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This section explains, through an example scenario, how the Polling Consumer EIP can be implemented using WSO2 ESB. The following topics are covered:

Introduction to Polling Consumer

The Polling Consumer EIP allows the ESB to explicitly make a call when the application wants to receive a message. For more information, refer to http://www.eaipatterns.com/PollingConsumer.html.

Figure 1: Polling Consumer EIP

Example scenario

This example scenario demonstrates the WSO2 ESB JMS proxy, which is a polling transport that is used to connect to various JMS providers. After configuring the JMS proxy, it will listen on a queue on the JMS server. Next, we send a message to the queue, and the JMS proxy in the ESB will pick up the message when it is ready for consumption.

The diagram below depicts how to simulate the example scenario using WSO2 ESB.

Figure 2: Example Scenario of the Polling Consumer EIP

Before digging into implementation details, let's take a look at the relationship between the example scenario and the Polling Consumer EIP by comparing their core components.

Polling Consumer EIP (Figure 1)Polling Consumer Example Scenario (Figure 2)
SenderSimple Stock Quote Client
MessageSimple Stock Quote Request
Polling ConsumerProxy Service using JMS Transport
ReceiverSimple Stock Quote Service

Environment setup

  1. Download and install WSO2 ESB from http://wso2.com/products/enterprise-service-bus. For a list of prerequisites and step-by-step installation instructions, refer to Getting Started in the WSO2 ESB documentation. 
  2. Download and install a JMS server. We use ActiveMQ as the JMS provider in this example.  
  3. Make the following edits to the <ESB_HOME>/repository/conf/axis2/axis2.xml file.
    • To enable the JMS transport, uncomment the Axis2 transport listener configuration for ActiveMQ as follows:
      <transportReceiver name="jms" class="org.apache.axis2.transport.jms.JMSListener">... 
     
    • Set the transport.jms.SessionTransacted parameter to true. After making this update, the transportReceiver section of axis2.xml should appear as follows:

      <transportReceiver name="jms" class="org.apache.axis2.transport.jms.JMSListener">
              <parameter name="myQueueConnectionFactory" locked="false">
                  <parameter name="java.naming.factory.initial" locked="false">org.apache.activemq.jndi.ActiveMQInitialContextFactory</parameter>
                  <parameter name="java.naming.provider.url" locked="false">tcp://localhost:61616</parameter>
                  <parameter name="transport.jms.ConnectionFactoryJNDIName" locked="false">QueueConnectionFactory</parameter>
              	<parameter name="transport.jms.ConnectionFactoryType" locked="false">queue</parameter>
              	<parameter name="transport.jms.SessionTransacted">true</parameter>
              </parameter>
      
              <parameter name="default" locked="false">
                  <parameter name="java.naming.factory.initial" locked="false">org.apache.activemq.jndi.ActiveMQInitialContextFactory</parameter>
                  <parameter name="java.naming.provider.url" locked="false">tcp://localhost:61616</parameter>
                  <parameter name="transport.jms.ConnectionFactoryJNDIName" locked="false">QueueConnectionFactory</parameter>
                  <parameter name="transport.jms.ConnectionFactoryType" locked="false">queue</parameter>
      			<parameter name="transport.jms.SessionTransacted">true</parameter>
              </parameter>
      </transportReceiver>


     
    • Uncomment the Axis2 transport sender configuration as follows:
      <transportSender name="jms" class="org.apache.axis2.transport.jms.JMSSender"/> 
      
  4. Copy the following ActiveMQ client JAR files to the <ESB_HOME>/repository/components/lib directory to allow the ESB to connect to the JMS provider.
    • activemq-core-5.2.0.jar
    • geronimo-j2ee-management_1.0_spec-1.0.jar
     
  5. Add a custom mediator called MessageCounterMediator. Download the MessageCounterMediator file, and save it in the <ESB_HOME>/repository/components/lib folder. To learn how to write custom mediators, refer to Writing Custom Mediator Implementations in the WSO2 ESB documentation. 
  6. Start ActiveMQ (or equivalent JMS Server) and WSO2 ESB.
  7. Start the sample Axis2 server. For instructions, refer to the section ESB Samples Setup - Starting Sample Back-End Services in the WSO2 ESB documentation.

ESB configuration

Log into the ESB's management console UI (https://localhost:9443/carbon), and navigate to the Main menu and click Source View in the Service Bus section. Next, copy and paste the following configuration, which helps you explore the example scenario, to the source view.

<definitions xmlns="http://ws.apache.org/ns/synapse">
    <proxy name="StockQuoteProxy" transports="jms">
        <target>
            <inSequence>
                <property action="set" name="OUT_ONLY" value="true"/>
            </inSequence>
            <endpoint>
                <address uri="http://localhost:9000/services/SimpleStockQuoteService"/>
            </endpoint>
            <outSequence>
                <send/>
            </outSequence>
        </target>
        <publishWSDL uri="file:repository/samples/resources/proxy/sample_proxy_1.wsdl"/>
        <parameter name="transport.jms.ContentType">
            <rules>
                <jmsProperty>contentType</jmsProperty>
                <default>application/xml</default>
            </rules>
        </parameter>
    </proxy>
</definitions>

Simulating the sample scenario

  1. Send a stock quote request on JMS, as follows:
    ant jmsclient -Djms_type=pox -Djms_dest=dynamicQueues/StockQuoteProxy -Djms_payload=MSFT
     
  2. Note a message on the console running the sample Axis2 server saying that the server has accepted an order. For example:
    Accepted order #1 for : 17718 stocks of MSFT at $ 79.83113379282025

    Also note that there is one message queued and one de-queued in the queue created in the ActiveMQ web console at http://localhost:8161/admin/queues.jsp.
  3. Next, stop the ESB server, and send a stock quote request again. In the ActiveMQ web console, you can see that there are two messages queued and only one message de-queued.
  4. Start the ESB server again, and view the console running the sample Axis2 server. You will see a message indicating that the server has accepted the second message.

How the implementation works

Let's investigate the elements of the ESB configuration in detail. The line numbers below are mapped with the ESB configuration shown above.

  • proxy [line 2 in ESB config] - A proxy service with a JMS transport. 
  • parameter [line 15 in ESB config] - Sets JMS transport parameter contentType to application/xml. 
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