PSA: Bicep templates run in parallel

I had a problem recently where my Bicep templates were failing with an obscure error message:

The incoming request is not recognized as a namespace policy put request.

The Bicep in question was attempting to assign an Azure Service Bus topic subscription’s forwardTo to another queue.

I had ordered everything in the Bicep file correctly, i.e.

1. Create the topic
2. Create its subscriptions
3. Create the queues
4. Tell the topic subscription to forward messages to the queue

However, when I looked at the Deployments in the Azure Resource Group, it appeared that they weren’t running in the order I had specified:

This is because by default Bicep templates will run in parallel, unless it detects dependencies. And because my templates were a bit too clever with variables and modules, Bicep was unable to detect my implicit dependencies.

The fix then was to be explicit with my dependencies, using the dependsOn keyword:

// create service bus topics And subscriptions
param topicsAndSubscriptions array = [
  {
	topicName: 'property~changed~v1'  // ~ is what Azure uses for a forward slash, so this topic is actually property/changed/v1
	sanitizedName: 'property-changed' // Azure doesn't like ~ or / in deployment names.
	subscriptions: [
	  'ozone'
	  'valor'
	]
  }
]

module serviceBusTopicsModule './serviceBusTopic.bicep' = [for item in topicsAndSubscriptions : {
  name: 'serviceBusTopic-${item.sanitizedName}-${deploymentNameSuffix}'
	params: {
	serviceBusName: serviceBusModule.outputs.serviceBusOutput.name
	topicName: item.topicName 
  }
}]

module topicsSubscriptionModule 'serviceBusTopicSubscription.bicep' = [ for item in topicsAndSubscriptions: {
  name: 'topicSubscription-${item.sanitizedName}-${deploymentNameSuffix}'
  params: {
	serviceBusName: serviceBusModule.outputs.serviceBusOutput.name
	topicName: item.topicName
	subscriptions: item.subscriptions
  }
  dependsOn: serviceBusTopicsModule
}]

// Create service bus queues
param queueSettings array = [
	{	
		name: 'ozone-property-changed-sbq'
		requiresDuplicateDetection: true
	}
	{
		name: 'valor-property-changed-sbq'
		requiresDuplicateDetection: false
	}
]

module serviceBusQueueModule './serviceBusQueue.bicep' = {
  name: 'serviceBusQueue-${deploymentNameSuffix}'
  params: {
	serviceBusName: serviceBusModule.outputs.serviceBusOutput.name
	queueSettings: queueSettings
  }
}

module serviceBusTopicSubsciptionForwardModule './serviceBusTopicSubscriptionForward.bicep' = {
  name: 'serviceBusTopicSubsciptionForward-${deploymentNameSuffix}'
  params: {
	serviceBusName: serviceBusModule.outputs.serviceBusOutput.name
	topicName: 'property~changed~v1'
	subscriptionName: 'valor'
	queueName: 'valor-property-changed-sbq'
  }
  dependsOn: [serviceBusQueueModule, topicsSubscriptionModule]
}