xeps/xep-0075.xml

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<!DOCTYPE xep SYSTEM 'xep.dtd'
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 <!ENTITY xmlrpc "XML-RPC<note><link url='http://www.xmlrpc.org/spec'>The XML-RPC Specification</link></note>">
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<xep>
  <header>
    <title>Jabber Object Access Protocol (JOAP)</title>
    <abstract>The Jabber Object Access Protocol, or JOAP, defines a
      mechanism for creating Jabber-accessible object servers, and
      manipulating objects provided by those servers. It is intended
      for development of business applications with Jabber.</abstract>
    &LEGALNOTICE;
    <number>0075</number>
    <status>Deferred</status>
    <type>Standards Track</type>
    <sig>Standards</sig>
    <dependencies/>
    <supersedes/>
    <supersededby/>
    <shortname>N/A</shortname>
    <author>
      <firstname>Evan</firstname>
      <surname>Prodromou</surname>
      <email>evan@prodromou.san-francisco.ca.us</email>
      <jid>EvanProdromou@jabber.org</jid>
    </author>
    <revision>
      <version>0.3</version>
      <date>2003-05-22</date>
      <initials>esp</initials>
      <remark>For consistency, renamed hyphenated elements
      'new-address' and 'return-type' to 'newAddress' and 'returnType'
      respectively. Added 'desc' element for human-readable
      descriptions to object servers and classes. Changed the
      'writeable' [sic] attribute to the more correct
      'writable'. Added experimental namespace recommendation in
      XMPP Registrar section.</remark>
    </revision>
    <revision>
      <version>0.2</version>
      <date>2003-03-05</date>
      <initials>esp</initials>
      <remark>Added a schema and DTD, a number of new examples, and
        ensured that all examples validate against the DTD and
        schema.</remark>
    </revision>
    <revision>
      <version>0.1</version>
      <date>2003-01-28</date>
      <initials>esp</initials>
      <remark>Initial version (unpublished).</remark>
    </revision>
  </header>
  <section1 topic='Introduction'>
    <p>This document defines the Jabber Object Access Protocol (JOAP)
      as an extension to the Jabber protocol. It outlines the
      addressing scheme and IQ stanzas that comprise the protocol as
      well as the data types that the protocol models. Example
      applications are discussed, as well as security
      considerations.</p>
    <p>Jabber has a number of attractive features that give it an
      advantage over existing frameworks for building multi-tier
      applications, such as the Simple Object Access Protocol (SOAP)
      or Java 2, Enterprise Edition (J2EE). Among these are:</p>
    <ul>
      <li><strong>Built-in authentication.</strong> All clients in the
        Jabber network must be authenticated with their server before
        sending messages to other Jabber entities. Inter-server
        communication requires additional authentication, in the form
        of dialback connections or other trust mechanisms. This
        ensures that, when a message is delivered to a JOAP object
        server, there is little doubt as to the authenticity of its
        originator.</li>
      <li><strong>Global namespace.</strong> Jabber allows namespacing
        of addresses according to domain names. This allows objects to
        be accessed globally, according to authorization rules.</li>
      <li><strong>An asynchronous messaging model.</strong> Jabber is
        built on a store-and-forward mechanism that allows a single
        client to send messages to multiple servers concurrently. Of
        course, synchronous messaging can be simulated on the client
        side.</li>
      <li><strong>Cross-enterprise messaging.</strong> Jabber is
        designed to allow cross-enterprise messaging. Using Jabber for
        multitier applications makes development of cross-enterprise
        systems as easy as intra-enterprise development.</li>
      <li><strong>Message routing.</strong> The architecture of Jabber
        is based on clients that connect to a local server, and then
        can send messages through that server to other clients,
        servers, or components. Topographically, this contrasts well
        with the one-to-one connections required with, for example,
        HTTP.</li>
      <li><strong>Factoring network connections out of
        scalability.</strong> Because messages in Jabber are routed,
        components need to maintain only one connection -- to their
        upstream Jabber server. This removes the number of network
        connections from the scalability equation for object
        servers.</li>
      <li><strong>Language independence.</strong> Jabber protocol
        implementations exist for Java, C, and C++ as well as a number
        of scripting languages such as Perl and Python.</li>
      <li><strong>Platform independence.</strong> The Jabber protocol
        is implemented on most major modern platforms.</li>
    </ul>
    <p>For existing Jabber development efforts, there are significant
      advantages to building applications within a JOAP
