Updating Azure app gateway rewrite rules with ARM template - azure

Question

I've been learning about how ARM templates work and how there is an incremental mode. Can I do a custom deployment that will update a resource with a template that only includes what I want to update?
What I'm trying to accomplish is to update or add rewrite rules for a specific set in our app gateway
When I try the following I get InvalidTemplateDeployment errors with messages about missing template properties i.e. "0 IP configuration specified for gateway".
For my template I was trying to follow an example here https://learn.microsoft.com/en-us/azure/architecture/guide/azure-resource-manager/advanced-templates/update-resource
{
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
...
},
"resources": [
{
"apiVersion": "2020-06-01",
"type": "Microsoft.Resources/deployments",
"name": "updateRewriteRules",
"properties": {
"mode": "Incremental",
"template": {
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.1",
"resources": [
{
"type": "Microsoft.Network/applicationGateways",
"apiVersion": "2020-05-01",
"name": "[parameters('applicationGatewayName')]",
"location": "[parameters('location')]",
"properties": {
"rewriteRuleSets": [
{
"name": "[parameters('rewriteSetName')]",
"properties": {
"rewriteRules": [
{
"ruleSequence": 300,
"conditions": [],
"name": "security-response-headers",
"actionSet": {
"requestHeaderConfigurations": [],
"responseHeaderConfigurations": [
{
"headerName": "Permissions-Policy",
"headerValue": "accelerometers=(), camera=(), geolocation=(), gyroscope=(), magnetometer=(), microphone=(), payment=(), usb=()"
}
]
}
}
]
}
}
]
}
}
],
"outputs": {}
}
}
}
],
"outputs": {}
}

Answer 1

Looks like for app gateways you need to deploy the entire template with all its properties to make changes.
Update existing Application Gateway via ARM
Not sure where this is documented though.