Azure Event Grid Subscriptions on custom topic in different resource group - azure-eventgrid

Question

I am trying to subscribe to a custom event grid topic that exists on a different resource group. For example if I have a custom event grid topic my-custom-topic in a resource group publisher-group. How do I create an event grid subscription to my topic from within the resource group subscriber-group?
The following ARM template only works if the my-custom-topic is in the same resource group that I am applying the template too
{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json",
"contentVersion": "1.0.0.0",
"parameters": {
"eventGridSubscriptionName": {
"type": "String",
"metadata": {
"description": "The name of the Event Grid custom topic's subscription."
}
},
"location": {
"defaultValue": "[resourceGroup().location]",
"type": "String",
"metadata": {
"description": "The location in which the Event Grid resources should be deployed."
}
}
},
"resources": [
{
"type": "Microsoft.EventGrid/topics/providers/eventSubscriptions",
"apiVersion": "2018-01-01",
"name": "[concat('my-custom-topic', '/Microsoft.EventGrid/', parameters('eventGridSubscriptionName'))]",
"location": "[parameters('location')]",
"properties": {
"destination": {
"endpointType": "EventHub",
"properties": {
"resourceId": "..."
}
},
"filter": {
"includedEventTypes": [
"All"
]
}
}
}
]
}
If I change name to be a the full path of the topic (e.g. subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx/resourceGroups/publisher-group/providers/Microsoft.EventGrid/topics/my-custom-topic) then the template complains I have too many segments
I would have thought this was a very common use case having topics and subscriptions in different resource groups but I am unable to find concrete examples of this
How do I create an ARM template to subscribe to an event grid topic on a different resource group?

Answer 1

This isn't possible - Event Grid Topics and Subscriptions have to be in the same resource group.
I would suggest creating a separate resource group just for containing your Event Grid Topic and all of it's Subscriptions.
Logically you shouldn't think of a single event publisher as owning the Event Grid Topic it publishes to - rather think of a Topic as a shared resource which many publishers and subscribers depend on.