      framework. It should go without saying that, for developers creating
      business applications on top of Jabber, a uniform object access
      protocol provides significant advantage for cross-product
      integration.</p>
    <p>In addition, implementers of special-purpose components, such as
      multi-user chat servers or whiteboarding components, can use an
      object-server interface to allow fine-grained control of the
      implementations, especially where such control is not specified
      by the applicable Jabber protocol.</p>
  </section1>
  <section1 topic='Requirements'>
    <p>JOAP has the following design goals:</p>
    <ul>
      <li>Create a protocol for client programs to access object
        server components.</li>
      <li>Create a model for addressing object servers, classes, and
        instances.</li>
      <li>Define a language for manipulating data.</li>
      <li>Define a language for describing data structures.</li>
      <li>Maintain compatibility with &xep0009;.</li>
      <li>Make classes and instances directly addressable.</li>
      <li>Allow human- and programming-language independence.</li>
      <li>Allow easy dynamic mapping of JOAP classes to local classes
        in Perl, Python, and other dynamic languages.</li>
    </ul>
    <p>The following are non-goals:</p>
    <ul>
      <li>Enable object-oriented access to all parts of the Jabber
        network (e.g., clients, other components).</li>
      <li>Define a language for creating or altering classes on an
        object server.</li>
      <li>Define a language for describing or defining the
        authorization of given users for given objects.</li>
      <li>Define a programming interface between object servers and
        classes in those servers (like Enterprise Java Beans).</li>
    </ul>
  </section1>
  <section1 topic='Overview'>
    <p>The JOAP interface is made up of three key parts:</p>
    <ul>
      <li>A scheme for defining the addresses of object servers,
        classes, and instances (collectively known as
        "objects").</li>
      <li>A set of message stanzas in the <tt>jabber:iq:joap</tt>
        namespace for manipulating data in object servers, classes,
        and instances. The stanzas allow client programs to analyze
        the structure of objects, to read object attributes, to edit
        object attributes, to add new instances, to delete instances,
        and to search classes.</li>
      <li>An application of XEP-0009 for calling methods on
        objects.</li>
    </ul>
  </section1>
  <section1 topic='Entities in the JOAP Universe'>
    <p>This section describes the various entities in the JOAP
      universe. Some entities are directly addressable with Jabber IDs
      (JIDs), as described below. Others are not considered outside of
      their enclosing entities.</p>
    <section2 topic='Object Server Component'>
      <p>An object server component is a Jabber component that
        provides object services. It is addressed like any other
        Jabber component, i.e., with a DNS hostname or
        pseudo-hostname. Some examples would be:</p>
      <ul>
        <li><tt>payroll.example.com</tt> - A payroll application
          server.</li>
        <li><tt>jukebox.example.com</tt> - An MP3 jukebox server.</li>
      </ul>
      <p>An object server has zero or more attributes, methods, and
        classes.</p>
    </section2>
    <section2 topic='Class'>
      <p>A class is a category of object instances. It defines the
        structure and interface of these instances. Each class is
        addressed using the class name as the node identifier, and the
        object server as the domain identifier. Class names must
        conform to the node identifier restrictions defined for
        XMPP. Class names must also be unique, regardless of case,
        within an object server.</p>
      <p>For example:</p>
      <ul>
        <li><tt>Employee@payroll.example.com</tt> - An employee class
          at the payroll.example.com server.</li>
        <li><tt>Song@jukebox.example.net</tt> - A song class on the
          jukebox server.</li>
        <li><tt>Board@circuit-design.example.com</tt> - A class for
          circuit boards.</li>
        <li><tt>Board@surf-shop.example.net</tt> - A class for
          surfboards -- distinct from above class!</li>
      </ul>
      <p>Beside uniqueness and XMPP compliance, no further requirements are
        made on class names. However, good design suggests mnemonic names.</p>
      <p>Classes define the attributes and methods of their instances. In
        addition, they can have attributes and methods of their own. Finally,
        classes can have superclasses, which indicate an inheritance structure
        as well as implementation of a defined interface.</p>
      <p>JOAP allows for no relative addressing of classes. Classes
        are always referred to by their full address (node identifier
        plus domain identifier).</p>
    </section2>
    <section2 topic='Instance'>
      <p>An instance is a collection of data with identity, state, and
        behavior. Each instance is a member of a class, which defines the
