ESB Mediators

A mediator is the basic message processing unit and a fundamental part the ESB profile. A mediator can take a message, carry out some predefined actions on it, and output the modified message. For example, the Clone mediator splits a message into several clones, the Send mediator sends the messages, and the Aggregate mediator collects and merges the responses before sending them back to the client.

The ESB profile ships with a range of mediators capable of carrying out various tasks on input messages, including functionality to match incompatible protocols, data formats and interaction patterns across different resources. Data can be split, cloned, aggregated, and enriched, allowing the ESB profile to match the different capabilities of services. XQuery and XSLT allow rich transformations on the messages. Rule-based message mediation allows users to cope with the uncertainty of business logic. Content-based routing using XPath filtering is supported in different flavors, allowing users to get the most convenient configuration experience. Built-in capability to handle Transactions allows message mediation to be done transactionally inside the ESB profile. With the eventing capabilities of the ESB profile, EDA based components can be easily interconnected, allowing the ESB profile to be used in the front-end of an organisation's SOA infrastructure.

A mediator is a full-powered processing unit in the ESB profile of WSO2 EI. At run-time, a mediator has access to all the parts of the ESB profile along with the current message and can do virtually anything with the message. At the run-time, a message is injected in to the mediator with the ESB profile information. Then this mediator can do virtually anything with the message. A user can write a mediator and put it into the ESB profile. This custom mediator and any other built-in mediator will be exactly the same as the API and the privileges (Refer to more information in Creating Custom Mediators).

A mediation sequence, commonly called a "sequence", is a list of mediators. That means, it can hold other mediators and execute them. It is part of the ESB profile's core and message mediation cannot live without this mediator. When a message is delivered to a sequence, it sends the message through all its child mediators. For more information, see Mediation Sequences.

The Process of message mediation is depicted in the diagram below.

In case an error occurs in the main sequence while processing, the message goes to the fault sequence.

When adding a mediator to a sequence, you can configure the mediator in design view or in source view. Usually, a mediator is configured using XML. Source view provides the XML representation of the configurations done in the UI. When you edit the source XML all changes immediately reflect in the design view as well.

Each mediator has its own XML configuration. You can change the following mediators using their source view:

Aggregate Mediator
Cache Mediator
Filter Mediator
In Mediator
Iterate Mediator
Out Mediator
Sequence Mediator
Synapse Mediator
Template Mediator
Validate Mediator

It is possible to comment out lines of code in the Synapse definition as well as in the source view of the complete EI configuration.

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.

Mediators are classified as follows based on whether or not they access the message's content:

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 EI. You can create custom mediators and add them to the ESB profile. This custom mediator and any other built-in mediator will be exactly the same as the API and the privileges.

The standard mediators in the ESB profile 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 WSO2 EI 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 profile
	In	Applies to messages that are in the In path of the ESB profile
	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	Evaluates messages based on whether the same message came to the ESB profile
	ForEach	Splits a message into a number of different messages by finding matching elements in an XPath expression of the original message.
	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
	Builder	Builds the actual SOAP message, from a message, which is coming into the ESB profile through the Binary Relay.
	Entitlement	Evaluates user actions against a XACML policy
	OAuth	2-legged OAuth support
	Smooks	Used to apply lightweight transformations on messages in an efficient manner.
	Data Mapper	Converts and transforms one data format to another, or changes the structure of the data in a message.
Extension	Bean	Manipulates JavaBeans
	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
Agent	Publish Event	Constructs events and publishes them to different systems such as WSO2 BAM/DAS/CEP/SP via event sinks.