The Call mediator is used to send messages out of the ESB to an endpoint. The Call mediator invokes the service in an asynchronous manner. Then, the underlying worker thread returns without waiting for the response. Mediation will be paused from that point. When the response is received, the mediation flow resumes from the next mediator in the sequence.
The Call mediator is similar to the Callout mediator, which performs a blocking external service invocation during mediation. Unlike the Callout mediator, the Call mediator leverages the non-blocking transports for much greater performance. Therefore, in most cases you should use the Call mediator instead of the Callout mediator. However, the Callout mediator is recommended in situations where you need to execute the mediation flow in a single thread.
The Send Mediator is another similar mediator. A send operation can be blocking or non-blocking depending on the actual transport implementation used. The default NIO-based http/s implementation does not block on a send.
The Call mediator is a content-unaware mediator.
Syntax
<call/>
If the message is to be sent to one or more endpoints, use the following syntax:
<call> (endpointref | endpoint)+ </call>
- The
endpointreftoken refers to the following:
<endpoint key="name"/>
- The
endpointtoken refers to an anonymous endpoint definition.
UI Configuration
Select one of the following options to define the endpoint to which the message should be delivered.
| Parameter Name | Description |
|---|---|
| None | Select this option if you do not want to provide an endpoint. The Call mediator will send the message using its wsa:to address. |
| Define Inline | If this is selected, the endpoint to which the message should be sent can be included within the Call mediator configuration. Click Add to add the required endpoint. See Adding an Endpoint for further details. |
| Pick From Registry | If this is selected, the message can be sent to a pre-defined endpoint which is currently saved as a resource in the registry. Click either Configuration Registry or Governance Registry as relevant to select the required endpoint from the resource tree. |
| XPath | If this is selected, the endpoint to which the message should be sent will be derived via an XPath expression. You are required to enter the relevant XPath expression in the text field that appears when this option is selected. You can click NameSpaces to add namespaces if you are providing an expression. Then the Namespace Editor panel would appear where you can provide any number of namespace prefixes and URLs used in the XPath expression. |
Example
Example 1 - Service orchestration
In this example, the Call mediator invokes a backend service. An Enrich mediator stores the response received for that service invocation.
The Filter Mediator added after the Call mediator carries out a filter to determine whether the first call has been successful. If it is successful, second backend service is invoked. The payload of the request to the second backend is the response of the first service invocation.
After a successful second backend service invocation, response of the first service is retrieved by the Enrich mediator from the property where it was formerly stored. This response is sent to the client by the Respond mediator.
If it is not successful, a custom JSON error message is sent with HTTP 500. If the first call itself is not successful, the output is just sent back with the relevant error code.
<target>
<inSequence>
<log/>
<call>
<endpoint>
<http method="get" uri-template="http://192.168.1.10:8088/mockaxis2service"/>
</endpoint>
</call>
<enrich>
<source type="body" clone="true"/>
<target type="property" action="child" property="body_of_first_call"/>
</enrich>
<filter source="get-property('axis2', 'HTTP_SC')" regex="200">
<then>
<log level="custom">
<property name="switchlog" value="Case: first call successful"/>
</log>
<call>
<endpoint>
<http method="get" uri-template="http://localhost:8080/MockService1"/>
</endpoint>
</call>
<filter source="get-property('axis2', 'HTTP_SC')" regex="200">
<then>
<log level="custom">
<property name="switchlog" value="Case: second call successful"/>
</log>
<enrich>
<source type="property" clone="true" property="body_of_first_call"/>
<target type="body"/>
</enrich>
<respond/>
</then>
<else>
<log level="custom">
<property name="switchlog" value="Case: second call unsuccessful"/>
</log>
<property name="HTTP_SC" value="500" scope="axis2"/>
<payloadFactory media-type="json">
<format>{ "status": "ERROR!"}</format>
<args/>
</payloadFactory>
<respond/>
</else>
</filter>
</then>
<else>
<log level="custom">
<property name="switchlog" value="Case: first call unsuccessful"/>
</log>
<respond/>
</else>
</filter>
</inSequence>
</target>
Example 2 - Continuing mediation without waiting for responses
In this example, the message will be cloned by the Clone Mediator and sent via the Call mediator. The Drop mediator drops the response so that no further mediation is carried out for the cloned message. However, since the continueParent attribute of the Clone mediator is set to true, the original message is mediated in parallel. Therefore, the Log Mediator at the end of the configuration will log the After call mediator log message without waiting for the Call mediator response.
... <log level="full"/> <clone continueParent="true"> <target> <sequence> <call> <endpoint> <address uri="http://localhost:8080/echoString"/> </endpoint> </call> <drop/> </sequence> </target> </clone> <log level="custom"> <property name="MESSAGE" value="After call mediator"/> </log> ...
Samples
For another example, see Sample 500: Call Mediator for Non-Blocking Synchronous Service Invocation.