Before deploying an Azure Resource Manager template (ARM template), you can preview the changes that will happen. Azure Resource Manager provides the what-if operation to let you see how resources will change if you deploy the template. The what-if operation doesn't make any changes to existing resources. Instead, it predicts the changes if the specified template is deployed.
You can use the what-if operation with Azure PowerShell, Azure CLI, or REST API operations. What-if is supported for resource group, subscription, management group, and tenant level deployments.
To deploy a Bicep file or ARM template, you need write access on the resources you're deploying and access to all operations on the Microsoft.Resources/deployments resource type. For example, to deploy a virtual machine, you need Microsoft.Compute/virtualMachines/write and Microsoft.Resources/deployments/* permissions. The what-if operation has the same permission requirements.
When you use what-if in PowerShell or Azure CLI, the output includes color-coded results that help you see the different types of changes.
The text output is:
PowerShell
Resource and property changes are indicated with these symbols:
- Delete
+ Create
~ Modify
The deployment will update the following scope:
Scope: /subscriptions/./resourceGroups/ExampleGroup
~ Microsoft.Network/virtualNetworks/vnet-001 [2018-10-01]
- tags.Owner: "Team A"
~ properties.addressSpace.addressPrefixes: [
- 0: "10.0.0.0/16"
+ 0: "10.0.0.0/15"
]
~ properties.subnets: [
- 0:
name: "subnet001"
properties.addressPrefix: "10.0.0.0/24"
]
Resource changes: 1 to modify.
Athugasemd
The what-if operation can't resolve the reference function. Every time you set a property to a template expression that includes the reference function, what-if reports the property will change. This behavior happens because what-if compares the current value of the property (such as true or false for a boolean value) with the unresolved template expression. Obviously, these values will not match. When you deploy the template, the property will only change when the template expression resolves to a different value.
You can use the --confirm-with-what-if switch (or its short form -c) to preview the changes and get prompted to continue with the deployment. Add this switch to:
For example, use az deployment group create --confirm-with-what-if or -c for resource group deployments.
The preceding commands return a text summary that you can manually inspect. To get a JSON object that you can programmatically inspect for changes, use the --no-pretty-print switch. For example, use az deployment group what-if --no-pretty-print for resource group deployments.
If you want to return the results without colors, open your Azure CLI configuration file. Set no_color to yes.
The what-if operation lists seven different types of changes:
Create: The resource doesn't currently exist but is defined in the template. The resource will be created.
Delete: This change type only applies when using complete mode for deployment. The resource exists, but isn't defined in the template. With complete mode, the resource will be deleted. Only resources that support complete mode deletion are included in this change type.
Ignore: The resource exists, but isn't defined in the template. The resource won't be deployed or modified. When you reach the limits for expanding nested templates, you will encounter this change type. See What-if limits.
NoChange: The resource exists, and is defined in the template. The resource will be redeployed, but the properties of the resource won't change. This change type is returned when ResultFormat is set to FullResourcePayloads, which is the default value.
NoEffect: The property is ready-only and will be ignored by the service. For example, the sku.tier property is always set to match sku.name in the Microsoft.ServiceBus namespace.
Modify: The resource exists, and is defined in the template. The resource will be redeployed, and the properties of the resource will change. This change type is returned when ResultFormat is set to FullResourcePayloads, which is the default value.
Deploy: The resource exists, and is defined in the template. The resource will be redeployed. The properties of the resource may or may not change. The operation returns this change type when it doesn't have enough information to determine if any properties will change. You only see this condition when ResultFormat is set to ResourceIdOnly.
Result format
You control the level of detail that is returned about the predicted changes. You have two options:
FullResourcePayloads - returns a list of resources that will change and details about the properties that will change
ResourceIdOnly - returns a list of resources that will change
The default value is FullResourcePayloads.
For PowerShell deployment commands, use the -WhatIfResultFormat parameter. In the programmatic object commands, use the ResultFormat parameter.
For Azure CLI, use the --result-format parameter.
The following results show the two different output formats:
Full resource payloads
PowerShell
Resource and property changes are indicated with these symbols:
- Delete
+ Create
~ Modify
The deployment will update the following scope:
Scope: /subscriptions/./resourceGroups/ExampleGroup
~ Microsoft.Network/virtualNetworks/vnet-001 [2018-10-01]
- tags.Owner: "Team A"
~ properties.addressSpace.addressPrefixes: [
- 0: "10.0.0.0/16"
+ 0: "10.0.0.0/15"
]
~ properties.subnets: [
- 0:
name: "subnet001"
properties.addressPrefix: "10.0.0.0/24"
]
Resource changes: 1 to modify.
