Recipient List

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

Introduction to Recipient List

The Recipient List EIP routes a message to a list of dynamically specified recipients. It processes an incoming message and identifies its list of recipients. Once the list is identified, the message will be sent to all recipient channels. For more information, refer to http://www.eaipatterns.com/RecipientList.html.

Figure 1: Recipient List EIP

Example scenario

This example scenario is a stock quote service sending a stock quote request to recipients that are instances of a sample Axis2 server. The Switch mediator identifies the content of the client request and distributes the content among the Recipient List endpoints.

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

Figure 2: Recipient List Example Scenario

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

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. Deploy the SimpleStockQuoteService and start three instances of Axis2 Server in ports 9000, 9001, 9002, and 9003. 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.

<!-- Would Route the Message Based on the List of Recipients-->
<definitions xmlns="http://ws.apache.org/ns/synapse">
    <registry provider="org.wso2.carbon.mediation.registry.ESBRegistry">
        <parameter name="root">file:repository/samples/resources/</parameter>
        <parameter name="cachableDuration">15000</parameter>
    </registry>
    <proxy name="RecipientListProxy">
        <target>
            <inSequence>
            <switch source="//m0:getQuote/m0:request/m0:symbol" xmlns:m0="http://services.samples">
            <!-- First the recipient list will be identified -->
                <case regex="foo">
               <send>
                  <!--Dynamic Recipient List-->
                        <endpoint>
                     	<recipientlist>
                        <endpoint xmlns="http://ws.apache.org/ns/synapse">
                                <address uri="http://localhost:9000/services/SimpleStockQuoteService"/>
                        </endpoint>
                        <endpoint xmlns="http://ws.apache.org/ns/synapse">
                                <address uri="http://localhost:9001/services/SimpleStockQuoteService"/>
                        </endpoint>
                       </recipientlist>
                    </endpoint>          
                </send>
                  <drop/>
            </case>    
            <case regex="WSO2">
               <send>
                  <!--Dynamic Recipient List-->
                        <endpoint>
                     <recipientlist>
                        <endpoint xmlns="http://ws.apache.org/ns/synapse">
                                <address uri="http://localhost:9002/services/SimpleStockQuoteService"/>
                        </endpoint>
                        <endpoint xmlns="http://ws.apache.org/ns/synapse">
                                <address uri="http://localhost:9003/services/SimpleStockQuoteService"/>
                        </endpoint>
                       </recipientlist>
                    </endpoint>          
                </send>
                  <drop/>
            </case>                
            <default>
              <!-- Message Should Be Discarded -->
            </default>
            </switch>       
            </inSequence>
            <outSequence>
                <send/>
            </outSequence>
        </target>   
     </proxy>
</definitions>

Simulating the sample scenario

Send a request using the Stock Quote client to WSO2 ESB in the following manner. For information about the Stock Quote client, refer to the Sample Clients section in the WSO2 ESB documentation.

ant stockquote -Dtrpurl=http://localhost:8280/services/RecipientListProxy -Dsymbol=WSO2

Note that the ESB sends the request to servers running on ports 9002 and 9003. If you change the symbol to foo, it will send the requests to servers running on port 9000 and 9001. 

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.

• Switch [line 10 in ESB config] - The Switch mediator performs a switch/case based on the symbol found inside the original request. In this example scenario, one of two send mediators are used, based on the value of the symbol element in the request. 
• recipientList [line 16 in ESB config] - the recipientList mediator lists several endpoints inside the send tags. ESB will forward the request to all endpoints in this list.