Introduction
As deployments grow in complexity, structuring ARM templates becomes increasingly important. While linked templates separate deployments into external files, nested templates allow you to organize deployments within a single template file.
Nested templates are a powerful feature in Microsoft Azure that help you manage complexity without introducing external dependencies.
What Are Nested ARM Templates
A nested template is an ARM template embedded inside another ARM template.
Instead of referencing an external file, the template is defined directly within the parent template using a deployment resource.
This allows you to:
- Group related resources
- Control deployment flow
- Keep everything in a single file
Why Use Nested Templates
Nested templates solve specific design and operational challenges.
1. Logical Grouping
You can group related resources together.
Example:
- Networking components in one nested template
- Compute resources in another
2. Controlled Deployment Flow
You can define:
- Execution order
- Dependency relationships
- Deployment boundaries
3. Single File Management
Everything is contained in one template:
- No external hosting required
- No URL dependencies
- Easier portability
4. Scoped Deployments
Nested templates can be deployed at different scopes:
- Resource group
- Subscription
- Management group
How Nested Templates Work
Nested templates are implemented using the same deployment resource used for linked templates.
The difference is:
- Linked templates use templateLink
- Nested templates use an inline template definition
Example of a Nested Template
{
"type": "Microsoft.Resources/deployments",
"apiVersion": "2021-04-01",
"name": "nestedNetworkDeployment",
"properties": {
"mode": "Incremental",
"template": {
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"vnetName": {
"type": "string"
}
},
"resources": [
{
"type": "Microsoft.Network/virtualNetworks",
"apiVersion": "2023-02-01",
"name": "[parameters('vnetName')]",
"location": "[resourceGroup().location]",
"properties": {
...
}
}
]
},
"parameters": {
"vnetName": {
"value": "[parameters('vnetName')]"
}
}
}
}
Key Components of Nested Templates
Inline Template Definition
The nested template is defined inside the parent template under:
- properties.template
Parameter Passing
Parameters must be passed explicitly from the parent to the nested template.
Deployment Resource
Each nested template is executed using:
- Microsoft.Resources/deployments
Dependency Management
Nested templates support dependency control using dependsOn.
Example
"dependsOn": [
"nestedNetworkDeployment"
]
What This Means
- The next deployment waits for the nested template to complete
Nested Templates vs Linked Templates
Understanding the difference is critical.
Nested Templates
- Defined inline
- No external files
- Easier portability
- Can become large if overused
Linked Templates
- Stored externally
- Referenced via URL
- Better for large-scale modular design
- Requires hosting and access management
When to Use Nested Templates
Use nested templates when:
- You want a single deployment file
- Logical grouping is needed
- External dependencies are not desirable
When Not to Use Nested Templates
Avoid nested templates when:
- Templates become too large
- Reusability across projects is required
- Multiple teams need to manage separate components
Common Mistakes
Over-Nesting
Creating too many nested levels:
- Reduces readability
- Makes troubleshooting difficult
Parameter Misalignment
- Parent and nested templates do not match parameters
Fix:
- Keep parameter names consistent
- Validate parameter flow
Ignoring Dependencies
- Incorrect deployment order
Fix:
- Use dependsOn correctly
Best Practices
Keep Nesting Logical
Group resources by function:
- Networking
- Compute
- Storage
Limit Depth
Avoid deeply nested structures.
Maintain Readability
- Use clear naming
- Keep structure organized
Validate Nested Templates Independently
Test logic before embedding it into parent templates.
Practical Use Case
Scenario:
You want to deploy:
- Virtual Network
- Subnet
- Virtual Machine
Approach:
- Nested template for networking
- Nested template for compute
- Parent template controls execution order
This keeps structure clean and manageable.
Conclusion
Nested ARM templates provide a way to structure complex deployments without relying on external files.
They allow you to:
- Organize resources logically
- Control deployment flow
- Keep deployments contained in a single file
However, they must be used carefully to avoid unnecessary complexity.
ARM templates are not just about deployment.
They are about structured and controlled infrastructure design.
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