Azure Resource Manager (ARM) VS Azure Service Manager (Classic/ASM)

In this topic, you will learn about Azure Resource Manager and classic deployment models, the state of your resources, and why your resources were deployed with one or the other. The Resource Manager deployment model contains important differences from the classic deployment model, and the two models are not completely compatible with each other.

For most resources, you can transition to Resource Manager without any issues. However, a few resource providers offer two versions of the resource (one for classic, and one for Resource Manager) because of the architectural differences between the models. The resource providers that differentiate between the two models are:

  • Compute – Supports instances of virtual machines and optional availability sets.
  • Storage – Supports required storage accounts that store the VHDs for virtual machines, including their operating system and additional data disks.
  • Network – Supports required NICs, virtual machine IP addresses, and subnets within virtual networks and optional load balancers, load balancer IP addresses, and Network Security Groups.

For these resource types, you must be aware of which version you are using because the supported operations will differ. To understand which model was used to deploy your resources, let’s review the two models.

Resource Manager characteristics:

Resources created through Resource Manager share the following characteristics:

  • Created through one of the following methods:
    • The Azure portal.Azure portalFor Compute, Storage, and Networking resources, you have the option of using either Resource Manager or Classic deployment. Select Resource Manager.

      Resource Manager deployment

    • For Azure PowerShell, use the Resource Manager version of commands. These commands have the format Verb-AzureRmNoun, as shown below.
      Copy
      Get-AzureRmResourceGroupDeployment
    • Azure Resource Manager REST API for REST operations.
    • Azure CLI commands run in the arm mode.
      Copy
      azure config mode arm
  • The resource type does not include (classic) in the name. The image below shows the type asStorage account.web app

The application shown in the following diagram shows how resources deployed through Resource Manager are contained in a single resource group.

Additionally, there are relationships between the resources within the resource providers:

  • A virtual machine depends on a specific storage account defined in the SRP to store its disks in blob storage (required).
  • A virtual machine references a specific NIC defined in the NRP (required) and an availability set defined in the CRP (optional).
  • A NIC references the virtual machine’s assigned IP address (required), the subnet of the virtual network for the virtual machine (required), and to a Network Security Group (optional).
  • A subnet within a virtual network references a Network Security Group (optional).
  • A load balancer instance references the backend pool of IP addresses that include the NIC of a virtual machine (optional) and references a load balancer public or private IP address (optional).

Classic deployment characteristics

You may also know the classic deployment model as the Service Management model.

In Azure Service Management, the compute, storage, or network resources for hosting virtual machines are provided by:

  • A required cloud service that acts as a container for hosting virtual machines (compute). Virtual machines are automatically provided with a network interface card (NIC) and an IP address assigned by Azure. Additionally, the cloud service contains an external load balancer instance, a public IP address, and default endpoints to allow remote desktop and remote PowerShell traffic for Windows-based virtual machines and Secure Shell (SSH) traffic for Linux-based virtual machines.
  • A required storage account that stores the VHDs for a virtual machine, including the operating system, temporary, and additional data disks (storage).
  • An optional virtual network that acts as an additional container, in which you can create a subnetted structure and designate the subnet on which the virtual machine is located (network).

Resources created in the classic deployment model share the following characteristics:

  • Created through one of the following methods:
    • Classic portalClassic portalOr, the Azure portal and you specify Classic deployment (for Compute, Storage, and Networking).

      Classic deployment

    • For Azure PowerShell, use the Service Management version of commands. These command names have the format Verb-AzureNoun, as shown below.
      Copy
      Get-AzureDeployment
    • Service Management REST API for REST operations.
    • Azure CLI commands run in asm or default mode.
  • The resource type includes (classic) in the name. The image below shows the type as Storage account (classic).classic type

You can still use the Azure portal to manage resources that were created through classic deployment.

Here are the components and their relationships for Azure Service Management.

Benefits of using Resource Manager and resource groups

Resource Manager added the concept of the resource group. Every resource you create through Resource Manager exists within a resource group. The Resource Manager deployment model provide several benefits:

  • You can deploy, manage, and monitor all of the services for your solution as a group, rather than handling these services individually.
  • You can repeatedly deploy your application throughout the app lifecycle and have confidence your resources are deployed in a consistent state.
  • You can use declarative templates to define your deployment.
  • You can define the dependencies between resources so they are deployed in the correct order.
  • You can apply access control to all resources in your resource group because Role-Based Access Control (RBAC) is natively integrated into the management platform.
  • You can apply tags to resources to logically organize all of the resources in your subscription.

Prior to Resource Manager, every resource you created through classic deployment did not exist within a resource group. When Resource Manager was added, all resources were retroactively added to default resource groups. If you create a resource through classic deployment now, the resource is automatically created within a default resource group for that service, even though you did not specify that resource group at deployment. However, just existing within a resource group does not mean that the resource has been converted to the Resource Manager model. For Virtual Machines, Storage, and Virtual Networks, if the resource was created through classic deployment, you must continue to operate on it through classic operations.

You can move resources to a different resource group, and add new resources to an existing resource group. So, your resource group can contain a mix of resources created through Resource Manager and classic deployment. This combination of resources can create unexpected results because the resources do not support the same operations.

By using declarative templates, you might be able to simplify your scripts for deployment. Instead of attempting to convert existing scripts from Service Management to Resource Manager, consider re-working your deployment strategy to take advantage of defining your infrastructure and configuration in the template.

Reference:

  1. Article from azure.microsoft.com
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