The assertion condition is specified as a nested extension element. The following example shows the postassertion condition schema definition in BPEL 2.0.

<invoke | receive | onMessage>
    standard-elements
    <bpelx:postAsserts> 
       <bpelx:postAssert faultName="QName">
         <bpelx:expression expressionLanguage="anyURI"?>expression
         </bpelx:expression>
         <bpelx:message expressionLanguage="anyURI"?>expression</bpelx:message>
       </bpelx:postAssert>
   </bpelx:postAsserts>
</invoke | receive | onMessage>

The following example shows the postassertion condition syntax in BPEL 2.0.

<bpelx:postAsserts>
   <bpelx:postAssert faultName="ns2:InvalidInput">
     <bpelx:expression>number(concat($inputVariable.payload/client:input,'2')) <  
      500</bpelx:expression>
      <bpelx:message>"AssertXpathPostInvoke_20 assert fired"</bpelx:message>
   </bpelx:postAssert>
</bpelx:postAsserts>

The following example shows the postassertion condition schema definition in BPEL 1.1. Note the differences between BPEL 1.1 and BPEL 2.0.

<invoke | receive | onMessage>
    standard-elements
    <bpelx:postAssert name="ncname" expression="boolean-expr" faultName="QName"+
 message="generic-expr"+/>
</invoke | receive | onMessage>

The following example shows the postassertion condition syntax in BPEL 1.1.

<bpelx:postAssert name="Assert_1"
   message='Post Invoke Multiple assert value fired'
   faultName="ns2:NegativeValue"
   expression="bpws:getVariableData('invar','payload','/ns1:process/ns1:input') >
0"/>

The following example shows the preassertion condition schema definition in BPEL 2.0.

<invoke | reply>
   standard-elements
   <bpelx:preAsserts>
     <bpelx:preAssert faultName="QName">
      <bpelx:expression expressionLanguage="anyURI"?>expression</bpelx:expression>
      <bpelx:message expressionLanguage="anyURI"?>expression</bpelx:message>
     </bpelx:preAssert>
   </bpelx:preAsserts>
</invoke | reply>

The following example shows the preassertion condition syntax in BPEL 2.0.

<bpelx:preAsserts>
    <bpelx:preAssert faultName="ns1:InvalidInput">
       <bpelx:expression>concat($inputVariable.payload/client:input,'2') >
       $inputVariable.payload/client:input</bpelx:expression>
       <bpelx:message>"AssertXpathPreInvoke_20 Assert test"</bpelx:message>
    </bpelx:preAssert>
</bpelx:preAsserts>

The following example shows the preassertion condition schema definition in BPEL 1.1. Note the differences between BPEL 1.1 and BPEL 2.0.

<invoke | reply>
    standard-elements
    <bpelx:preAssert name="NCName" expression="string" message="string"
    faultName="QName"/>
</invoke | reply>

The following example shows the preassertion condition syntax in BPEL 1.1.

<bpelx:preAssert name="Assert_1"
   expression="bpws:getVariableData('invar','payload','/ns1:process/ns1:input') > 
   0"
   message='pre invoke assert NegativeInput fired'
   faultName="ns4:NegativeInput"/>

The bpelx:postAssert extension specifies the XPath expression to evaluate upon receipt of a callback message from a partner. If the assertion expression returns a false boolean value, the specified fault is thrown from the activity. If the assertion expression returns a true boolean value, no fault is thrown and the activities following the invoke activity, receive activity, or the onMessage branch of pick and scope activities are executed as in a normal BPEL process flow.

The bpelx:preAssert or bpelx:postAssert extension is similar to the Java assert statement. In Java, if the assert expression does not evaluate to true, an error is reported by the JVM. Similarly, the expression in the bpelx:preAssert or bpelx:postAssert extension must evaluate to true; otherwise, the specified fault is thrown.

For example, with the BPEL 1.1 invoke activity shown in the following example, if the XPath expression specified in the assertion condition returns false, the NegativeCredit fault is thrown.

<scope>
    <faultHandlers>
        <catch faultName="services:NegativeCredit" faultVariable="crError">
            <empty/>
        </catch>
    </faultHandlers>
    <sequence>
        <invoke name="invokeCR" partnerLink="creditRatingService"
                portType="services:CreditRatingService" operation="process"
                inputVariable="crInput" outputVariable="crOutput">
            <bpelx:postAssert name="negativeCredit"
 expression="$crOutput.payload/tns:rating > 0"
                          faultName="services:NegativeCredit" message="'Negative
 Credit'" />
        </invoke>
    </sequence>
</scope>

The optional name attribute for bpelx:preAssert or bpelx:postAssert is used while creating the audit trail event message. The name in this instance enables you to identify the assertion element in case multiple assertions are specified. If no name attribute is specified, the line number of the assertion element in the BPEL file may be used.