JEST response is either XML or JSON.


h2. XML Representation
		
JEST represents persistent Java object graph in a schema-compliant XML. The schema is metamodel-driven and hence invariant to particular domain model. 
The schema {{jest-instance.xsd}} is as follows:
		
{code:xml|title=Metamodel-driven schema for JEST XML}
<!-- ========================================================================= -->
<!-- Schema for serialized persistence instance graph                          -->
<!-- ========================================================================= -->
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
	attributeFormDefault="unqualified" elementFormDefault="qualified"
	version="1.0">

	<xsd:annotation>
		<xsd:documentation><![CDATA[
         Describes closure of managed persistence instance.
         Each instance is described by all its loaded persistent attribute.
         The related instances are resolved within the document root.
         Document root represents zero or more instances. 
          
         The file must be named "jest-instance.xsd".
         ]]>
		</xsd:documentation>
	</xsd:annotation>

    <!-- The root element of the document contains zero or more instances -->
	<xsd:element name="instances">
		<xsd:complexType>
			<xsd:sequence>
			    <xsd:element name="uri"         minOccurs="1" maxOccurs="1" type="xsd:anyURI"/>
			    <xsd:element name="description" minOccurs="0" maxOccurs="1" type="xsd:string"/>
				<xsd:element name="instance"    minOccurs="0" maxOccurs="unbounded" type="instance-type" />
			</xsd:sequence>
			<xsd:attribute name="version" type="xsd:string" />
		</xsd:complexType>
	</xsd:element>

	<!-- The root element for a single instance. Children of this element are persistent attribute -->
	<!-- Persistent Attributes occur in order. The order is determined by the attribute category.  -->
	<!-- Attribute category is determined by the enumerated PersistentAttributeType defined in     -->
	<!-- javax.persistence.metamodel and then further refined by id, version, lob and enum.        -->
	<!-- See org.apache.openjpa.persistence.jest.MetamodelHelper for further details.              -->
	<xsd:complexType name="instance-type">
		<xsd:sequence>
			<xsd:element name="id"           type="basic-attr-type"      minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="version"      type="basic-attr-type"      minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="basic"        type="basic-attr-type"      minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="enum"         type="basic-attr-type"      minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="embedded"     type="instance-type"        minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="lob"          type="lob-attr-type"        minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="one-to-one"   type="singular-attr-type"   minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="many-to-one"  type="singular-attr-type"	 minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="element-collection" type="collection-attr-type" minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="one-to-many"  type="collection-attr-type" minOccurs="0" maxOccurs="unbounded"/>
			<xsd:element name="many-to-many" type="map-attr-type"        minOccurs="0" maxOccurs="unbounded"/>
		</xsd:sequence>
		<xsd:attribute name="id" type="xsd:ID" use="required" />
	</xsd:complexType>

	<!-- A reference to another instance within the same(?) document -->
	<xsd:complexType name="ref-type">
		<xsd:simpleContent>
			<xsd:extension base="xsd:string">
				<xsd:attribute name="id" type="xsd:IDREF" />
			</xsd:extension>
		</xsd:simpleContent>
	</xsd:complexType>
	
	<!-- A null reference                                            -->
	<xsd:complexType name="ref-null">
		<xsd:simpleContent>
			<xsd:extension base="xsd:string">
			</xsd:extension>
		</xsd:simpleContent>
	</xsd:complexType>

	<!-- Basic Attribute has a name and its runtime type   -->
	<!-- non-null value appears as text content.           -->
	<!-- null value appears as attribute with empty text . -->
	<xsd:complexType name="basic-attr-type">
		<xsd:simpleContent>
			<xsd:extension base="xsd:string">
				<xsd:attribute name="name" type="xsd:string" use="required" />
				<xsd:attribute name="type" type="xsd:string" use="required" />
				<xsd:attribute name="null" type="xsd:boolean" />
			</xsd:extension>
		</xsd:simpleContent>
	</xsd:complexType>
	
	<!-- Large Binary Objects (LOB) represented as hex array -->
	<xsd:complexType name="lob-attr-type">
		<xsd:simpleContent>
			<xsd:extension base="xsd:hexBinary">
				<xsd:attribute name="name" type="xsd:string" use="required" />
				<xsd:attribute name="type" type="xsd:string" use="required" />
				<xsd:attribute name="null" type="xsd:boolean" />
			</xsd:extension>
		</xsd:simpleContent>
	</xsd:complexType>

