Designed for collaboration between business-oriented services, NServiceBus is not a replacement for RPC technologies like WCF. Successful SOA and DDD projects use a mix of approaches and technologies - not just NServiceBus for communications. Here you'll find the similarities and differences between NServiceBus and its Microsoft counterparts.
Closer to WCF than to BizTalk
When many people hear the term "service bus" they picture a central box which all communication goes through, like BizTalk. That's actually a description of the Broker architectural style, not the Bus architectural style. A bus isn't necessarily a physical entity. In that respect, NServiceBus is more similar to WCF than it is to BizTalk.
There is no physical WCF one can point to in the network topology. WCF is part of the infrastructure that is run in-process with a given application's code. NServiceBus is the same.
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