Mediators

A mediator is a full-powered processing unit in the ESB. At run-time, a mediator has access to all the parts of the ESB along with the current message and can do virtually anything with the message.

When adding a mediator to a sequence, you can configure the mediator in design view or in source view, which allows you to edit the source XML (different mediators have their own XML configurations). 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.

The message content is accessed by some mediators in some mediation scenarios while it is not accessed in the other scenarios. Mediators can be classified as follows based on this aspect.

Content-aware mediators: These mediators always access the message content when mediating messages (e.g., Enrich mediator).
Content-unaware mediators: These mediators never access the message content when mediating messages (e.g., Send mediator).
Conditionally content-aware mediators: These mediators could be either content-aware or content-unaware depending on their exact instance configuration. For an example a simple Log Mediator instance (i.e. configured as <log/>) is content-unaware. However a log mediator configured as <log level=”full”/> would be content-aware since it is expected to log the message payload.

Mediators are considered to be one of the main mechanisms for extending an ESB. You can write a custom mediator and add it to the ESB. This custom mediator and any other built-in mediator will be exactly the same as the API and the privileges.

The standard mediators in WSO2 ESB are listed in the table below. Click a link for details on that mediator. There are also many samples that demonstrate how to use mediators.

The Mediator Catalog

Category	Name	Description
Core	Call	Invoke a service in non blocking synchronous manner
	Enqueue	Uses a priority executor to ensure high-priority messages are not dropped
	Send	Sends a message
	Loopback	Moves the message from the In flow to the Out flow, skipping all remaining configuration in the In flow
	Sequence	Inserts a reference to a sequence
	Respond	Stops processing on the message and sends it back to the client
	Event	Sends event notifications to an event source, publishes messages to predefined topics
	Drop	Drops a message
	Call Template	Constructs a sequence by passing values into a sequence template
	Enrich	Enriches a message
	Property	Sets or remove properties associated with the message
	Log	Logs a message
Filter	Filter	Filters a message using XPath, if-else kind of logic
	Out	Applies to messages that are in the Out path of the ESB
	In	Applies to messages that are in the In path of the ESB
	Validate	Validates XML messages against a specified schema.
	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	Performs XSLT transformations on the XML payload
	FastXSLT	Performs XSLT transformations on the message stream
	URLRewrite	Modifies and rewrites URLs or URL fragments
	XQuery	Performs XQuery transformation
	Header	Sets or removes SOAP headers
	Fault (also called Makefault)	Create SOAP Faults
	PayloadFactory	Transforms or replaces message content in between the client and the backend server
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
	DBReport	Writes data to database
	DBLookup	Retrieves information from database
	EJB	Calls an external Enterprise JavaBean(EJB) and stores the result in the message payload or in a message context property.
	Rule	Executes rules
	Entitlement	Evaluates user actions against a XACML policy
	OAuth	2-legged OAuth support
	RMSequence
	Smooks	Used to apply lightweight transformations on messages in an efficient manner.
Extension	Bean
	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
Agent	BAM	Captures data events and sends them to the BAM server