Using messaging within flows

Sending a message from within a flow

The POSTMessage activity can be used within a flow to send a message to a queue. This activity uses the Connection API to send its input to a connection destination, using the messaging protocol.

The message sent by the POSTMessage activity will include a header, contentType, containing the content type associated with the input. If none is supplied, a default value, text/plain; charset=UTF-8, will be used.

To use the POSTMessage activity, you will first need to:

• Include both zero.assemble.flow and zero.messaging.connection in your application dependencies.
• Configure the connection to the messaging broker. For more information, see Configuring connections to messaging brokers.
• Optionally, configure a connection destination definition describing the target queue.

The following example flow source uses the POSTMessage activity to send a message:

<POSTMessage name="POSTMessage_1" destination="myQueue" broker="myBroker">
   <input value="${receiveMessage_0}" content-type="text/plain"/>
</POSTMessage>

The following parameters may be used with the POSTMessage activity:

destination

The message will be posted to the connection destination named by this required parameter. If a matching connection destination is not configured or does not specify a queue name, the destination name is used as the queue name.

broker

This optional parameter can be used to specify the broker connection configuration. If this is not specified, a matching connection destination with a valid connectionConfig is required.

Starting a flow when a message arrives

The IBM Reliable Transport Extension messaging receiver includes two components that may be used to start a flow to process a message that has arrived on a queue:

• The FlowMessageHandler is used to handle the message event, generated by the messaging receiver when a message arrives on a monitored queue, and start a flow.
• The receiveMessage activity is used within a flow started by the FlowMessageHandler to accept the message and passes the message contents as the activity output.

The output of the receiveMessage activity is annotated with the content type specified by the message header, contentType. If no contentType header is included with the message, the default value, text/plain; charset=UTF-8, is used.

To use the FlowMessageHandler and the receiveMessage activity, you will need to:

• Include both zero.assemble.flow and zero.messaging.receiver in your application dependencies.
• Configure a messaging kicker to monitor the queue and inform the receiver application when a message arrives.
• Configure the messaging receiver and its connection to the messaging broker.
• Configure the FlowMessageHandler to handle the message event for messages arriving on the queue.
• Provide a flow file that uses the receiveMessage activity to accept the message.

See Using the messaging kicker and receiver for more information about these concepts and their configuration.

The following configuration extract specifies that the FlowMessageHandler should handle a message event associated with the specified queue:

/config/handlers += [{
  "events" : "message",
  "handler" : "zero.messaging.receiver.flow.FlowMessageHandler.class",
  "conditions" : "/request/messaging/queue =~ myQueue",
  "instanceData" : { "view" : "myMessageFlow.flow" }
}]

Under the above configuration, the FlowMessageHandler will be invoked when a message arrives for the queue, myQueue. The view property in the handler instance data identifies the flow that will be invoked, myMessageFlow.flow.

Here is an example flow that can be used with the above handler configuration. The flow is saved as /app/view/myMessageFlow.flow.

<process name="myMessageFlow" expressionLanguage="Groovy">
  <receiveMessage name="receiveMessage_0"/>
  <POSTMessage name="POSTMessage_0" destination="targetQueue">
    <input value="${receiveMessage_0}"/>
  </POSTMessage>
</process>

In this simple flow, the receieveMessage activity is used to accept the message. The output of the receiveMessage activity is then used as the input to a POSTMessage activity which forwards the message to another destination.