Message Dispatcher

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

Introduction to Message Dispatcher

The Message Dispatcher EIP consumes messages from a single channel and distributes them among performers. It allows multiple consumers on a single channel to coordinate their message processing. For more information, refer to http://www.eaipatterns.com/MessageDispatcher.html.

Figure 1: Message Dispatcher EIP

Example scenario

This example scenario demonstrates how to distribute messages among performers using the weighted load balance mediator. We have several Axis2 server instances, each considered to be a performer.

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

Figure 2: Example Scenario of the Message Dispatcher EIP

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

Message Dispatcher EIP (Figure 1)	Message Dispatcher Example Scenario (Figure 2)
Sender	Simple Stock Quote Client
Messages	Simple Stock Quote Requests
Message Dispatcher	Message Endpoint, Load-Balance Endpoint
Performers	Simple Stock Quote Server Instances

Environment setup

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.
Start three sample Axis2 server instances on ports 9000, 9001, and 9002. For instructions, refer to the section ESB Samples Setup - Starting Sample Back-End Services in the WSO2 ESB documentation.

ESB configuration

Start the ESB server and log into its management console UI (https://localhost:9443/carbon). In the management console, navigate to Main Menu, click Service Bus and then Source View. 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">
   <sequence name="fault">
      <log level="full">
         <property name="MESSAGE" value="Executing default &#34;fault&#34; sequence"/>
         <property name="ERROR_CODE" expression="get-property('ERROR_CODE')"/>
         <property name="ERROR_MESSAGE" expression="get-property('ERROR_MESSAGE')"/>
      </log>
      <drop/>
   </sequence>
   <sequence name="main">
      <in>
         <send>
            <endpoint>
               <loadbalance algorithm="org.apache.synapse.endpoints.algorithms.WeightedRoundRobin">
                  <endpoint>
                     <address uri="http://localhost:9000/services/SimpleStockQuoteService/"/>
                     <property name="loadbalance.weight" value="1"/>
                  </endpoint>
                  <endpoint>
                     <address uri="http://localhost:9001/services/SimpleStockQuoteService/"/>
                     <property name="loadbalance.weight" value="2"/>
                  </endpoint>
                  <endpoint>
                     <address uri="http://localhost:9002/services/SimpleStockQuoteService/"/>
                     <property name="loadbalance.weight" value="3"/>
                  </endpoint>
               </loadbalance>
            </endpoint>
         </send>
      </in>
      <out>
         <send/>
      </out>
   </sequence>
</definitions>

Simulating the sample scenario

Repeatedly send several requests to the ESB using the Stock Quote client as follows:

ant stockquote -Dtrpurl=http://localhost:8280/ -Dsymbol=Foo

Following is the request sent by the Stock Quote client in this example:

<soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope" xmlns:ser="http://services.samples" xmlns:xsd="http://services.samples/xsd">
   <soapenv:Header />
   <soapenv:Body>
        <ser:getQuote>        
         <ser:request>
             <ser:symbol>Foo</ser:symbol>
         </ser:request>      
      </ser:getQuote>
   </soapenv:Body>
</soapenv:Envelope>

Note in each Axis2 server console that the requests are distributed among several servers in a weighted manner. Servers running on port 9000, 9001, and 9002 receive the request in that order until the process starts over again in a round-robin manner.

How the implementation works

Let's investigate the elements of the ESB configuration in detail. The line numbers below refer to the ESB configuration shown above.

endpoint [line 14 in ESB config] - Defines the endpoint where the request should be sent.
loadbalance [line 15 in ESB config] - Defines a set of endpoints where incoming requests are distributed using a particular algorithm. In this example, the algorithm distributes messages in a weighted round-robin manner.