[[common]]
= Common Schema

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The "common" schema defines a number of complex types that are used by other higher-level schemas.


== `oidDto`

The `oidDto` complex type captures an object's type and its identifier.
This is basically a formal XML equivalent to the `Bookmark` object obtained from the xref:refguide:applib:index/services/bookmark/BookmarkService.adoc[BookmarkService].

Although simple, this is an enormously powerful concept, in that it represents a URI to any domain object managed by a given Apache Isis application.
With it, we have the ability to lookup any arbitrary object.
Further discussion and examples can be found xref:userguide:fun:overview.adoc#oid[here].

The `oidDto` complex type is defined as:

[source,xml]
----
<xs:schema targetNamespace="http://isis.apache.org/schema/common"                           <!--.-->
           elementFormDefault="qualified"
           xmlns="http://isis.apache.org/schema/common"
           xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:complexType name="oidDto">                                                          <!--.-->
        <xs:sequence/>
        <xs:attribute name="type" type="xs:string"/>                                        <!--.-->
        <xs:attribute name="id" type="xs:string"/>                                          <!--.-->
        <xs:attribute name="objectState" type="bookmarkObjectState"/>
    </xs:complexType>

    <xs:simpleType name="bookmarkObjectState">                                              <!--.-->
        <xs:restriction base="xs:string">
            <xs:enumeration value="persistent"/>
            <xs:enumeration value="transient"/>
            <xs:enumeration value="viewModel"/>
        </xs:restriction>
    </xs:simpleType>

    <xs:complexType name="oidsDto">                                                         <!--.-->
        <xs:sequence>
            <xs:element name="oid" type="oidDto" minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
    ...
</xs:schema>
----
<.> the common schema has a namespace URI of "http://isis.apache.org/schema/common".
Although URIs are not the same as URLs, you will find that the schemas are also downloadable from this location.
<.> the `oidDto` complex type defines the unique identifier for any domain object: its type, and an identifier.
The `objectState` attribute can usually be omitted (indicating a persistent object)
<.> the object type, corresponding to either the xref:refguide:applib:index/annotation/DomainObject.adoc#logicalTypeName[@DomainObject#logicalTypeName()] attribute, or to the (JDO)
xref:refguide:applib-ant:PersistenceCapable.adoc[@PersistenceCapable] annotation (`schema` and/or `table` attributes), or to the (JDO) xref:refguide:applib-ant:Discriminator.adoc[@Discriminator] annotation.
If none is specified, then the fully qualified class name will be used.
<.> the object identifier (aka primary key), converted to string form.
<.> the `bookmarkObjectState` enumerates the possible persistence states of the referenced object.
In previous versions of the schema the attribute was defaulted to "persistent"; the "persistent" state is assumed if the attribute is omitted.
<.> Models a list of OIDs.
This is used by the xref:refguide:schema:cmd.adoc["cmd" schema] to represent the
intention to perform a bulk actions (against a number of selected objects).

In previous versions of the schema the object type and object identifers of `oidDto` were modelled as an element rather than an attribute.
The element form can still be used, but is deprecated.

The `oidDto` complex type is used in a number of places by the framework:

* first, as a means of serializing JAXB view model/DTOs (annotated with
xref:refguide:applib-ant:XmlRootElement.adoc[@XmlRootElement]), that reference domain entities.
+
These references are serialized instead into OIDs

* second, as references to the target of a command representing the _intention_ to invoke an action or edit a property, as described by the xref:refguide:schema:cmd.adoc["cmd" (command)] schema.
+
They are also used to represent references to any action arguments/properties that take domain object entities/view models.

* third, as references to the target of an interaction capturing the actual execution of an action invocation or property edit, as described by the xref:refguide:schema:ixn.adoc["ixn" (interaction)] schema.


== `collectionDto` etc

The `collectionDto` type defines a collection of values, also capturing the type of those values (for example ``string``s, or ``OidDto``s).
It is used primarily to model invocations of actions with collection parameters.

[source,xml]
----
<xs:schema targetNamespace="http://isis.apache.org/schema/common" ... >
    ...
    <xs:complexType name="collectionDto">
        <xs:sequence>
            <xs:element name="value" type="valueDto" minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="type" use="required" type="valueType"/>
        <xs:attribute name="null" use="optional" type="xs:boolean"/>
    </xs:complexType>
    ...
</xs:schema>
----


== `valueDto` etc

The common schema also defines two types representing values: the `valueDto` complex type, the `valueType` simple type and the `valueWithTypeDto` complex type:

