:
  # Compute custom variables
  ansible_port: 22
  ssh_key: "'~/.ssh/ansible_key'"

  # Format hostnames
  short_name: inventory_hostname.split('.')[0]

The constructed plugin evaluates expressions against existing host variables and creates new groups and variables dynamically.

keyed_groups

keyed_groups examples


keyed_groups:
  # Group by a variable value
  - key: ansible_distribution
    prefix: distro
    separator: "_"

  # Group by a tag
  - key: tags.Environment
    prefix: env
    separator: "_"

  # Group by availability zone
  - key: placement.availability_zone
    prefix: az
    separator: "_"

  # Group by instance type with transform
  - key: instance_type | regex_replace('t3\.', 't3-')
    prefix: type
    separator: "_"

keyed_groups create dynamic groups by extracting values from host attributes. The prefix and separator control group naming.

compose Expressions

compose examples


compose:
  # Conditional variable
  ansible_user: "'admin' if ansible_os_family == 'Debian' else 'ec2-user'"

  # Computed value
  memory_gb: ansible_memtotal_mb | int / 1024

  # List from fact
  installed_services: ansible_facts.services.keys() | list

  # Boolean flag
  is_container: ansible_virtualization_type == "docker"

compose expressions use Jinja2 syntax to compute new variables from existing facts. These variables are available in playbooks.

composite Variables

composite example


composite:
  # Build a host metadata dictionary
  host_metadata:
    hostname: inventory_hostname
    ip: ansible_default_ipv4.address | default(ansible_host)
    os: ansible_distribution ~ " " ~ ansible_distribution_version
    role: tags.Role | default("unknown")
    env: tags.Environment | default("unknown")

  # Build a list of all hosts in the same group
  peer_hosts: groups['webservers'] | map('extract', hostvars, 'ansible_host') | list

composite variables let you build complex data structures from multiple sources. They are useful for templates and reporting.

Merging Multiple Sources

ansible.cfg with multiple inventories


[defaults]
inventory = ./static.yml, ./dynamic.yml

Ansible merges multiple inventory sources. Static and dynamic inventories are combined, with later sources overriding earlier ones.

Constructed inventory is powerful

Constructed inventory lets you restructure any inventory source. Use it to create logical groups from cloud tags, compute variables from facts, or merge multiple sources into a unified view.

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Stage 4 · Provision

Dynamic Inventory & Scale