Because Web Services are essentially software interfaces, they are inherently stateless. Therefore, traditional ESBs must maintain state information in their underlying execution environment, typically with threads. This approach leads to two issues: first, composing Services across ESBs is a hassle, and second, you need to write the thread state to a database for long-running processes, and then rehydrate that state when the process resumes.

Read the entire post at http://www.devx.com/blog/looking-for-a-restful-soa-intermediary-part-one.html