On Windows Azure is there a difference between VM role and vm instances?

Question

In windows azure I see references to vm roles and vm instances. Are they different ? one it seems is to allow uploading of a vm image for use in the platform as as a service (PaaS) and the other allows on demand creation of vm images for installation of applications like traditional VPS.

Is this just out of date documentation ?

Answer 1

It will depend upon the context. Do you have a link to the documentation you are referring to?

In the context of a Windows Azure Virtual Machine the phrase VM Instance does refer to the ability to create a Virtual Machine either from the gallery or from the ground up.

In the context of a Cloud Service (the PaaS model) both the phrase VM Role and VM Instance apply. The VM Role in this context is a definition of a virtual machine that will run a specific set of code. For example, you might have a VM Role defined that processes orders or one that computes number crunching algorithms. This is a template if you will, or a role that defined for your application. When you define a role you also indicate the number of instances of that role. So in the context of a Cloud Service the VM Instance is a Virtual Machine that is running the code defined for the role it belongs to, but there might be ten more running the exact same code.

Answer 2

You're right and the documentation isn't old (or isn't incorrect anyways).

Here's a decent short article on the difference that is about 8 months old - The difference between Azure VMs and Azure VM Role

My quick take on it:

The VM Role (Paas) was introduced first and was the initial way to get users who need a customized build of the image that was to be launched for their application in the cloud. The building of the VM image locally then uploading was a bit complicated BUT it provided the means to scale out that image rather quickly in a Paas type environment (even though it was also the OS in this case).

The VM instance was the whole new way to get the same thing - but also control more and also be in an Iaas scenario instead of the Paas. This option gives you the ability to build the image the way you want and have it persisted across reboots of the machine. However this new more-control-Iaas means you have to do more (or at least create and run more scripts) in order to scale out an image to be running multiple instances quickly.