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

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This example scenario demonstrates an implementation of Scatter-Gather EIP , which that broadcasts a message to multiple recipients , using WSO2 ESB. The ESB uses an Aggregator the Aggregate mediator to collect the responses and distill merge them into a single response message.

We use a sample Stock Quote service as the service provided by the vendors. In this scenario, you send a quote request to 3 three vendors, get quotes for certain items, and return the best quote to the client. We assume that all 3 three vendors implement the same service contract. If the service contracts are different, appropriate transformations must be done the ESB must transform the messages before sending the requests them to the vendor services , and then transform the responses backbefore returning them to the client. The XSLT mediator can be used to do that is designed to handle these transformations.

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

Figure 2: Scatter-Gather Example Scenario

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

Environment setup

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Start the ESB server and log into its management console UI (https: //localhost:9443/carbon ). In the management console, navigate to Main Menu, click Services, then -> Services -> Add and then click Proxy Service. Next, copy and paste the following configuration, which helps you explore the example scenario, to a new Pass Through Proxy Service named ScatterGatherProxy. 

 
<definitions xmlns="http://ws.apache.org/ns/synapse">
  <proxy xmlns="http://ws.apache.org/ns/synapse" name="ScatterGatherProxy" transports="https http" startOnLoad="true" trace="disable">  
      <description/>  
      <target>  
          <inSequence>  
              <clone>  
                  <target>  
                      <endpoint name="vendorA">  
                          <address uri="http://localhost:9000/services/SimpleStockQuoteService/"/>  
                      </endpoint>  
                  </target>  
                  <target>  
                      <endpoint name="vendorB">  
                          <address uri="http://localhost:9001/services/SimpleStockQuoteService/"/>  
                      </endpoint>  
                  </target>  
                  <target>  
                      <endpoint name="vendorC">  
                          <address uri="http://localhost:9002/services/SimpleStockQuoteService/"/>  
                      </endpoint>  
                  </target>  
              </clone>  
          </inSequence>  
          <outSequence>  
              <log level="full"/>  
              <aggregate>  
                  <completeCondition>  
                      <messageCount min="3"/>  
                  </completeCondition>  
                  <onComplete xmlns:m1="http://services.samples/xsd" xmlns:m0="http://services.samples" expression="//m0:return">  
                      <enrich>  
                          <source xmlns:m1="http://services.samples/xsd" clone="true" xpath="//m0:return[not(preceding-sibling::m0:return/m1:last &lt;= m1:last) and not(following-sibling::m0:return/m1:last &lt; m1:last)]"/>  
                          <target type="body"/>  
                      </enrich>  
                      <send/>  
                  </onComplete>  
              </aggregate>  
          </outSequence>  
      </target>  
  </proxy>  
</definitions>

Simulating the sample scenario

1. Use a SOAP client like

SOAP UI

2. SoapUI to send the following request to the ScatterGatherProxy service.

   Code Block
   language html/xml
   <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services.samples"> <soapenv:Header/> <soapenv:Body> <ser:getSimpleQuote> <ser:

symbol>Foo<

3. symbol>foo</ser:symbol> </ser:getSimpleQuote> </soapenv:Body> </soapenv:Envelope>

Since
4. Because the log mediator is enabled inside the outSequence, there will be
3
5. three responses from the
3
6. three vendors. The logs will be similar to the following:
   

   Image Modified

7. In
SOAP UI
8. SoapUI, you will get the response from the vendor providing the best quote as follows:
   

   Image Modified

Comparison of
9. Compare the logged response messages with the response received by the client
shows
10. to see that the ScatterGatherProxy service returns the best quote to the client
(soapUI)
11. .

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 illustrated in step 3 shown above.

• clone [line 6 8 in ESB config] - In the inSequence of the ScatterGatherProxy service, we use the Clone mediator to make three copies of the request. Those requests are then forwarded to the three vendor services (SimpleStockeQuoteService). The responses to those three requests are received at the outSequence. The Clone mediator is similar to the Splitter EIP. It clones the incoming request and passes the requests parallelly in parallel to several endpoints.     
• log [line 27 in ESB config] - All received responses are logged before the Aggregate mediator merges them.
• aggregate [line 26 28 in ESB config] - The Aggregate mediator aggregates response messages for requests made by the Iterate or Clone mediators mediator. The completion condition specifies the minimum or maximum number of messages to be collected.
• onComplete [line 32 in ESB config] - When all messages are aggregated, the sequence inside onComplete will be run. 
• In the inSequence of the ScatterGatherProxy service, we are simply cloning, or making 3 copies of the request to be sent by the client using the Clone mediator. Those requests are then forwarded to the 3 vendor services (SimpleStockeQuoteService). The responses to those 3 requests are received at the outSequence.
• All received responses are logged before aggregating, using the Aggregate mediator of the ESB. The onComplete sequence of the Aggregate mediator onComplete sequence of the Aggregate mediator will run. This sequence is called once all 3 responses are received or the specified completion condition is met. The responses are aggregated based on the value of the return element in the response.
• Next, the enrich [line 34 in ESB config] - The Enrich mediator is used to extract the response, which contains the best quote. The following XPath 1.0 expression is used for this purpose:
  
  //m0:return[not(preceding-sibling::m0:return/m1:last <= m1:last) and not(following-sibling::m0:return/m1:last < m1:last)]
  
  In essence, this

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• expression instructs the ESB to pick the response

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• that has the lowest last value.

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• (The XPath 2.0 min function

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• could reduce the complexity of the above

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• expression, but XPath 1.0 is the current default supported by WSO2 ESB

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• .

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• ) Once the proper response is found, we enrich the SOAP body with it and send that response back to the client

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• .