Dictionary Merging in Ansible Inventories

Projects: c2platform/ansible , c2platform/rws/ansible-gis , c2platform/rws/ansible-collection-wincore , c2platform/rws/ansible-collection-gis

Problem

Dictionary merging can reduce duplication, but it also makes data flow less obvious. When merges are undocumented or applied too broadly, it becomes hard to see which values win and why.

Context

In C2 Platform inventory projects, hash_behaviour = merge in ansible.cfg allows dictionaries from multiple variable sources to be combined. This can be useful for layered configuration, but only when the merged structure stays easy to reason about.

Solution

1. Examine and Document Your Inventory Structure: Clearly document where and how hash_behaviour = merge is applied within your Ansible projects. It’s crucial to maintain an updated inventory structure documentation, noting any instances of merged dictionaries.
2. Use Merging Judiciously: Employ dictionary merging primarily for host or group variables that benefit from this method. Overuse can lead to complexity and obscure data relationships.
3. Thoughtful and Unique Keys: When using dictionaries ensure the keys are well chosen and unique within your inventory project.
4. Rigorous Testing: Thoroughly test playbooks to validate that merged dictionaries behave as expected. This step is vital to catch any merging issues or unintended consequences early in the development cycle.
5. Understand Variable Precedence: Familiarize yourself with the rules of variable precedence in Ansible. Knowledge of how variables from different levels (e.g., group_vars, host_vars) merge is critical for predicting the final configuration state.
6. Conflict Resolution Strategy: Develop and document strategies for handling key conflicts in merged dictionaries. This plan ensures consistent handling of conflicts across projects.
7. Secure Sensitive Data: When merging dictionaries involves sensitive information, use encryption methods like Ansible Vault. This practice ensures that sensitive data is securely managed while allowing non-sensitive information to be merged as needed.

Practical Examples

Merging win_chocolatey Configurations

When utilizing win_chocolatey variable of the c2platform.wincore.win Ansible role for different server groups within your Ansible inventory, utilizing “dictionaries” enables a more dynamic and flexible approach compared to simple “lists”. This method ensures servers in multiple groups receive all necessary packages without configuration conflicts.

Example Configuration in group_vars/windows/main.yml:

win_chocolatey:
  windows:
    - name: nuget.commandline

Example Configuration in group_vars/age_pro/main.yml:

win_chocolatey:
  age_pro:
    - name: firefox

When a server belongs to both the windows and age_pro groups, the merged configuration for win_chocolatey will include both the NuGet command line and Firefox, ensuring comprehensive package management across server roles.

Merged Configuration Result

win_chocolatey:
  windows:
    - name: nuget.commandline
  age_pro:
    - name: firefox

Flexibility in Key Naming

You can use any descriptive key names for categorizing configurations within dictionaries, as long as the keys remain unique at the level where the merge happens. For example, the keys can be whatever and some_other_key.

win_chocolatey:
  whatever:
    - name: nuget.commandline

win_chocolatey:
  some_other_key:
    - name: firefox

Limitations in Key Naming

You can choose any key name you want, but only dictionaries merge predictably in this pattern. Simple lists do not merge.

1. In group_vars/windows/main.yml:

win_chocolatey:
  tools:
    - name: nuget.commandline

1. In group_vars/age_pro/main.yml:

win_chocolatey:
  tools:
    - name: firefox

Additional Information

• Review Dictionary Merging in Ansible Projects