[source,xml]
----
<xs:schema targetNamespace="http://isis.apache.org/schema/common" ... >
    ...
    <xs:complexType name="valueDto">                                <!--.-->
        <xs:choice minOccurs="0" maxOccurs="1">
            <xs:element name="string" type="xs:string"/>
            <xs:element name="byte" type="xs:byte"/>
            <xs:element name="short" type="xs:short"/>
            ...
            <xs:element name="timestamp" type="xs:dateTime"/>
            <xs:element name="enum" type="enumDto"/>
            <xs:element name="reference" type="oidDto"/>
            <xs:element name="collection" type="collectionDto"/>
            <xs:element name="blob" type="blobDto"/>
            <xs:element name="clob" type="clobDto"/>
        </xs:choice>
    </xs:complexType>

    <xs:simpleType name="valueType">                                <!--.-->
        <xs:restriction base="xs:string">
            <xs:enumeration value="string"/>
            <xs:enumeration value="byte"/>
            <xs:enumeration value="short"/>
            ...
            <xs:enumeration value="enum"/>
            <xs:enumeration value="reference"/>
            <xs:enumeration value="collection"/>
            <xs:enumeration value="blob"/>
            <xs:enumeration value="clob"/>
            <xs:enumeration value="void"/>                          <!--.-->
        </xs:restriction>
    </xs:simpleType>

    <xs:complexType name="valueWithTypeDto">                        <--.-->
        <xs:complexContent>
            <xs:extension base="valueDto">
                <xs:attribute name="type" use="required" type="valueType"/>
                <xs:attribute name="null" use="optional" type="xs:boolean"/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
    ...
</xs:schema>
----
<.> Intended to hold any valid value, eg of an argument to an action or a new value of a property.
<.> Enumerates the full set of types understood by the framework; note that these also include references to entities or view models, and to enums.
<.> Not valid to be used as the parameter type of an action; can be used as its return type.
<.> Inherits from `valueDto`, capturing both a value and its corresponding type.
Used for the return value of action invocations, and for the new value in property edits.


These type definitions are just building blocks.
The first, `valueDto` is The second, `valueType`, enumerates the different types of vales, eg of a formal parameter to an action.


[IMPORTANT]
====
When used as a parameter, blob and clob arguments are _not_ serialized.
Instead these are persisted only as references.
This is primarily to save storage space if the resultant XML is persisted as a memento (eg `CommandDto`).
====


== Ancillary types

The common schema also defines a number of ancillary types, used either by the common schema itself (see above) or by the xref:refguide:schema:cmd.adoc["cmd"] and xref:refguide:schema:ixn.adoc["ixn"] schemas.

[source,xml]
----
<xs:schema targetNamespace="http://isis.apache.org/schema/common" ... >
    ...
    <xs:complexType name="enumDto">                                 <!--.-->
        <xs:sequence>
            <xs:element name="enumType" type="xs:string"/>
            <xs:element name="enumName" type="xs:string"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="blobDto">                                 <!--.-->
        <xs:sequence>
            <xs:element name="name" type="xs:string"/>
            <xs:element name="mimeType" type="xs:string"/>
            <xs:element name="bytes" type="xs:hexBinary"/>
        </xs:sequence>
        <xs:attribute name="type" use="required" type="valueType"/>
        <xs:attribute name="null" use="optional" type="xs:boolean"/>
    </xs:complexType>

    <xs:complexType name="clobDto">                                 <!--.-->
        <xs:sequence>
            <xs:element name="name" type="xs:string"/>
            <xs:element name="mimeType" type="xs:string"/>
            <xs:element name="chars" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="type" use="required" type="valueType"/>
        <xs:attribute name="null" use="optional" type="xs:boolean"/>
    </xs:complexType>

    <xs:complexType name="periodDto">                               <!--.-->
        <xs:sequence>
            <xs:element name="startedAt" type="xs:dateTime"/>
            <xs:element name="completedAt" type="xs:dateTime"
                        minOccurs="0" maxOccurs="1"/>
        </xs:sequence>
    </xs:complexType>

    <xs:complexType name="differenceDto">                           <!--.-->
        <xs:sequence/>
        <xs:attribute name="before" type="xs:int"/>
        <xs:attribute name="after" type="xs:int"/>
    </xs:complexType>

    <xs:simpleType name="interactionType">                          <!--.-->
        <xs:restriction base="xs:string">
            <xs:enumeration value="action_invocation" />
            <xs:enumeration value="property_edit" />
        </xs:restriction>
    </xs:simpleType>

    <xs:complexType name="oidsDto">                                 <!--.-->
        <xs:sequence>
            <xs:element name="oid" type="oidDto" minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>

</xs:schema>
----
<.> Models an instance member of an enum (eg `Color.RED`).
<.> Models a `Blob`
<.> Models a `Clob`
<.> Captures a period of time, eg for capturing metrics/timings.
<.> Captures a pair of numbers representing a difference.
Used for example to capture metrics (number objects modified before and after).
<.> Whether this command/interaction with a member is invoking an action, or editing a property.
Used by both the xref:refguide:schema:cmd.adoc["cmd"] and xref:refguide:schema:ixn.adoc["ixn"] schemas.
<.> Contains a list of OIDs, eg for use in "bulk" actions that apply to multiple targets.

