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The Architecture section in the WSO2 ESB documentation describes how the ESB processes a message it receives. When a message arrives, before it is sent through mediation, it will be transformed to a SOAP envelope using message builders. This way, the message will be in a standard format for the message mediation engine to do the processing. Once the response is ready to be sent back to the receiver, it will be changed back to its original format through message formatters.