Publish-Subscribe with JMS

JMS supports two models for messaging as follows:  

• Queues: point-to-point
• Topics: publish and subscribe
  

There are many business use cases that can be implemented using the publisher-subscriber (pub-sub) pattern. For example, consider a blog with subscribed readers. The blog author posts a blog entry, which the subscribers of that blog can view. In other words, the blog author publishes a message (the blog post content) and the subscribers (the blog readers) receive that message. Popular publisher/subscriber patterns like these can be implemented using JMS topics, as described in the following sections.

Scenario overview

In this sample scenario, we create a JMS Topic in a broker server such as ActiveMQ or the WSO2 Message Broker and then add proxy services that act as the publisher and subscribers in WSO2 ESB. This example assumes you have already downloaded and installed WSO2 ESB (see Getting Started).

Configuring the broker server

For this example, we will use ActiveMQ as our broker server. Follow the instructions in Configure with ActiveMQ to set up ActiveMQ for use with WSO2 ESB.

Configuring the publisher

1. Open the <ESB_HOME>/repository/conf/JNDI.properties file and specify the JNDI designation of the topic (in this example, SimpleStockQuoteService). For example:

   # register some queues in JNDI using the form
   # queue.[jndiName] = [physicalName]
   queue.MyQueue = example.MyQueue
    
   # register some topics in JNDI using the form
   # topic.[jndiName] = [physicalName]
   topic.MyTopic = example.MyTopic
   topic.SimpleStockQuoteService = SimpleStockQuoteService

2. Next, add a proxy service named StockQuoteProxy and configure it to publish to the topic SimpleStockQuoteService. You can add the proxy service to the ESB using the management console, either by building the proxy service in the design view or by copying the XML configuration into the source view. Alternatively, you can add an XML file named StockQuoteProxy.xml to <ESB_HOME>/repository/deployment/server/synapse-configs/default/proxy-services. A sample XML code segment that defines the proxy service is given below. Notice that the address URI specifies properties for configuring the JMS transport.

   <definitions xmlns="http://ws.apache.org/ns/synapse">
      <proxy name="StockQuoteProxy"
             transports="http"
             startOnLoad="true"
             trace="disable">
         <target>
            <endpoint>
   
               <address 
   uri="jms:/SimpleStockQuoteService?transport.jms.ConnectionFactoryJNDIName=TopicConnectionFactory&amp;java.naming.factory.initial=org.apache.activemq.jndi.ActiveMQInitialContextFactory&amp;java.naming.provider.url=tcp://localhost:61616&amp;transport.jms.DestinationType=topic"/>
            </endpoint>
            <inSequence>
               <property name="OUT_ONLY" value="true"/>
            </inSequence>
            <outSequence>
               <send/>
            </outSequence>
         </target>
      </proxy>
   </definitions>

   If you are using the source view in the management console, you must use '&' instead of '&' in endpoint URLs, as shown in the example above. If you are saving this proxy service configuration as an XML file in the proxy-services directory, use '&' instead.

Configuring the subscribers

Next, you configure two proxy services that subscribe to the JMS topic SimpleStockQuoteService, so that whenever this topic receives a message, it is sent to these subscribing proxy services. Following is the sample configuration for these proxy services.

<definitions xmlns="http://ws.apache.org/ns/synapse">
    <proxy name="SimpleStockQuoteService1"
          transports="jms"
          startOnLoad="true"
          trace="disable">
      <description/>
      <target>
         <inSequence>
            <property name="OUT_ONLY" value="true"/>
            <log level="custom">
               <property name="Subscriber1" value="I am Subscriber1"/>
            </log>
            <drop/>
         </inSequence>
         <outSequence>
            <send/>
         </outSequence>
      </target>
      <parameter name="transport.jms.ContentType">
         <rules>
            <jmsProperty>contentType</jmsProperty>
            <default>application/xml</default>
         </rules>
      </parameter>
      <parameter name="transport.jms.ConnectionFactory">myTopicConnectionFactory</parameter>
      <parameter name="transport.jms.DestinationType">topic</parameter>
      <parameter name="transport.jms.Destination">SimpleStockQuoteService</parameter>
   </proxy>

   <proxy name="SimpleStockQuoteService2"
          transports="jms"
          startOnLoad="true"
          trace="disable">
      <description/>
      <target>
         <inSequence>
            <property name="OUT_ONLY" value="true"/>
            <log level="custom">
               <property name="Subscriber2" value="I am Subscriber2"/>
            </log>
            <drop/>
         </inSequence>
         <outSequence>
            <send/>
         </outSequence>
      </target>
      <parameter name="transport.jms.ContentType">
         <rules>
            <jmsProperty>contentType</jmsProperty>
            <default>application/xml</default>
         </rules>
      </parameter>
      <parameter name="transport.jms.ConnectionFactory">myTopicConnectionFactory</parameter>
      <parameter name="transport.jms.DestinationType">topic</parameter>
      <parameter name="transport.jms.Destination">SimpleStockQuoteService</parameter>
   </proxy>
</definitions>

Publishing to the topic

1. Start the ESB with one of the following commands:

   • <ESB_HOME>/bin/wso2server.sh (on Linux)

   • <ESB_HOME>/bin/wso2server.bat (on Windows)

   A log message similar to the following will appear:

   INFO {org.wso2.andes.server.store.CassandraMessageStore} -  Created Topic : SimpleStockQuoteService
   INFO {org.wso2.andes.server.store.CassandraMessageStore} -  Registered Subscription tmp_127_0_0_1_44759_1 for Topic SimpleStockQuoteService

2. To invoke the publisher, use the sample stockquote client service by navigating to <ESB_HOME>/samples/axis2Client and running the following command:

   ant stockquote -Daddurl=http://localhost:8280/services/StockQuoteProxy -Dmode=placeorder -Dsymbol=MSFT 

   The message flow is executed as follows:

• • When the stockquote client sends the message to the StockQuoteProxy service, the publisher is invoked and sends the message to the JMS topic.

  • The topic delivers the message to all the subscribers of that topic. In this case, the subscribers are ESB proxy services.