DS-COMPOSITE datastream

Content Models may contain this reserved datastream. It lists the datastreams that must exist in subscribing data objects, and the few requirements on them.

DSCompositeSchema

The old schema for the datastream can be seen below.

<xsd:schema
        targetNamespace="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified"
        attributeFormDefault="unqualified">
    <xsd:element name="dsCompositeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="dsTypeModel"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="dsTypeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="form"/>
            </xsd:sequence>
            <xsd:attribute name="ID" use="required" type="xsd:NCName"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="form">
        <xsd:complexType>
            <xsd:attribute name="FORMAT_URI" use="optional" type="xsd:anyURI"/>
            <xsd:attribute name="MIME" use="optional"/>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

The content of a DS-COMPOSITE datastream could look like this

<dsCompositeModel
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#">
    <dsTypeModel ID="DC">
        <form MIME="text/xml"/>
    </dsTypeModel>
    <dsTypeModel ID="ORIGIN">
        <form MIME="text/xml"/>
    </dsTypeModel>
</dsCompositeModel>

Optional datastreams

First, we will ammend the schema to support the property "optional" to datastream declarations. The JIRA issue for this is http://fedora-commons.org/jira/browse/FCREPO-531

The semantic meaning of optional is:
1. The datastream can exist in the substribing objects but does not have to
2. If the datastream exist, it must adhere to the specification in the content model

This will allow you to express optional datastreams like this

<dsCompositeModel
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#">
    <dsTypeModel ID="DC">
        <form MIME="text/xml"/>
    </dsTypeModel>
    <dsTypeModel ID="ORIGIN" optional="true">
        <form MIME="text/xml"/>
    </dsTypeModel>
</dsCompositeModel>

To allow this, the schema is now

<xsd:schema
        targetNamespace="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified"
        attributeFormDefault="unqualified">
    <xsd:element name="dsCompositeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="dsTypeModel"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="dsTypeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="form"/>
            </xsd:sequence>
            <xsd:attribute name="ID" use="required" type="xsd:NCName"/>
            <xsd:attribute name="optional" use="optional" type="xsd:boolean" default="false"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="form">
        <xsd:complexType>
            <xsd:attribute name="FORMAT_URI" use="optional" type="xsd:anyURI"/>
            <xsd:attribute name="MIME" use="optional"/>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

Allowing extensions in DS-COMPOSITE

Since Fedora already use DS-COMPOSITE to declare the existence of datastreams, it is the natural location to specify restrictions on the contents of datastreams. Unfortunately, the schema for the DS-COMPOSITE datastream does not allow for any extra content, To that effect, we have made a small change to the schema.

<xsd:schema
        targetNamespace="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        elementFormDefault="qualified"
        attributeFormDefault="unqualified">
    <xsd:element name="dsCompositeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="dsTypeModel"/>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="dsTypeModel">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="form"/>
                <xsd:element minOccurs="0" maxOccurs="unbounded" ref="extension"/>
            </xsd:sequence>
            <xsd:attribute name="ID" use="required"/>
            <xsd:attribute name="optional" use="optional" type="xsd:boolean" default="false"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="extension">
        <xsd:complexType>
            <xsd:sequence>
                <xsd:any namespace="##any" processContents="skip" minOccurs="0" maxOccurs="unbounded"/>
            </xsd:sequence>
            <xsd:attribute name="name" use="required"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="form">
        <xsd:complexType>
            <xsd:attribute name="FORMAT_URI" use="optional" type="xsd:anyURI"/>
            <xsd:attribute name="MIME" use="optional"/>
        </xsd:complexType>
    </xsd:element>
    <xsd:element name="reference">
        <xsd:complexType>
            <xsd:attribute name="type"/>
            <xsd:attribute name="value"/>
        </xsd:complexType>
    </xsd:element>
</xsd:schema>

With this changed schema, the contents could look like this:

<dsCompositeModel
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#">

    <dsTypeModel ID="DC">
        <form MIME="text/xml"/>
        <extension name="SCHEMA">

        </extensions>
    </dsTypeModel>
    <dsTypeModel ID="ORIGIN">
        <form MIME="text/xml"/>
        <extension name="SCHEMA">

        </extensions>
    </dsTypeModel>
</dsCompositeModel>

What is noteworthy here is that the <dsTypeModel> and the <form> elements are left unchanged. The Fedora code, working with DS-COMPOSITE only looks for these tags, so the new schema will not cause conflicts, and the extensions will be quietly ignored. This is exactly as we want, this change should not make our objects incompatible with an older Fedora.

Reserved extensions in DS-COMPOSITE

We have reserved two extention names.

"Schema" - used to specify that the datastream content must adhere to a schema
"Ontology" - used to specify constraints on the relations from this datastream as defined in RELS-INT

That leaves the question on where to place the content (schema/ontology). Embedding it directly in DS-COMPOSITE makes for a very unreadable datastream. Alternatively, you could just specify an URL to the schema, but this approach have problems too. Having the Content Model depend on schemas defined elsewhere, perhaps on remote servers, mean that the content models could break by actions totally unrelated to the repository. The best way, we have found, is to embed the schema in a datastream in the content model. To do this, we make use of the <reference> element, which we added the the schema above.

<dsCompositeModel
        xmlns="info:fedora/fedora-system:def/dsCompositeModel#"
        xmlns:schema="http://ecm.sourceforge.net/types/dscompositeschema/0/1/#">

    <!-- The DC datastream is declared. It's mime type must be text/xml. It must adhere to the xml schema residing in the DC_SCHEMA datastream in this content model. -->
    <dsTypeModel ID="DC">
        <form MIME="text/xml"/>
        <extension name="SCHEMA">
            <reference type="datastream" value="DC_SCHEMA"/>
        </extensions>
    </dsTypeModel>

    <!-- The ORIGIN datastream is declared. It's mime type must be text/xml. It must adhere to the xml schema residing in the ORIGIN_SCHEMA datastream in this content model-->
    <dsTypeModel ID="ORIGIN">
        <form MIME="text/xml"/>
        <extension name="SCHEMA">
            <reference type="datastream" value="ORIGIN_SCHEMA"/>
        </extensions>
    </dsTypeModel>

</dsCompositeModel>

Schema Extension

There are three kinds of datastreams in a Fedora object:

1. xml embedded in the object (inline content)
2. bytes embedded in the object (managed content)
3. external file referenced by URL. (External and Redirect content)

The schema extension does not care what kind of datastream is is applied to. But it only makes sense to apply it to a datastream with xml content, even if this xml content is externally referenced

DSCOMPOSITE-MODEL