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feat: added sample code for configuration heirarchy
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‎_posts/2025-12-10-how-we-used-configuration-heirarchies-to-support-scale-of-1million+-users-per-day-in-our-saas-infra.md‎

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### converged configuration
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convergence of configuration came out as a side effect of the `inventory/hosts` pattern that was gave with `parent/child` relationship
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convergence of configuration came out as a side effect of the `inventory/hosts` pattern that came with `parent/child` relationship
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ansible merges configuration finally at [per host level](https://docs.ansible.com/projects/ansible/latest/inventory_guide/intro_inventory.html#inheriting-variable-values-group-variables-for-groups-of-groups) which you specify in your `tasks` file which can be observed as below
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# ansible-playbook -i inventories/DEV/dev01/user1 playbooks/aws/control-plane/create.yml
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```
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## show me the code 🧑‍💻
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sample code to understand the pattern is available here ->
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## value additions
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✅ no config sprawl

‎code/configuration-heirarchies/README.md‎

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# configuration heirarchies
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this repo gives a sample of the configuration heirarchy pattern that we used to scale our infra code base for our app. blog post ->
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## structure
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```text
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.
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├── inventories
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│   ├── DEV
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│   │   └── dev01
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│   │   ├── group_vars
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│   │   │   ├── DEV -> ../../../group_vars/DEV/
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│   │   │   └── dev01
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│   │   │   ├── db.yaml
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│   │   │   └── eks.yaml
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│   │   ├── hosts
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│   │   └── user1
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│   │   ├── group_vars
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│   │   │   ├── DEV -> ../../../../group_vars/DEV/
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│   │   │   ├── dev01 -> ../../group_vars/dev01/
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│   │   │   └── user1
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│   │   │   └── feature_flags.yaml
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│   │   └── hosts
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│   ├── group_vars
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│   │   ├── aws_us_east_1_pre_account
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│   │   │   └── common.yaml
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│   │   ├── aws_us_east_1_prod_account
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│   │   │   └── common.yaml
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│   │   ├── aws_us_east_2_pre_account
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│   │   │   └── common.yaml
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│   │   ├── aws_us_east_2_prod_account
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│   │   │   └── common.yaml
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│   │   ├── DEV
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│   │   │   └── eks.yaml
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│   │   ├── PRE
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│   │   │   └── eks.yaml
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│   │   └── PROD
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│   │   ├── eks.yaml
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│   │   └── feature_flags.yaml
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│   ├── PRE
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│   │   └── pre01
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│   │   ├── group_vars
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│   │   │   ├── PRE -> ../../../group_vars/PRE/
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│   │   │   └── pre01
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│   │   │   └── feature_flags.yaml
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│   │   └── hosts
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│   └── PROD
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│   └── prod
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│   ├── group_vars
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│   │   └── PROD -> ../../../group_vars/PROD/
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│   └── hosts
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├── playbooks
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├── README.md
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└── roles
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```
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## how the heirarchy works
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- Dev env: DEV -> dev01 -> user1
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- Pre env: PRE -> pre01
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- Prod env: Prod -> prod
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## how to view the inheritance of vars
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- ensure to have ansible installed
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- clone/fork the repo and navigate to `./code/configuration-heirarchies`
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- for `DEV -> dev01`:
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```shell
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ansible-inventory -i inventories/DEV/dev01/hosts --host deploy_host_local
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# output:
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# {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "1.34",
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# "eks_k8s_version": "1.34",
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# "eks_network_calico_version": "3.31.2",
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# "eks_nodes_ami_type": "AL2023_x86_64_STANDARD",
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# "eks_nodes_instance_types": [
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# "m7i-flex.xlarge"
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# ],
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# "eks_nodes_version": "1.34",
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# "eks_ssm_parameter_name": "/aws/service/eks/optimized-ami/{{ eks_cluster_version }}/amazon-linux-2023/x86_64/standard/recommended/release_version",
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# "rds_allocated_storage": 32,
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# "rds_engine_version": "15.7",
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# "rds_max_allocated_storage": 64,
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# "rds_parameter_group_family": "postgres15"
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# }
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```
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- for `DEV -> dev01 -> user1`:
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```shell
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ansible-inventory -i inventories/DEV/dev01/user1/hosts --host deploy_host_local
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# NOTES:
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# - notice 2 vars: feature_flags_enable_feature_x, feature_flags_enable_feature_y got added into the list
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# output:
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# {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "1.34",
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# "eks_k8s_version": "1.34",
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# "eks_network_calico_version": "3.31.2",
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# "eks_nodes_ami_type": "AL2023_x86_64_STANDARD",
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# "eks_nodes_instance_types": [
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# "m7i-flex.xlarge"
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# ],
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# "eks_nodes_version": "1.34",
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# "eks_ssm_parameter_name": "/aws/service/eks/optimized-ami/{{ eks_cluster_version }}/amazon-linux-2023/x86_64/standard/recommended/release_version",
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# "feature_flags_enable_feature_x": false,
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# "feature_flags_enable_feature_y": false,
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# "rds_allocated_storage": 32,
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# "rds_engine_version": "15.7",
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# "rds_max_allocated_storage": 64,
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# "rds_parameter_group_family": "postgres15"
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# }
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```
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- for `PRE -> pre01`
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```shell
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ansible-inventory -i inventories/PRE/pre01/hosts --host deploy_host_aws_us_east_1
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# output
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# {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "v2",
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# "eks_k8s_version": "1.33",
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# "feature_flags_enable_feature_x": false,
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# "feature_flags_enable_feature_y": true
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# }
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```
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- for `PROD -> prod`
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```shell
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ansible-inventory -i inventories/PROD/prod/hosts --host deploy_host_aws_us_east_1
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# output:
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# {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "v2",
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# "eks_k8s_version": "1.33",
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# "feature_flags_enable_feature_x": true,
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# "feature_flags_enable_feature_y": true
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# }
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```
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## how to view the parent-child relationship
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```shell
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ansible-inventory -i inventories/DEV/dev01/hosts --list
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ansible-inventory -i inventories/DEV/dev01/user1/hosts --list
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ansible-inventory -i inventories/PRE/pre01/hosts --list
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ansible-inventory -i inventories/PROD/prod/hosts --list
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# output
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# {
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# "PRE": {
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# "children": [
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# "pre01"
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# ]
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# },
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# "_meta": {
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# "hostvars": {
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# "deploy_host_aws_us_east_1": {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "v2",
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# "eks_k8s_version": "1.33",
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# "feature_flags_enable_feature_x": false,
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# "feature_flags_enable_feature_y": true
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# },
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# "deploy_host_aws_us_east_2": {
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# "ansible_connection": "local",
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# "ansible_python_interpreter": "/usr/bin/env python3",
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# "eks_cluster_version": "v2",
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# "eks_k8s_version": "1.33",
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# "feature_flags_enable_feature_x": false,
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# "feature_flags_enable_feature_y": true
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# }
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# }
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# },
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# "all": {
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# "children": [
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# "ungrouped",
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# "PRE"
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# ]
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# },
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# "aws_us_east_1_pre_account": {
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# "hosts": [
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# "deploy_host_aws_us_east_1"
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# ]
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# },
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# "aws_us_east_2_pre_account": {
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# "hosts": [
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# "deploy_host_aws_us_east_2"
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# ]
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# },
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# "pre01": {
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# "children": [
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# "aws_us_east_1_pre_account",
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# "aws_us_east_2_pre_account"
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# ]
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# }
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# }
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```
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## at what levels is variables can be placed
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- DEV account
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```text
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# NOTE:
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- this means the vars are inherited from DEV -> dev01 -> user1 where it allows users to set only required vars at each level
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DEV -> vars for account level
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dev01 -> vars for dev01 env
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user1 -> vars for dev01/user1 env
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```
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- PRE account
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```text
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# NOTE:
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- this means the vars are inherited from PRE -> pre01 where it allows users to set only required vars at each level
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PRE -> vars for account level
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pre01 -> vars for pre01 env
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```
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- PROD account
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```text
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# NOTE:
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- this means the vars are inherited from PROD -> prod where it allows users to set only required vars at each level
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PROD -> vars for account level
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prod -> vars for prod env
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```

