I am doing some research on service-oriented architecture (SOA) this month and turned to a guy who has been closely tied to SOA since "way back" when the concept, but not the term, was introduced as part of early B2B efforts in the 1990s. Paul Fremantle is currently CTO of WSO2, where he led the development of the Apache-Synapse-based WSO2 Enterprise Service Bus (ESB). At IBM for a decade before co-founding WSO2, Paul was on the team that put the Service Integration Bus technology into WebSphere Application Server 6.
All of which makes Paul a great guy to ask the $64,000 question:
"Do I need an ESB to have SOA?"
His answer is that maybe you don't need something with that label on it. But you need key functionality in the areas of managing distributed computing and exposing services functionality (i.e., registry or repository) that people have come to think of as an ESB. Unfortunately (my opinion, not Paul's), the term ESB got caught up in marketing wars between Progress and TIBCO a few years back and therefore a lot of IT folks dismissed ESB functionality as buzzword hype.
Paul explains some important ESB/SOA concepts in a great essay on WSO2's community Web site (called the Oxygen Tank, by the way, as in WS-O2). The article, "Reclaiming the ESB" does a great job of comparing theory with reality and explaining what to look for in an ESB. The gist of the article, leading from its title, is that some ESBs are just renamed enterprise application integration (EAI) engines; in acquiring an ESB, make sure you get the distributed computing management and services exposition features you really need to implement SOA.
As an aside, I reminded Paul that I had been nibbling around the edges of SOA since "way back" when they called it electronic data interchange (EDI). He instantly came up with some great parallels of why SOA succeeds where EDI failed, which I include in my upcoming research. Articles about that research will appear here on IT Business Edge. If you can contribute historical information about SOA, please e-mail me as soon as possible.