For a complete list of the co-authors and contributors, see the end of the article.

A cloud service in Windows Azure will typically have multiple concurrent instances. Each instance may be running all or a part of the service's codebase. As a developer, you control the number and type of roles that you want running your service.

Web Roles and Worker Roles
Windows Azure roles are comparable to standard Visual Studio projects, where each instance represents a separate project. These roles represent different types of applications that are natively supported by Windows Azure. There are two types of roles that you can use to host services with Windows Azure:

• Web roles
• Worker roles

Web roles provide support for HTTP and HTTPS through public endpoints and are hosted in IIS. They are most comparable to regular ASP.NET projects, except for differences in their configuration files and the assemblies they reference.

Worker roles can also expose external, publicly facing TCP/IP endpoints on ports other than 80 (HTTP) and 443 (HTTPS); however, worker roles do not run in IIS. Worker roles are applications comparable to Windows services and are suitable for background ­processing.

Virtual Machines
Underneath the Windows Azure platform, in an area that you and your service logic have no control over, each role is given its own virtual machine or VM. Each VM is created when you deploy your service or service-oriented solution to the cloud. All of these VMs are managed by a modified hypervisor and hosted in one of Microsoft's global data centers.

Each VM can vary in size, which pertains to the number of CPU cores and memory. This is something that you control. So far, four pre-defined VM sizes are provided:

• Small - 1.7ghz single core, 2GB memory
• Medium - 2x 1.7ghz cores, 4GB memory
• Large - 4x 1.7ghz cores, 8GB memory
• Extra large - 8x 1.7ghz cores, 16GB memory

Notice how each subsequent VM on this list is twice as big as the previous one. This simplifies VM allocation, creation, and management by the hypervisor.

Windows Azure abstracts away the management and maintenance tasks that come along with traditional on-premise service implementations. When you deploy your service into Windows Azure and the service's roles are spun up, copies of those roles are replicated automatically to handle failover (for example, if a VM were to crash because of hard drive failure). When a failure occurs, Windows Azure automatically replaces that "unreliable" role with one of the "shadow" roles that it originally created for your service. This type of failover is nothing new. On-premise service implementations have been leveraging it for some time using clustering and disaster recovery solutions. However, a common problem with these failover mechanisms is that they are often server-focused. This means that the entire server is failed over, not just a given service or service composition.

When you have multiple services hosted on a Web server that crashes, each hosted service experiences downtime between the current server crashing and the time it takes to bring up the backup server. Although this may not affect larger organizations with sophisticated infrastructure too much, it can impact smaller IT enterprises that may not have the capital to invest in setting up the proper type of failover infrastructure.

Also, suppose you discover in hindsight after performing the failover that it was some background worker process that caused the crash. This probably means that unless you can address it quick enough, your failover server is under the same threat of crashing.

Windows Azure addresses this issue by focusing on application and hosting roles. Each service or solution can have a Web frontend that runs in a Web role. Even though each role has its own "active" virtual machine (assuming we are working with single instances), Windows Azure creates copies of each role that are physically located on one or more servers. These servers may or may not be running in the same data center. These shadow VMs remain idle until they are needed.

Should the background process code crash the worker role and subsequently put the underlying virtual machine out of commission, Windows Azure detects this and automatically brings in one of the shadow worker roles. The faulty role is essentially discarded. If the worker role breaks again, then Windows Azure replaces it once more. All of this is happening without any downtime to the solution's Web role front end, or to any other services that may be running in the cloud.

Input Endpoints
Web roles used to be the only roles that could receive Internet traffic, but now worker roles can listen to any port specified in the service definition file. Internet traffic is received through the use of input endpoints. Input endpoints and their listening ports are declared in the service definition (*.csdef) file.

Keep in mind that when you specify the port for your worker role to listen on, Windows Azure isn't actually going to assign that port to the worker. In reality, the load balancer will open two ports-one for the Internet and the other for your worker role. Suppose you wanted to create an FTP worker role and in your service definition file you specify port 21. This tells the fabric load balancer to open port 21 on the Internet side, open pseudo-random port 33476 on the LAN side, and begin routing FTP traffic to the FTP worker role.

In order to find out which port to initialize for the randomly assigned internal port, use the RoleEnvironment.CurrentRoleInstance.InstanceEndpoints["FtpIn"].IPEndpoint object.

Inter-Role Communication
Inter-Role Communication (IRC) allows multiple roles to talk to each other by exposing internal endpoints. With an internal endpoint, you specify a name instead of a port number. The Windows Azure application fabric will assign a port for you automatically and will also manage the name-to-port mapping.

