Designing the Application Architecture - Compute

https://www.windowsazure.com/en-us/home/tour/compute/

Windows Azure offers highly-available Compute capacity that enables you to run application code in the cloud, and allows you to quickly scale your applications up or down to meet your customer
needs. Each Compute instance is a virtual machine that isolates you from other customers. Once Compute resources are assigned to your application, Windows Azure automatically handles network load balancing and failover to provide continuous availability.
Windows Azure provides a 99.95% monthly SLA. for
Compute services.

Applications in Windows Azure utilize Compute resources through one or more Compute containers called “roles”. Roles come in three different types: Web, Worker, and Virtual Machine (VM). Some common uses or application patterns for these roles include:

Web applications

Web roles in Windows Azure are special purpose, and provide a dedicated Internet Information Services (IIS) web-server used for hosting front-end web applications. You can quickly and easily deploy web applications to Web Roles and then scale your Compute capabilities
up or down to meet demand.

Backend applications

Applications hosted within Worker roles can run asynchronous, long-running or perpetual tasks independent of user interaction or input. When you separate your application’s background processes in a Worker role and host the front-end in a Web role, you can
better distribute your application logic and have more fine grain control over how your application scales.

Legacy applications

Virtual Machine (VM) roles, now in Beta,
enable you to deploy a custom Windows Server 2008 R2 (Enterprise or Standard) image to Windows Azure. You can use the VM role when your application
requires a large number of server OS customizations and cannot be automated. The VM Role gives you full control over your application environment and lets you migrate existing applications
to the cloud. For more information about the VM role, gohere.

Capabilities and Benefits of Compute

Great application hosting environment

Web roles are pre-configured with IIS7 making it easy to create applications using popular web technologies like ASP.NET, PHP, and Node.js.

Worker roles let you host any type of application, including Apache Tomcat and Java Virtual Machines (JVM).

Applications are commonly composed of both Web and Worker roles.
A common implementation in Windows Azure takes input from a Web role, sends those requests through a Queue to a Worker role, then processes the requests and stores the output.

A VM role runs a virtual hard drive (VHD) image of a Windows Server 2008 R2 virtual machine. This VHD can be created using an on-premise Windows Server machine, and then uploaded to Windows Azure.

Focus on applications, not operations

Windows Azure is designed to let you build applications that are continuously available, even during system upgrades and hardware failures.

Network load balancing and failover are handled automatically, reducing the effort and cost of administering the application environment.

Windows Azure handles automatically patching the OS your applications run on top of, removing the need for you to manually coordinate or script this.

Compute capabilities in Windows Azure allow you to scale applications up or down when necessary, so they use only the resources they need.

Pricing and Metering for Compute:

Each compute instance is a virtual server. There are 5 compute virtual server sizes you can choose from. The table below summarizes the resources provided by each Compute instance size.

Virtual Machine Size	CPU Cores	Memory	Cost Per Hour
Extra Small	Shared	768 MB	$0.04
Small	1	1.75 GB	$0.12
Medium	2	3.5 GB	$0.24
Large	4	7 GB	$0.48
Extra Large	8	14 GB	$0.96

Details on the Virtual machine sizes can be found here

Compute hours are charged only when your application is deployed. Compute hours are billed based on the number of clock hours your service was deployed multiplied by the number of compute
instances. If you have two tenants deployed for a hosted service, one for staging and one for production, both will be charged as both are utilizing Windows Azure resources. When developing and testing your application remove the compute instances that are
not being used to minimize compute hour billing. All compute hours, except for extra small compute instances that are billed separately, are converted into small instance hours when presented on your bill.