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This project (Synapse) is related to ESB, but it is not in itself an ESB. What Synapse brings to the table is a mediation framework that allows users to get in the middle between service requesters and providers and perform various tasks including transformation and routing and that helps to promote loose coupling between services
The distinction David makes goes to the heart of both questions. First, Synapse is not a full ESB in the most common sense of the term. Secondly, because the design of Apache Synapse is different from that of Apache ServiceMix, the two solutions have different capabilities and limitations.
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In providing management through WS-* to Apache Axis, the Apache Synapse project provides a very important and useful capability to that specific Web service brokerservices. However, in its current implementation Apache Synapse would be overextended as an "architectural" solution. Building an SOA on Apache Synapse would presume that all exchanges in the SOA would be Web Service exchanges based on SOAPmade through SOAP-based Web Services, that the management of the exchanges would be invoked exclusively by means of WS-*, and that the underlying SOAP technology at each brokered endpoint would be Apache Axis2 *ANY* SOAP stack which implements stack(s) implement the relevant protocols.
ServiceMix is a full ESB that can work with many different SOAP Stacks such as Axis, WSIF, XFire, ActiveSOAP and JAX-WS. ServiceMix also has full support for Routing, Transformation and Orchestration. Like Apache Synapse, Apache ServiceMix provides mediation for web services exchanges and is designed to support transformation, management and routing through WS-* standards. However, this capability is provided without dependency on the SOAP stack, and most importantly, is provided through the same services architecture used for all message formats and transports, including, but certainly not limited to, Web services.
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