When you are using box_carring feature, you have to maintain same session across all the operation calls. Otherwise, it will evaluates as separated calls and will not be atomic. Here is a sample synapse config that can be used to maintain the same session.
<?xml version="1.0" encoding="UTF-8"?>
<proxy xmlns="http://ws.apache.org/ns/synapse"
name="ESBService"
transports="https http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<inSequence>
<transaction action="new"/>
<property name="id" expression="json-eval($.id)"/>
<property name="userName" expression="json-eval($.userName)"/>
<property name="firstName" expression="json-eval($.firstName)"/>
<property name="lastName" expression="json-eval($.lastName)"/>
<property name="address" expression="json-eval($.address)"/>
<enrich>
<source type="body" clone="true"/>
<target type="property" property="FirstBody"/>
</enrich>
<property name="messageType" value="application/xml" scope="axis2"/>
<header name="Action" value="urn:begin_boxcar"/>
<payloadFactory media-type="xml">
<format>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:dat="http://ws.wso2.org/dataservice">
<soapenv:Header/>
<soapenv:Body>
<dat:begin_boxcar/>
</soapenv:Body>
</soapenv:Envelope>
</format>
<args/>
</payloadFactory>
<call>
<endpoint>
<address uri="http://localhost:9764/services/testService.SOAP11Endpoint/"/>
</endpoint>
</call>
<property name="setCookieHeader" expression="$trp:Set-Cookie"/>
<property name="Cookie"
expression="get-property('setCookieHeader')"
scope="transport"/>
<property name="OUT_ONLY" value="true"/>
<payloadFactory media-type="xml">
<format>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:dat="http://ws.wso2.org/dataservice">
<soapenv:Header/>
<soapenv:Body>
<p:insert_employee xmlns:p="http://ws.wso2.org/dataservice">
<xs:UserId xmlns:xs="http://ws.wso2.org/dataservice">$1</xs:UserId>
<xs:userName xmlns:xs="http://ws.wso2.org/dataservice">$2</xs:userName>
<xs:firstName xmlns:xs="http://ws.wso2.org/dataservice">$3</xs:firstName>
<xs:lastName xmlns:xs="http://ws.wso2.org/dataservice">$4</xs:lastName>
</p:insert_employee>
</soapenv:Body>
</soapenv:Envelope>
</format>
<args>
<arg evaluator="xml" expression="get-property('id')"/>
<arg evaluator="xml" expression="get-property('userName')"/>
<arg evaluator="xml" expression="get-property('firstName')"/>
<arg evaluator="xml" expression="get-property('lastName')"/>
</args>
</payloadFactory>
<property name="Content-Encoding" scope="transport" action="remove"/>
<property name="Cookie"
expression="get-property('setCookieHeader')"
scope="transport"/>
<call>
<endpoint>
<address uri="http://localhost:9764/services/testService.SOAP11Endpoint/"/>
</endpoint>
</call>
<payloadFactory media-type="xml">
<format>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:dat="http://ws.wso2.org/dataservice">
<soapenv:Header/>
<soapenv:Body>
<dat:end_boxcar/>
</soapenv:Body>
</soapenv:Envelope>
</format>
<args/>
</payloadFactory>
<property name="Content-Encoding" scope="transport" action="remove"/>
<property name="Cookie"
expression="get-property('setCookieHeader')"
scope="transport"/>
<call>
<endpoint>
<address uri="http://localhost:9764/services/testService.SOAP11Endpoint/"/>
</endpoint>
</call>
<respond/>
</inSequence>
<faultSequence>
<log>
<property name="END" value="****ROLLBACK****"/>
</log>
<transaction action="rollback"/>
<respond/>
</faultSequence>
</target>
</proxy>
However, you can use request_box feature, where you do not have to maintain the session across operations.
Thanks