The Azure Fabric Controller (FC) is the service that automatically manages all roles and resources in Azure, is in charge of :

• Mapping declarative service specifications to available resources and allocate them
• Assuring system health and satisfies SLAs
• Performing application and OS updates

Keeping in mind that, there are two concepts that are particularly important for assuring the availability of your services :

• Fault domains are a physical unit of failure (you could associate that concept with a server rack for instance),the FC distributes the services in different update domains for assuring that if one physical unit becomes unavailable , your service will remain available .
• Update domains are a logical unit, that allows you to specify how your service will be updated(for an application or OS update).When the FC is updating a service, it updates the roles in the same update domain at the same time and in a transactional way,so if you distribute your roles in different update domains,your service wont’ become unavailable during an update.You can distribute the roles in different update domains specifying the upgradeDomainCount attribute in the service definition file.

You can associate these two concepts with a vertical and horizontal partition.The following diagram illustrates a Azure service distributed in 2 fault domains and 3 update domains.

A new interesting feature ,you can now choose when your applications will receive operating system updates and patches by selecting which version of the operating system your applications will run on in Windows Azure.

This feature allows you to test your application before upgrading your production deployment.

To select an operating system version for your application, add the new osVersion attribute to your service configuration file.  The full list of available operating system versions is maintained in the Configuring Operating System Versions topic in the Windows Azure MSDN documentation.

If you missed the The Business of Windows Azure: What you should know about Windows Azure Platform pricing and SLAs PDC session,it gives a clear vision of Azure business model.

Some important highlights about this session :

• There will be three purchasing models :
  • Pay as you go
  • Subscription licensing are plans for payment predictability
  • Volume licensing :Available through centralized purchasing experience and will have volume discounts
  • Additionally,there are some offers that you can join with important benefits (some of them requires an MSDN subscription)
• The TCO Analyzer Tool helps you to  quickly determine the value of using Windows Azure.
• SLAs : One of the key parts of the Windows Azure Platform is the Service Level Agreements,you can check the terms of the agreement in the link.

I would recommend you to attend this session,it’s really worthless!

I wanted to start from the beginning describing what cloud computing is, just using a set of "magic words",it's complicated to resume this computing paradigm using a reduced set of words, but here it's my try:

• Computing as utility
• Accessible through broad networks like Internet
• "Unlimited" pool of resources
• On-demand self-service
• Growing on demand
• Pay as you go

I know that there is much more to talk about cloud computing, but these are the essential ideas.

I also liked the NIST Notional Definition of Cloud Computing, it's quite a complete description and it can be read in less than 2 minutes...

In the next post I will show you how these concepts are made true in Microsoft Azure…hope to see you then!