Scatter-Gather

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

Introduction to Scatter-Gather

The Scatter-Gather EIP maintains the overall message flow when a message needs to be sent to multiple recipients, each of which may send a reply back. For more information, refer to http://www.eaipatterns.com/BroadcastAggregate.html.

Figure 1: Scatter-Gather EIP

Example scenario

This example scenario demonstrates an implementation of Scatter-Gather EIP that broadcasts a message to multiple recipients using WSO2 ESB. The ESB uses the Aggregate mediator to collect the responses and 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 three vendors, get quotes for certain items, and return the best quote to the client. We assume that all three vendors implement the same service contract. If the service contracts are different, the ESB must transform the messages before sending them to the vendor services and then transform the responses before returning them to the client. The XSLT mediator is designed to handle these transformations.

The diagram below depicts how to simulate the example scenario using 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

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 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. 

<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>  

Simulating the sample scenario

1. Use a SOAP client like SoapUI to send the following request to the ScatterGatherProxy service.

   <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services.samples">  
      <soapenv:Header/>  
      <soapenv:Body>  
         <ser:getSimpleQuote>  
            <ser:symbol>Foo</ser:symbol>  
         </ser:getSimpleQuote>  
      </soapenv:Body>  
   </soapenv:Envelope>  

2. Because the log mediator is enabled inside the outSequence, there will be three responses from the three vendors. The logs will be similar to the following:
   

   

3. In SoapUI, you will get the response from the vendor providing the best quote as follows:
   

   

4. Compare the logged response messages with the response received by the client to see that the ScatterGatherProxy service returns the best quote to the client.

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.

• clone [line 6 in ESB config] - The Clone mediator is similar to the Splitter EIP. It clones the incoming request and passes the requests in parallel to several endpoints.   
• aggregate [line 26 in ESB config] - The Aggregate mediator aggregates response messages for requests made by the Iterate or Clone mediator. The completion condition specifies the minimum or maximum number of messages to be collected. When all messages are aggregated, the sequence inside onComplete will be run. 
• 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.
• All received responses are logged before the Aggregate mediator merges them. The onComplete sequence of the Aggregate mediator is called once all three 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.
• 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 message instructs the ESB to pick the response that has the lowest last value. (The XPath 2.0 min function could reduce the complexity of the above command, but XPath 1.0 is the current default supported by WSO2 ESB.) Once the proper response is found, we enrich the SOAP body with it and send that response back to the client.