Here is an example of how you would specify an internal endpoint for IRC:

<ServiceDefinition xmlns=
"http://schemas.microsoft.com/ServiceHosting/2008/10/
ServiceDefinition" name="HelloWorld">
<WorkerRole name="WorkerRole1">
<Endpoints>
<InternalEndpoint name="NotifyWorker" protocol="tcp" />
</Endpoints>
</WorkerRole>
</ServiceDefinition>

Example 1
In this example, NotifyWorker is the name of the internal endpoint of a worker role named WorkerRole1. Next, you need to define the internal endpoint, as follows:

RoleInstanceEndpoint internalEndPoint =
RoleEnvironment.CurrentRoleInstance.
InstanceEndpoints["NotificationService"];
this.serviceHost.AddServiceEndpoint(
typeof(INameOfYourContract),
binding,
String.Format("net.tcp://{0}/NotifyWorker",
internalEndPoint.IPEndpoint));
WorkerRole.factory = new ChannelFactory<IClientNotification>(binding);

Example 2
You only need to specify the IP endpoint of the other worker role instances in order to communicate with them. For example, you could get a list of these endpoints with the following routine:

var current = RoleEnvironment.CurrentRoleInstance;
var endPoints = current.Role.Instances
.Where(instance => instance != current)
.Select(instance => instance.InstanceEndpoints["NotifyWorker"]);

Example 3
IRC only works for roles in a single application deployment. Therefore, if you have multiple applications deployed and would like to enable some type of cross-application role communication, IRC won't work. You will need to use queues instead.

Summary of Key Points

• Windows Azure roles represent different types of supported applications or services.
• There are two types of roles: Web roles and worker roles.
• Each role is assigned its own VM.

•   •   •

This excerpt is from the book, "SOA with .NET & Windows Azure: Realizing Service-Orientation with the Microsoft Platform", edited and co-authored by Thomas Erl, with David Chou, John deVadoss, Nitin Ghandi, Hanu Kommapalati, Brian Loesgen, Christoph Schittko, Herbjörn Wilhelmsen, and Mickie Williams, with additional contributions from Scott Golightly, Daryl Hogan, Jeff King, and Scott Seely, published by Prentice Hall Professional, June 2010, ISBN 0131582313, Copyright 2010 SOA Systems Inc. For a complete Table of Contents please visit: www.informit.com/title/0131582313

Authors
David Chou is a technical architect at Microsoft and is based in Los Angeles. His focus is on collaborating with enterprises and organizations in such areas as cloud computing, SOA, Web, distributed systems, and security.

John deVadoss leads the Patterns & Practices team at Microsoft and is based in Redmond, WA.

Thomas Erl is the world's top-selling SOA author, series editor of the Prentice Hall Service-Oriented Computing Series from Thomas Erl (www.soabooks.com), and editor of the SOA Magazine (www.soamag.com).

Nitin Gandhi is an enterprise architect and an independent software consultant, based in Vancouver, BC.

Hanu Kommalapati is a Principal Platform Strategy Advisor for a Microsoft Developer and Platform Evangelism team based in North America.

Brian Loesgen is a Principal SOA Architect with Microsoft, based in San Diego. His extensive experience includes building sophisticated enterprise, ESB and SOA solutions.

Christoph Schittko is an architect for Microsoft, based in Texas. His focus is to work with customers to build innovative solutions that combine software + services for cutting edge user experiences and the leveraging of service-oriented architecture (SOA) solutions.

Herbjörn Wilhelmsen is a consultant at Forefront Consulting Group, based in Stockholm, Sweden. His main areas of focus are Service-Oriented Architecture, Cloud Computing and Business Architecture.

Mickey Williams leads the Technology Platform Group at Neudesic, based in Laguna Hills,

Contributors
Scott Golightly is currently an Enterprise Solution Strategist with Advaiya, Inc; he is also a Microsoft Regional Director with more than 15 years of experience helping clients to create solutions to business problems with various technologies.

Darryl Hogan is an architect with more than 15 years experience in the IT industry. Darryl has gained significant practical experience during his career as a consultant, technical evangelist and architect.

As a Senior Technical Product Manager at Microsoft, Kris works with customers, partners, and industry analysts to ensure the next generation of Microsoft technology meets customers' requirements for building distributed, service-oriented solutions.

Jeff King has been working with the Windows Azure platform since its first announcement at PDC 2008 and works with Windows Azure early adopter customers in the Windows Azure TAP

Scott Seely is co-founder of Tech in the Middle, www.techinthemiddle.com, and president of Friseton, LLC.