It would seem to me that the cost of having a team of developers build and maintain all the components necessary to provide routing, workflow orchestration, durability, security and the other features provided by a commercial ESB would be equivalent to, or even more than, licensing a commercial ESB. Other than up front licensing costs and perhaps architectural preference, what are important factors to consider when designing an integration architecture around building a message bus using SOAP services over licensing message bus software from a commercial vendor and integrating your business components with it?
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It becomes an issue of flexibility – an ESB is all well and good, but ties you to the infrastructure. A SOA based approach (with web services as an example) gives you more flexibility as the components are more independent and can be redeployed or reused.
One issue is with the time taken to develop the solution, but in my experience you end up writing roughly the same amount of code with both systems (after you’ve developed 1 service, the others tend to be easily repeatable).