A mediator is a full-powered processing unit in the ESB. In run-time, it has access to all the parts of the ESB along with the current message. Usually a mediator is configured using XML. Different mediators have their own XML configurations.
At the run-time, a message is injected into the mediator with the ESB's run-time information. Then this mediator can do virtually anything with the message.
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Also mediators are considered as one of the main mechanisms for extending an ESB. A user can write a mediator and put it into the ESB. This custom mediator and any other built in mediator will be exactly the same as the API and the privileges (see Writing a WSO2 ESB Mediator).
There is a wide range of standard mediators in the WSO2 ESB.
The Mediator Catalog
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