Selective Consumer

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

Introduction to Selective Consumer

The Selective Consumer EIP allows a message consumer to select which messages it will receive. It filters the messages delivered by its channel so that it only receives the ones that match its criteria. For more information, refer to http://www.eaipatterns.com/MessageSelector.html.

Figure 1: Selective Consumer EIP

Example scenario

This example scenario demonstrates how a specific receiver only processes messages that are pre-filtered based on certain criteria. We have an Axis2 server as the consumer. The consumer criteria is specified through an XML schema validation, which is stored as a local entry in the registry. We use the Validate mediator to check whether the messages that are sent to the ESB match the criteria of the schema. The Axis2 server can consume the message only if the message meets the validation criteria.

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

Figure 2: Example Scenario of the Selective Consumer EIP

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

Selective Consumer EIP (Figure 1)	Selective Consumer Example Scenario (Figure 2)
Specifying Producer	Simple Stock Quote Client
Messages with Selection Values	Simple Stock Quote Request
Selective Consumer	Schema Validator (Validate Mediator)
Receiver	Simple Stock Quote Server

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 the sample Axis2 server. 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 the Main menu and click Source View in the Service Bus section. 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">
   <localEntry key="selective_criteria">
      <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
                 xmlns="http://www.apache-synapse.org/test"
                 elementFormDefault="qualified"
                 attributeFormDefault="unqualified"
                 targetNamespace="http://services.samples">
         <xs:element name="getQuote">
            <xs:complexType>
               <xs:sequence>
                  <xs:element name="request">
                     <xs:complexType>
                        <xs:sequence>
                           <xs:element name="stockvalue" type="xs:string"/>
                        </xs:sequence>
                     </xs:complexType>
                  </xs:element>
               </xs:sequence>
            </xs:complexType>
         </xs:element>
      </xs:schema>
   </localEntry>
   <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>
         <validate>
            <schema key="selective_criteria"/>
            <on-fail>
               <makefault>
                  <code xmlns:tns="http://www.w3.org/2003/05/soap-envelope" value="tns:Receiver"/>
                  <reason value="Invalid custom quote request"/>
               </makefault>
               <property name="RESPONSE" value="true"/>
               <header name="To" expression="get-property('ReplyTo')"/>
               <drop/>
            </on-fail>
         </validate>
         <send>
            <endpoint>
               <address uri="http://localhost:9000/services/SimpleStockQuoteService?wsdl"/>
            </endpoint>
         </send>
      </in>
      <out>
         <send/>
      </out>
   </sequence>
</definitions>

Simulating the sample scenario

Send the following request using a SOAP client like SoapUI.

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services.samples" xmlns:xsd="http://services.samples/xsd">
   <soapenv:Header/>
   <soapenv:Body>
      <ser:getQuote>
         <!--Optional:-->
         <ser:request>
            <!--Optional:-->
            <ser:symbol>IBM</ser:symbol>
         </ser:request>
      </ser:getQuote>
   </soapenv:Body>
</soapenv:Envelope>

Note that the stock quote request is not processed. Send the following message to the ESB server.

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ser="http://services.samples" xmlns:xsd="http://services.samples/xsd">
   <soapenv:Header/>
   <soapenv:Body>
      <ser:getQuote>
         <!--Optional:-->
         <ser:request>
            <!--Optional:-->
            <ser:stockvalue></ser:stockvalue>
         </ser:request>
      </ser:getQuote>
   </soapenv:Body>
</soapenv:Envelope>

The consumer has specified the criteria of using a schema validation. Only the messages that meet this criteria will be consumed.

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.

localEntry [line 2 in ESB config] - A local registry entry with key selective_criteria is used to define the XML schema used for validation inside the main sequence.
validate [line 33] in ESB config] - This mediator is used to define the portion of a message used for validation. In this example, no source attribute is specified using an XPath expression, so the ESB performs the validation on the first child element of the SOAP body.
scehma [line 34 in ESB config] - Defines which schema to use for validation. In this example, the local registry entry definition in line 2 is used.
on-fail [line 35 in ESB config] - Defines the action to take on failure of a validation. In this example, a fault is created and the message is dropped.