Managing dependencies in ARM template

Managing dependencies in ARM template

Managing dependencies in an Azure Resource Manager (ARM) template is crucial for ensuring that resources are deployed in the correct order, particularly when one resource depends on another. ARM templates allow you to specify and manage these dependencies to ensure the right sequence of actions is followed during deployment.

In ARM templates, dependencies can be managed using the dependsOn property, which ensures that certain resources are created or updated before others. Let's explore how to use dependsOn effectively and also look at implicit dependencies.

1. dependsOn Property

The dependsOn property explicitly defines the order in which resources are deployed in an ARM template. This is useful when one resource depends on another, such as when a virtual machine (VM) requires a network interface card (NIC) to be created first, or a virtual network (VNet) must be deployed before a subnet.

Syntax of dependsOn

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"dependsOn": [
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  "<resourceId>"
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]

The dependsOn property is an array of resource IDs that defines the order of resource deployment.

If a resource in the dependsOn array fails, the deployment of the dependent resource will not proceed.

Example of dependsOn

Suppose you want to deploy a virtual machine that depends on a virtual network and a network interface. Here's how you would define that dependency:

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"resources": [
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  {
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    "type": "Microsoft.Network/virtualNetworks",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "myVnet",
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    "properties": {
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      "addressSpace": {
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        "addressPrefixes": ["10.0.0.0/16"]
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      }
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    }
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  },
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  {
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    "type": "Microsoft.Network/networkInterfaces",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "myNIC",
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    "dependsOn": [
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      "[resourceId('Microsoft.Network/virtualNetworks', 'myVnet')]"
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    ],
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    "properties": {
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      "ipConfigurations": [
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        {
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          "name": "ipconfig1",
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          "properties": {
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            "subnet": {
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              "id": "[resourceId('Microsoft.Network/virtualNetworks/subnets', 'myVnet', 'default')]"
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            }
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          }
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        }
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      ]
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    }
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  },
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  {
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    "type": "Microsoft.Compute/virtualMachines",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "myVM",
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    "dependsOn": [
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      "[resourceId('Microsoft.Network/networkInterfaces', 'myNIC')]"
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    ],
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    "properties": {
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      "hardwareProfile": {
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        "vmSize": "Standard_DS1_v2"
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      },
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      "osProfile": {
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        "computerName": "myVM",
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        "adminUsername": "adminuser",
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        "adminPassword": "password123"
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      },
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      "networkProfile": {
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        "networkInterfaces": [
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          {
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            "id": "[resourceId('Microsoft.Network/networkInterfaces', 'myNIC')]"
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          }
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        ]
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      }
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    }
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  }
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]

Explanation:

1. Virtual Network (myVnet): This resource is created first, and there's no dependsOn because it's the first resource.

2. Network Interface (myNIC): This depends on the virtual network myVnet (specified in the dependsOn array), as the NIC needs to be associated with a subnet in the virtual network.

3. Virtual Machine (myVM): This depends on the network interface myNIC (specified in the dependsOn array), because the VM needs the NIC to be created and associated with it.

2. Implicit Dependencies

In addition to dependsOn, ARM templates also have implicit dependencies. Azure implicitly knows the order in which resources should be deployed based on their references to other resources.

Example of Implicit Dependency

Consider the following example where a virtual machine depends on a network interface, but no dependsOn property is used:

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"resources": [
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  {
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    "type": "Microsoft.Network/networkInterfaces",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "myNIC",
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    "properties": {
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      "ipConfigurations": [
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        {
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          "name": "ipconfig1",
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          "properties": {
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            "subnet": {
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              "id": "[resourceId('Microsoft.Network/virtualNetworks/subnets', 'myVnet', 'default')]"
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            }
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          }
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        }
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      ]
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    }
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  },
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  {
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    "type": "Microsoft.Compute/virtualMachines",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "myVM",
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    "properties": {
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      "hardwareProfile": {
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        "vmSize": "Standard_DS1_v2"
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      },
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      "osProfile": {
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        "computerName": "myVM",
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        "adminUsername": "adminuser",
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        "adminPassword": "password123"
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      },
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      "networkProfile": {
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        "networkInterfaces": [
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          {
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            "id": "[resourceId('Microsoft.Network/networkInterfaces', 'myNIC')]"
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          }
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        ]
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      }
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    }
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  }
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]

In this case:

1. The network interface (myNIC) is created before the virtual machine (myVM), because the VM references the NIC via the networkInterfaces property.

2. Azure will implicitly understand that the network interface must be deployed before the VM, even though no dependsOn is explicitly provided.

3. Circular Dependencies

Circular dependencies occur when two or more resources depend on each other, creating an endless loop. ARM templates do not allow circular dependencies, and you will receive an error if such a situation is detected.

For example, consider the following scenario:

1. Resource A depends on Resource B.

2. Resource B depends on Resource A.

This is a circular dependency, and ARM will not allow it.

To avoid circular dependencies:

1. Reassess the resources' relationships.

2. Ensure that dependencies are only unidirectional (i.e., one resource depends on another, not vice versa).

4. Implicit Ordering of Resources

Although resources are deployed in parallel by default, ARM ensures that resources that have dependencies (either implicit or explicit) are deployed in the correct order.

For example:

1. If a virtual machine requires a storage account, the storage account will be created before the virtual machine, even if no dependsOn is explicitly defined.

2. If two resources are independent (i.e., they don’t reference each other), they will be deployed in parallel, unless otherwise specified by dependsOn.

5. Using dependsOn with Loops or Multiple Resources

In more complex templates, you may want to use loops (like copy loops) to deploy multiple similar resources, and sometimes the order of those resources matters.

For example, deploying multiple NICs and ensuring they are created before VMs:

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"resources": [
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  {
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    "type": "Microsoft.Network/networkInterfaces",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "[concat('nic', copyIndex())]",
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    "copy": {
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      "name": "nicLoop",
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      "count": 3
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    },
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    "properties": {
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      "ipConfigurations": [
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        {
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          "name": "ipconfig1",
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          "properties": {
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            "subnet": {
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              "id": "[resourceId('Microsoft.Network/virtualNetworks/subnets', 'myVnet', 'default')]"
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            }
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          }
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        }
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      ]
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    }
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  },
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  {
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    "type": "Microsoft.Compute/virtualMachines",
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    "apiVersion": "2023-03-01",
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    "location": "East US",
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    "name": "[concat('myVM', copyIndex())]",
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    "dependsOn": [
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      "[resourceId('Microsoft.Network/networkInterfaces', concat('nic', copyIndex()))]"
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    ],
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    "copy": {
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      "name": "vmLoop",
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      "count": 3
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    },
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    "properties": {
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      "hardwareProfile": {
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        "vmSize": "Standard_DS1_v2"
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      },
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      "osProfile": {
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        "computerName": "[concat('myVM', copyIndex())]",
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        "adminUsername": "adminuser",
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        "adminPassword": "password123"
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      },
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      "networkProfile": {
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        "networkInterfaces": [
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          {
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            "id": "[resourceId('Microsoft.Network/networkInterfaces', concat('nic', copyIndex()))]"
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          }
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        ]
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      }
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    }
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  }
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]

Here:

The dependsOn ensures that each virtual machine depends on the NIC created in the same copy loop.

6. Summary

Managing dependencies in ARM templates is essential for ensuring that resources are deployed in the correct order. You can use the dependsOn property to explicitly manage dependencies, but implicit dependencies often make this less necessary. For complex scenarios, such as loops or conditional dependencies, dependsOn can help you define precise deployment sequences. Circular dependencies should be avoided, and careful consideration of resource relationships will ensure smooth deployments.