‎code/configuration-heirarchies/inventories/DEV/dev01/group_vars/DEV‎

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../../../group_vars/DEV/

‎code/configuration-heirarchies/inventories/DEV/dev01/group_vars/dev01/db.yaml‎

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rds_parameter_group_family: postgres15
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rds_allocated_storage: 32
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rds_max_allocated_storage: 64
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rds_engine_version: "15.7"

‎code/configuration-heirarchies/inventories/DEV/dev01/group_vars/dev01/eks.yaml‎

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eks_cluster_version: "1.34"
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eks_ssm_parameter_name: "/aws/service/eks/optimized-ami/{{ eks_cluster_version }}/amazon-linux-2023/x86_64/standard/recommended/release_version"
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eks_network_calico_version: 3.31.2
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eks_nodes_version: "1.34"
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eks_nodes_instance_types:
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- m7i-flex.xlarge
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eks_nodes_ami_type: AL2023_x86_64_STANDARD

‎code/configuration-heirarchies/inventories/DEV/dev01/hosts‎

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---
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all:
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hosts:
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deploy_host_local:
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ansible_connection: local
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children:
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DEV:
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children:
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dev01:
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hosts:
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deploy_host_local:
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vars:
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ansible_python_interpreter: /usr/bin/env python3

‎code/configuration-heirarchies/inventories/DEV/dev01/user1/group_vars/DEV‎

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../../../../group_vars/DEV/

‎code/configuration-heirarchies/inventories/DEV/dev01/user1/group_vars/dev01‎

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../../group_vars/dev01/

‎code/configuration-heirarchies/inventories/DEV/dev01/user1/group_vars/user1/feature_flags.yaml‎

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feature_flags_enable_feature_x: false
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feature_flags_enable_feature_y: false

‎code/configuration-heirarchies/inventories/DEV/dev01/user1/hosts‎

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---
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all:
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hosts:
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deploy_host_local:
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ansible_connection: local
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children:
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DEV:
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children:
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dev01:
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children:
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user1:
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hosts:
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deploy_host_local:
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vars:
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ansible_python_interpreter: /usr/bin/env python3

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