It is amazing how, once a rumour gets established, it is almost impossible to stop it.
A couple of years back, as Enterprise Service Buses (ESBs) became all the rage, I started to have users ask me whether SOA required an ESB. No doubt this was due to interested parties in the marketplace encouraging this view. I wrote a paper at the time that addressed the combination of SOAs and ESBs, to see whether they fit, whether they were immutably tied and where there were advantages in the combination.
I had hoped that this would put to bed once and for all this misconception about SOAs and ESBs, but I find plenty of evidence that there is still a lot of confusion over this today. Let me reiterate what the paper concludes, although it takes a lot longer to do so!
You can build an SOA without an ESB. What an SOA DOES need is a number of integration functions in the SOA ecosystem, namely communications, mediation services and adapters/gateways. These provide the mechanisms for different components and services in the SOA to be linked together. As it happens, ESBs provide much of this function, and have other benefits when combined with SOA principles such as being standards-based. Therefore, ESBs are a good fit for SOA, and well worth considering. But they are not REQUIRED. For example, a traditional message broker could provide the same functionality.
I hope this helps to clarify the confusion. This is similar to the stupid rumours about SOA requiring the use of web services…