	<!-- Singular attribute is a reference to another instance or a null reference. -->
	<xsd:complexType name="singular-attr-type">
		<xsd:choice>
			<xsd:element name="null" type="ref-null" />
			<xsd:element name="ref" type="ref-type" />
		</xsd:choice>
		<xsd:attribute name="name" type="xsd:string" use="required" />
		<xsd:attribute name="type" type="xsd:string" use="required" />
	</xsd:complexType>

	<!-- Collection attributes list their members with their runtime type -->
	<!-- Members can be basic or other managed instance                   -->
	<xsd:complexType name="collection-attr-type">
		<xsd:sequence>
			<xsd:element name="member" type="member-type" minOccurs="0"
				maxOccurs="unbounded" />
		</xsd:sequence>
		<xsd:attribute name="name" type="xsd:string" use="required" />
		<xsd:attribute name="type" type="xsd:string" use="required" />
		<xsd:attribute name="member-type" type="xsd:string" use="required" />
	</xsd:complexType>

	<!-- Map attributes list their entries with runtime type of key and value    -->
	<!-- Both key and value can be independently basic or other managed instance -->
	<xsd:complexType name="map-attr-type">
		<xsd:sequence>
			<xsd:element name="entry" type="entry-type" />
		</xsd:sequence>
		<xsd:attribute name="name" type="xsd:string" use="required" />
		<xsd:attribute name="type" type="xsd:string" use="required" />
		<xsd:attribute name="key-type" type="xsd:string" use="required" />
		<xsd:attribute name="value-type" type="xsd:string" use="required" />
	</xsd:complexType>

	<!-- Value of a member of basic type. -->
	<xsd:complexType name="basic-value-type">
		<xsd:simpleContent>
			<xsd:extension base="xsd:string">
				<xsd:attribute name="null" type="xsd:boolean" />
			</xsd:extension>
		</xsd:simpleContent>
	</xsd:complexType>

	<!-- Value of a member of a collection/map -->
	<xsd:complexType name="member-type">
		<xsd:choice>
			<xsd:element name="basic" type="basic-value-type" />
			<xsd:element name="null" type="ref-null" />
			<xsd:element name="ref" type="ref-type" />
		</xsd:choice>
	</xsd:complexType>

	<!-- Denotes entry of a map element -->
	<xsd:complexType name="entry-type">
		<xsd:sequence>
			<xsd:element name="key"   type="member-type" minOccurs="1" maxOccurs="1" />
			<xsd:element name="value" type="member-type" minOccurs="1" maxOccurs="1"  />
		</xsd:sequence>
	</xsd:complexType>
	
</xsd:schema>
{code}

h3. Example of a serialized object graph
The example shows a Person (primary key p1) who is her own partner.

{code:xml|title=Example of Object Graph with Circular Reference}
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<instances>
  <instance id="Person-p1">
    <basic name="id" type="String">p1</basic>
    <basic name="dob" type="Date">Fri Sep 08 00:00:00 CDT 1989</basic>
    <basic name="firstName" type="String">Mary</basic>
    <basic name="gender" type="Gender">Female</basic>

    <basic name="lastName" type="String">Smith</basic>
    <singular name="address" type="Address">
      <instance id="Address-101">
        <basic name="id" type="long">101</basic>
        <basic name="country" type="String">C1</basic>
        <basic name="state" type="String">State1</basic>
        <basic name="street" type="String">Street1</basic>

        <basic name="zip" type="int">10001</basic>
      </instance>
    </singular>
    <singular name="partner" type="Person">
      <ref id="Person-p1"/>
    </singular>
  </instance>
</instances>
{code}

		 
h2. JSON Representation

String-based representation of persistent Java object graphs in either XML or more succinct JSON form has to address the critical issue of circular reference, because persistent object graphs
often contains circular reference. JSON, at this point of writing, does not address circular reference. Hence JEST introduces special semantics within JSON format to refer to 
instances in a serialized graph. However, the enhanced JSON can be parsed by standard JSON parsers.

Enhanced JSON representation in JEST introduces two _special_ fields named {{$id}} and {{$ref}} to address circular reference. Each JSON object carries a {{$id}} field whose value is the persistent identity of the corresponding entity. When JSON serialization encounters a circular reference a