        attributes (data) and methods (behavior) of the instance
        itself.</p>
      <p>An instance is addressed using the node plus server that identifies
        its class, as well as a unique string that occupies the resource
        identifier section of the Jabber ID. The resource is only unique over
        the space of the corresponding class. Some example instance addresses:</p>
      <ul>
        <li><tt>Room@hotel.example.com/103</tt> - Room 103 in the
          Example Hotel.</li>
        <li><tt>Element@periodic-table.example.net/103</tt> - Element
          103 (rutherfordium) in the periodic table.</li>
        <li><tt>Employee@payroll.example.com/JohnSmith</tt> - An
          employee named "John Smith".</li>
        <li><tt>Customer@videorental.example.net/JohnSmith</tt> - A
          customer named "John Smith" (not necessarily the
          same person as the above employee!).</li>
      </ul>
      <p>Besides uniqueness within a class, and compliance with the
        rules for resource identifiers in the XMPP standard, there are
        no further requirements on instance identifiers in JOAP. In
        particular, the instance identifier is opaque -- that is, no
        further information about the state of the object can or
        should be discerned from the identifier. What visible part of
        the instance, if any, makes up the unique resource identifier
        is implementation dependent.</p>
      <p>That said, it is recommended that the instance identifier be
        persistent through the life of the instance. In addition, using
        mnemonic identifiers can greatly enhance the usability of JOAP
        objects.</p>
      <p>As with other resource identifiers, instance identifiers are
        case-sensitive.</p>
      <p>The instance identifier roughly corresponds to a primary key in a
        relational database, and for object servers that provide access to
        relational databases, it is recommended to use the primary key of a
        table as the instance identifier. For tables with a compound key, a
        comma (',') dash ('-'), or other non-alphanumeric character can be
        used to separate parts of the key for better readability. For
        example:</p>
      <ul>
        <li><tt>Date@calendar.example.net/2003-01-26</tt> -- The date
          January 26th, 2003.</li>
        <li><tt>City@canada.example.com/Montréal,QC</tt> -- The city
          of Montréal, in the province of Québec.</li>
      </ul>
      <p>JOAP allows for no relative addressing of
        instances. Instances are always referred to using their full
        address (node identifier plus domain identifier plus resource
        identifier).</p>
    </section2>
    <section2 topic='Attribute'>
      <p>An attribute is a unit of state that makes up part of an object
        server, instance, or class. Each attribute has a name and a
        type.</p>
      <p>Attribute names must be strings of characters containing only
        the characters [a-zA-Z0-9_]. The first character must be an
        underscore or alphabetic character. <note>This requirement is
          intended to allow easy mapping of attributes in JOAP to
          attributes of objects in client programming languages. The
          restriction is the lowest common denominator for variable
          names in most modern programming languages.</note></p>
      <p>Attributes cannot be addressed individually. Attributes are
        manipulated by sending JOAP messages to the object that owns
        them.</p>
    </section2>
    <section2 topic='Method'>
      <p>A method is a unit of behavior that makes up part of an
        object. Methods in JOAP are compatible with &xmlrpc;, as
        specified in &xep0009;. In particular, methods have a name, a
        return type, and 0 or more parameters, each of which has a
        type.</p>
      <p>The one exception to XML-RPC compatibility is that method
        names for JOAP are restricted to the characters
        [a-zA-z0-9_]. <note>This is to avoid conceptual mismatch in
          programming languages where the other three characters allowed
          by XML-RPC, namely ".", ":", and
          "/", are used to separate class or instance names
          from methods.</note></p>
      <p>Methods cannot be directly addressed using JOAP. Methods are
        described and executed by sending messages to the object
        server, class, or instance that owns them.</p>
    </section2>
  </section1>
  <section1 topic='JOAP Data Types'>
    <p>The range of JOAP data types is borrowed directly from
      XML-RPC.</p>
    <section2 topic='Scalar Types'>
      <p>The scalar types include the following:</p>
      <ul>
        <li><strong>int</strong> or <strong>i4</strong>: a 32-bit
          signed integer</li>
        <li><strong>boolean</strong>: a one-digit integer representing
          "true" (1) or "false" (0)</li>
        <li><strong>string</strong>: a string of characters</li>
        <li><strong>double</strong>: double-precision signed
          floating-point number</li>
        <li><strong>datetime.iso8601</strong>: a date value, in ISO
          8601 format</li>