Resource ID only
PowerShell
Resource and property changes are indicated with this symbol:
! Deploy
The deployment will update the following scope:
Scope: /subscriptions/./resourceGroups/ExampleGroup
! Microsoft.Network/virtualNetworks/vnet-001
Resource changes: 1 to deploy.
Run what-if operation
Set up environment
To see how what-if works, let's runs some tests. First, deploy a template that creates a virtual network. You'll use this virtual network to test how changes are reported by what-if.
az group create \
--name ExampleGroup \
--location"Central US"az deployment group create \
--resource-group ExampleGroup \
--template-uri"https://raw.githubusercontent.com/Azure/azure-docs-json-samples/master/azure-resource-manager/what-if/what-if-before.json"
Test modification
After the deployment completes, you're ready to test the what-if operation. This time you deploy a template that changes the virtual network. It's missing one the original tags, a subnet has been removed, and the address prefix has changed.
az deployment group what-if \
--resource-group ExampleGroup \
--template-uri"https://raw.githubusercontent.com/Azure/azure-docs-json-samples/master/azure-resource-manager/what-if/what-if-after.json"
The what-if output appears similar to:
The text output is:
PowerShell
Resource and property changes are indicated with these symbols:
- Delete
+ Create
~ Modify
The deployment will update the following scope:
Scope: /subscriptions/./resourceGroups/ExampleGroup
~ Microsoft.Network/virtualNetworks/vnet-001 [2018-10-01]
- tags.Owner: "Team A"
~ properties.addressSpace.addressPrefixes: [
- 0: "10.0.0.0/16"
+ 0: "10.0.0.0/15"
]
~ properties.subnets: [
- 0:
name: "subnet001"
properties.addressPrefix: "10.0.0.0/24"
]
Resource changes: 1 to modify.
Notice at the top of the output that colors are defined to indicate the type of changes.
At the bottom of the output, it shows the tag Owner was deleted. The address prefix changed from 10.0.0.0/16 to 10.0.0.0/15. The subnet named subnet001 was deleted. Remember these changes weren't deployed. You see a preview of the changes that will happen if you deploy the template.
Some of the properties that are listed as deleted won't actually change. Properties can be incorrectly reported as deleted when they aren't in the template, but are automatically set during deployment as default values. This result is considered "noise" in the what-if response. The final deployed resource will have the values set for the properties. As the what-if operation matures, these properties will be filtered out of the result.
Programmatically evaluate what-if results
Now, let's programmatically evaluate the what-if results by setting the command to a variable.
results=$(az deployment group what-if--resource-group ExampleGroup --template-uri"https://raw.githubusercontent.com/Azure/azure-docs-json-samples/master/azure-resource-manager/what-if/what-if-after.json"--no-pretty-print)
Confirm deletion
The what-if operation supports using deployment mode. When set to complete mode, resources not in the template are deleted. The following example deploys a template that has no resources defined in complete mode.
To preview changes before deploying a template, use the confirm switch parameter with the deployment command. If the changes are as you expected, respond that you want the deployment to complete.
az deployment group create \
--resource-group ExampleGroup \
--mode Complete \
--confirm-with-what-if \
--template-uri"https://raw.githubusercontent.com/Azure/azure-docs-json-samples/master/azure-resource-manager/what-if/azuredeploy.json"
Because no resources are defined in the template and the deployment mode is set to complete, the virtual network will be deleted.
The text output is:
PowerShell
Resource and property changes are indicated with this symbol:
- Delete
The deployment will update the following scope:
Scope: /subscriptions/./resourceGroups/ExampleGroup
- Microsoft.Network/virtualNetworks/vnet-001
id:
"/subscriptions/./resourceGroups/ExampleGroup/providers/Microsoft.Network/virtualNet
works/vnet-001"
location: "centralus"
name: "vnet-001"
tags.CostCenter: "12345"
tags.Owner: "Team A"
type: "Microsoft.Network/virtualNetworks"
Resource changes: 1 to delete.
Are you sure you want to execute the deployment?
[Y] Yes [A] Yes to All [N] No [L] No to All [S] Suspend [?] Help (default is "Y"):
You see the expected changes and can confirm that you want the deployment to run.
SDKs
You can use the what-if operation through the Azure SDKs.
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