XML Reference Guide

Mar 14, 2003

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What do you think when I say "Service Oriented Architecture"? Do you think "buzzword for web services"? Well, you're only partially correct.

A Service Oriented Architecture is one in which various components are loosely coupled and in which communication between them is carried out via messaging. Sure, that sounds like web services, but it also sounds like CORBA and other programming practices. So where do you draw the line?

Well, you don't. It's like this: An SOA is, as I said, a message-enabled system, but there's no telling what those messages must be. For example, you COULD implement an SOA using a distributed technology such as DCOM.

Chances are, though, that you won't. This is because an SOA requires a great deal of interoperability between systems, for one thing, but it also includes a number of other infrastructure requirements for which web services are particularly well suited.

But before we go into that, let's talk about what an SOA actually looks like. For a truly service-oriented architecture, a system should have the following attributes:

Interaction is accomplished through messaging. The actual method of sending and receiving messages isn't important, but web services is an obvious choice.

Systems are loosely-coupled. In other words, the sender and receiver are independent of each other; the source can send the message independently of the target.

The system is scalable. If each component in a system had to talk directly to every other component, you would need to manage n*(n-1) connections for n components, so adding even one more player to a large system would be prohibitively complex. To prevent this, components communicate using standard protocols and messages for all exchanges.

Services are dynamically discoverable. In the case of web services, this is accomplished through services such as a directory of WSDL descriptions.

The system includes an Enterprise Service Bus. In computing terms, a "bus" passes messages. When we talk about an Enterprise Service Bus, we are talking about a centralized system that is capable of converting messages between formats and protocols. For example, you might have a sales application that communicates via SOAP over HTTP trying to communicate with a legacy database that is expecting a text message over UDP. The Enterprise Service Bus is responsible for managing all of these interactions.

There are many different possibilities for developing an SOA, such as web services, RMI, COM/DCOM, and CORBA, but web services is the most common because it eliminations many of the interoperability problems. For example, RMI is limited to Java, while COM/DCOM is limited to Microsoft platforms. What's more, web services provide many of the necessary "side" standards that are crucial for making a system like this work.

Some of the standards involved include:

XML
SOAP
WSDL
UDDI
WS-Addressing
WS-Notifications
WS-Coordination
WS-ReliableMessaging
WS-Security
Business Process Execution Language (BPEL)

Here are some resources to further your explorations.

Articles

"Service Oriented Architecture: Inventory of Distributed Computing Concepts" by Dirk Krafzig, Karl Banke and Dirk Slama a sample chapter from Enterprise SOA: Service-Oriented Architecture Best Practices (Prentice Hall, 2004, ISBN: 0-13-146575-9), talks about the communication that goes on in a Service-Oriented Architecture.

"Software Futures: Architecture by Stephen Morris explains the process of analyzing and designing the architecture of your systems and looks at SOA from an architectural point of view.

"Web Services and Contemporary SOA (Part II: Advanced Messaging, Metadata, and Security) by Thomas Erl looks at some of the technologies you will need to think about once you've got your basic infrastructure in place, such as security and metadata.

Books and eBooks

Service-Oriented Architecture: A Field Guide to Integrating XML and Web Services by Thomas Erl (Prentice Hall, 2004, ISBN: 0-13-142898-5) provides a complete look at creating a web-services-based SOA, from creating the initial services to integrating the SOA into the business. (Read online)

Enterprise SOA: Service-Oriented Architecture Best Practices by Dirk Krafzig, Karl Banke and Dirk Slama (Prentice Hall, 2004, ISBN: 0-13-146575-9) looks at implementing SOA in the real world, from technology to corporate politics. It includes case studies from several major implementations. (Read online)

Service-Oriented Architecture Compass: Business Value, Planning, and Enterprise Roadmap by Norbert Bieberstein, Sanjay Bose, Marc Fiammante, Keith Jones and Rawn Shah (IBM Press, 2005, ISBN: 0-13-187002-5) provides a look at the various concepts involved in the creation of an SOA, but also looks at the very real problems of managing an existing system. (Read online)

Understanding SOA with Web Services by Eric Newcomer and Greg Lomow (Addison Wesley Professional, 2004, ISBN: 0321180860) talks about general SOA concepts, but focuses on the actual implementation using web services, including tutorials on tutorials on WS-Security, WS-Reliable Messaging, WS-AtomicTransactions, WS-Composite Application Framework, WS-Addressing, WS-Policy, and WS-BPEL. (Read online)

Documentation

There is no "standard" for Service-Oriented Architectures, but many of the standards involved in implementing SOA using web services can be found here.

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