        <li><strong>base64</strong>: binary data, base64-encoded for
          transmission</li>
      </ul>
    </section2>
    <section2 topic='Instance Addresses'>
      <p>Instance addresses are a special type of string used for
        referring to instance objects. They can be passed as
        parameters to methods, or set as attribute values.</p>
      <p>If a value can contain an object instance, its type is the
        address of a class. The address of any object instance that is
        an instance of that class, or any of its subclasses, can be
        used in that value.</p>
      <p>For example, if <tt>Boxcar@trainset.example.com</tt> is a
        subclass of <tt>Car@trainset.example.com</tt>, then
        <tt>Boxcar@trainset.example.com/569</tt> can be used as a
        method parameter, or set as an attribute, where
        <tt>Car@trainset.example.com</tt> is the defined type.</p>
      <p>Because addresses are used for instance values, all methods
        involving instances are implicitly pass-by-reference. If a
        pass-by-value functionality is needed, a struct (see below)
        should be used instead.</p>
      <p>Note that attribute and method param types can use classes
        and instances from other object servers (that is, with
        different domain identifiers). For instance, an
        <tt>Employee@payroll.example.com</tt> class could have an
        attribute of type <tt>Job@hr.example.com</tt>.</p>
    </section2>
    <section2 topic='Compound Types'>
      <p>There are two compound types defined in XML-RPC.</p>
      <section3 topic='Arrays'>
        <p>An <em>array</em> is an ordered list of values. An array
          can contain values of any type, including other compound
          types.</p>
        <p>In JOAP, as with XML-RPC, it is not possible to address,
          set, or delete elements of an array. To set values in an
          array, the entire new array must be specified.</p>
      </section3>
      <section3 topic='Structs'>
        <p>A <em>struct</em> is a set of name-value pairs organized into
          a logical grouping. A struct can contain values of any type,
          including other compound types.</p>
        <p>In JOAP, as with XML-RPC, it is not possible to address,
          set, or delete elements of a struct. To set values in an
          struct, the entire new struct must be specified.</p>
        <p>Structs are useful mainly for groupings of data that do not
          have independent identity or behavior.  Where an object
          needs identity or behavior, an instance should be used
          instead of a struct.</p>
      </section3>
    </section2>
    <section2 topic='Specifying Types'>
      <p>Types are specified by a string name of the type. This can be
        one of the XML-RPC types described above, or a class
        address.<note>Implementers can determine if a specified type
        is valid by checking it against a list of the XML-RPC
        types. If it does not match, it should be checked to see if
        matches the syntax for a class address (node identifier
        plus domain identifier). Otherwise, it is not a valid
        type.</note></p>
    </section2>
  </section1>
  <section1 topic='JOAP Stanzas'>
    <p>This section defines the Jabber stanzas that make up the JOAP
      protocol.</p>
    <p>Each stanza is an information query (IQ). Except for method
      calls, the stanzas are all in the 'jabber:iq:joap' namespace. Each
      of the following sections describes a stanza in that namespace,
      herein called a "verb". The verbs allow basic access
      to object servers, classes, and instances.</p>
    <p>Not all verbs can be sent to all JOAP entities. The appropriate JOAP
      entity a verb should be addressed to is noted under the description of
      the verb.</p>
    <section2 topic='&lt;describe&gt;'>
      <p>The &lt;describe&gt; verb requests the interface -- that is, methods,
        attributes, and classes -- of a given object server or class. The IQ
        type is "get".</p>
      <p>The &lt;describe&gt; verb is useful for creating wrapper
        classes in JOAP clients, either at runtime or at compile
        time. It can also be used for object browsers, or for client
        programs to ascertain that the interface they assume for an
        object is still valid.</p>
      <section3 topic='Targets'>
        <p>&lt;describe&gt; verbs can be sent to object servers, classes, and
          instances. Each will return different data.</p>
        <ul>
          <li>Object servers return zero or more descriptive texts,
            zero or more attribute definitions, zero or more method
            definitions, zero or more class names, and a
            timestamp.</li>
          <li>Classes return zero or more descriptive texts, zero or
            more attribute definitions, zero or more method
            definitions, and a timestamp.</li>
          <li>Instances return the exact results of sending the
            &lt;describe&gt; method to their class. This is for convenience
            only; it is preferable to send &lt;describe&gt; to the class
            directly.</li>
        </ul>
      </section3>
