Mediators

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 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.

Mediators in a sequence can be one of the following types:

Node mediators - Contains child mediators.
Leaf mediators - Does not hold any other child mediators.

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

Category	Name	Description
Core	Send	Sends a message out
	Log	Logs a message
	Property	Sets or remove properties associated with the message
	Sequence	Refers a sequence
	Event	Sends event notifications to an event source, publishes messages to predefined topics
	Drop	Drops a message
	Enrich	Enriches a message
Filter	Filter	Filters a message using XPath, if-else kind of logic
	Out	Inbuilt filter for choosing messages in ESB out path
	In	Inbuilt filter for choosing messages in ESB in path
	Switch	Filters messages using XPath, switch logic
	Router	Routes messages based on XPath filtering
	Conditional Router	Implements complex routing rules (Header based routing, content based routing and other rules)
Transform	XSLT	XSLT transformations
	URLRewrite	Modifies and rewrites URLs or URL fragments
	XQuery	XQuey
	Header	Sets or removes SOAP headers
	Fault	Create SOAP Faults
Advanced	Cache	Evaluate messages based on whether the same message came to the ESB
	Clone	Clones a message
	Store	Stores messages in a predefined message store
	Iterate	Splits a message
	Aggregate	Combines a message
	Callout	Blocks web services calls
	Transaction	Executes a set of mediators transactionally
	Throttle	Limits the number of messages
	RMSequence	Reliable messaging
	DBReport	Writes data to database
	DBLookup	Retrieves information from database
	Rule	Executives rules
	Entitlement	Fines grain authorization
	OAuth	2-legged OAuth support
Extension	Class	Creates and executes a custom mediator
	POJOCommand	Executes a custom command
	Script	Executes a mediator written in Scripting language
	Spring	Creates a mediator managed by Spring
	Smooks	Used to apply lightweight transformations on messages in an efficient manner