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. There are multiple mediators that serve different purposes. They are grouped into three categories as follows:
- Content-aware mediators: Always access the message’s content when mediating messages (e.g., Enrich mediator).
- Content-unaware mediators: Never access the message’s content when mediating messages (e.g., Send mediator).
- Conditionally content-aware mediators: Either content-aware or content-unaware depending on their exact instance configuration. For example, a simple Log mediator instance (configured as <log/>) is content-unaware. However, a Log mediator configured as <log level=”full”/> is content-aware as it is expected to log the message payload.
When adding a mediator to a , you can configure the mediator in design view or in source view. 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. In the source view of the following mediators, you can comment out required lines if necessary when you edit the XML.
- Aggregate Mediator
- Cache Mediator
- Filter Mediator
- In Mediator
- Iterate Mediator
- Out Mediator
- Sequence Mediator
- Synapse Mediator
- Template Mediator
- Validate Mediator
It is also possible to comment out required lines 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.
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 EI. You can write a custom mediator and add it 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
|Core||Call||Invoke a service in non blocking synchronous manner|
|Enqueue||Uses a priority executor to ensure high-priority messages are not dropped|
Sends a message
Moves the message from the In flow to the Out flow, skipping all remaining configuration in the In flow
Inserts a reference to a sequence
|Respond||Stops processing on the message and sends it back to the client|
Sends event notifications to an event source, publishes messages to predefined topics
Drops a message
|Call Template||Constructs a sequence by passing values into a sequence template|
Enriches a message
Sets or remove properties associated with the message
Logs a message
Filters a message using XPath, if-else kind of logic
Applies to messages that are in the Out path of the ESB profile
Applies to messages that are in the In path of the ESB profile
|Validate||Validates XML messages against a specified schema.|
Filters messages using XPath, switch logic
Routes messages based on XPath filtering
Implements complex routing rules (Header based routing, content based routing and other rules)
Performs XSLT transformations on the XML payload
|FastXSLT||Performs XSLT transformations on the message stream|
Modifies and rewrites URLs or URL fragments
|XQuery||Performs XQuery transformation|
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|
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.|
Clones a message
Stores messages in a predefined message store
Splits a message
Combines a message
Blocks web services calls
Executes a set of mediators transactionally
Limits the number of messages
Writes data to database
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.|
Evaluates user actions against a XACML policy
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.|
Creates and executes a custom mediator
Executes a custom command
Executes a mediator written in Scripting language
Creates a mediator managed by Spring
|Agent||BAM||Captures data events and sends them to the BAM server|