Cloud Computing, SOA and Windows Azure - Part 2
SOA Principles and Patterns
By: Thomas Erl
Aug. 6, 2010 09:00 AM
For a complete list of the co-authors and contributors, see the end of the article.
Windows Azure Platform Overview
The infrastructure and service architectures that underlie many of these native services (as well as cloud-based services in general) are based on direct combined application of Stateful Services  and Redundant Implementation . This is made possible by leveraging several of the built-in extensions and mechanisms provided by the Windows Azure platform (as explained in this chapter and Chapter 16).
The Windows Azure platform is part of the Microsoft cloud, which consists of multiple categories of services:
Figure 3 illustrates the service categories related to the Windows Azure platform. Given that Windows Azure is itself a platform, let's explore it as an implementation of the PaaS delivery model.
Figure 3: A high-level representation of categories of services available in the Windows Azure cloud
The Windows Azure platform was built from the ground up using Microsoft technologies, such as the Windows Server Hyper-V-based system virtualization layer. However, the Windows Azure platform is not intended to be just another off-premise Windows Server hosting environment. It has a cloud fabric layer, called the Windows Azure Fabric Controller, built on top of its underlying infrastructure.
The Windows Azure Fabric Controller pools an array of virtualized Windows Server instances into a logical entity and automatically manages the following:
These are managed without requiring the hosted applications to explicitly deal with the details. The fabric layer provides a parallel management system that abstracts the complexities in the infrastructure and presents a cloud environment that is inherently elastic. As a form of PaaS, it also supports the access points for user and application interactions with the Windows Azure platform.
The Windows Azure platform essentially provides a set of cloud-based services that are symmetric with existing mainstream on-site enterprise application platforms (see Figure 4).
Figure 4: An overview of common Windows Azure platform capabilities
All of these capabilities can be utilized individually or in combination.
Windows Azure (Application Container)
A Windows Azure instance represents a unit of deployment, and is mapped to specific virtual machines with a range of variable sizes. Physical provisioning of the Windows Azure instances is handled by the cloud fabric. We are required only to specify, by policy, how many instances we want the cloud fabric to deploy for a given service.
We have the ability to manually start and shut down instances, and grow or shrink the deployment pool; however, the cloud fabric also provides automated management of the health and lifecycles of instances. For example, in the event of an instance failure, the cloud fabric would automatically shut down the instance and attempt to bring it back up on another node.
Windows Azure also provides a set of storage services that consumers can use to store and manage persistent and transient data. Storage services support geo-location and offer high durability of data by triple-replicating everything within a cluster and across data centers. Furthermore, they can manage scalability requirements by automatically partitioning and load balancing services across servers.
Also supported by Windows Azure is a VHD-based deployment model as an option to enable some IaaS requirements. This is primarily geared for services that require closer integration with the Windows Server OS. This option provides more control over the service hosting environment and can better support legacy applications.
Specifically, it elevates the importance of the Standardized Service Contract (693), Service Loose Coupling (695), and Service Abstraction (696) principles that, through collective application, shape and position service contracts to maximize abstraction and cross-service standardization, while minimizing negative forms of consumer and implementation coupling. Decoupled Contract  forms an expected foundation for Windows Azure-hosted service contracts, and there will generally be the need for more specialized contract-centric patterns, such as Validation Abstraction , Canonical Schema , and Schema Centralization .
In addition to reliability and scalability improvements, SQL Azure's replication mechanism can be used to apply Service Data Replication  in support of the Service Autonomy (699) principle. This is significant, as individual service autonomy within cloud environments can often fluctuate due to the heavy emphasis on shared resources across pools of cloud-based services.
A SQL Azure database instance is actually implemented as three replicas on top of a shared SQL Server infrastructure managed by the cloud fabric. This cloud fabric delivers high availability, reliability, and scalability with automated and transparent replication and failover. It further supports load-balancing of consumer requests and the synchronization of concurrent, incremental changes across the replicas. The cloud fabric also handles concurrency conflict resolutions when performing bi-directional data synchronization between replicas by using built-in policies (such as last-writer-wins) or custom policies.
Because SQL Azure is built on SQL Server, it provides a familiar relational data model and is highly symmetric to on-premise SQL Server implementations. It supports most features available in the regular SQL Server database engine and can also be used with tools like SQL Server 2008 Management Studio, SQLCMD, and BCP, and SQL Server Integration Services for data migration.
Windows Azure Platform AppFabric
Windows Azure platform AppFabric helps connect services within or across clouds and enterprises. It provides a Service Bus for connectivity across networks and organizational boundaries, and an Access Control service for federated authorization as a service.
The Service Bus acts as a centralized message broker in the cloud to relay messages between services and service consumers. It has the ability to connect to on-premise services through firewalls, NATs, and over any network topology.
Its features include:
The Windows Azure Service Bus complies to the familiar Enterprise Service Bus  compound pattern, and focuses on realizing this pattern across network, security, and organizational domains. Service Bus also provides a service registry to provide registration and discovery of service metadata, which allows for the application of Metadata Centralization  and emphasizes the need to apply the Service Discoverability (702) principle.
Access Control acts as a centralized cloud-based security gateway that regulates access to cloud-based services and Service Bus communications, while integrating with standards-based identity providers (including enterprise directories such as Active Directory and online identity systems like Windows Live ID). Access Control and other Windows Azure-related security topics are covered in Chapter 17.
Unlike Windows Azure and SQL Azure, which are based on Windows Server and SQL Server, Access Control Service is not based on an existing server product. It uses technology included in Windows Identity Foundation and is considered a purely cloud-based service built specifically for the Windows Azure platform environment.
Summary of Key Points
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
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,
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,
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