      <section3 topic='Descriptive Texts'>
        <p>Each object description can contain one or more strings of
        descriptive text. This is to indicate the
        purpose and usage of the object in human-readable form.</p>
        <p>Multiple descriptions are allowed in the hope that they
           will be used to describe the attribute in multiple
           languages (differentiated using the xml:lang
           attribute).</p>
      </section3>
      <section3 topic='Attribute Definitions'>
        <p>Attribute definitions have the following parts:</p>
        <ul>
          <li>A name, which is a legal attribute name as described
            above.</li>
          <li>A type, which is a legal JOAP type as described
            above.</li>
          <li>A flag indicating if the attribute is an attribute of the
            class itself, or of individual instances.</li>
          <li>A flag indicating if the attribute is writable.</li>
          <li>A flag indicating if the attribute is required.</li>
          <li>One or more strings of descriptive text, to indicate the
            purpose and usage of this attribute. Multiple
            descriptions are allowed in the hope that they will be
            used to describe the attribute in multiple languages
            (differentiated using the 'xml:lang' attribute).</li>
        </ul>
        <p>The attribute definitions returned to a client should 
          include only attributes the user is authorized to access.</p>
      </section3>
      <section3 topic='Method Definitions'>
        <p>Method definitions have the following parts:</p>
        <ul>
          <li>A name, which is a legal method name as described
            above.</li>
          <li>A return type, which is a legal JOAP type as described
            above.</li>
          <li>A flag indicating if the method is a method of the class
            itself, or of individual instances.</li>
          <li>Zero or more parameters, each of which has a name, a
            type, and one or more strings of descriptive text.</li>
          <li>One or more strings of descriptive text, indicating the
            use and behavior of this method.</li>
        </ul>
        <p>The method definitions returned to a client should 
          include only methods the user is authorized to access.</p>
      </section3>
      <section3 topic='Class References'>
        <p>Classes, in superclass definitions and object server
          interfaces, are always referred to by their full
          address.</p>
      </section3>
      <section3 topic='Timestamps'>
        <p>The timestamp is a date-time value in ISO 8601 format,
          UTC. The timestamp indicates the last time an interface was
          changed, if that information is available.</p>
      </section3>
      <section3 topic='Superclasses'>
        <p>The main point of describing the superclasses a class has
          is to allow clients to make typing distinctions: that is, to
          determine if a class presents a given interface, or may be
          provided as a parameter or attribute in another JOAP
          call.</p>
        <p>The list of superclasses given in a class description is
          flat, not hierarchical. No provision is made to
          indicate which of a class's superclasses are superclasses of
          each other, nor is there any implied precedence order in the
          order of the classes in the returned description.</p>
        <p>In addition, no provision is made to define which
          superclass actually implements any methods or attributes
          defined.</p>
      </section3>
      <section3 topic='Flattening'>
        <p>When a class receives a &lt;describe&gt; verb, it must
          return all its superclasses, including multiple
          ancestors. It must as well return all the attributes and
          methods that it responds to, including those defined in
          its superclasses. This is called a "flattened"
          description of the class. <note>Flattening the class
            interface reduces the need for making multiple
            "describe" verb calls just to find the interface
            for one class.</note></p>
      </section3>
      <section3 topic='Examples'>
        <p>The following examples illustrate the use of the &lt;describe&gt;
          verb. <note>All extended examples in this document refer
            to a particular object domain, based on a fictional model
            train set. A UML description of the object domain is
            available in Appendix D.</note></p>
        <p>To describe a server, the JOAP client sends this
          stanza.</p>
        <example caption='Describing An Object Server'><![CDATA[
          <iq type='get'
              id='joap_describe_1'
              from='Client@example.com'
              to='trainset.example.com'>
            <describe xmlns='jabber:iq:joap' />
          </iq>]]></example>
        <p>The object server returns this response:</p>
        <example caption='Description of an Object Server'><![CDATA[
          <iq type='result'
              id='joap_describe_1'
              from='trainset.example.com'
              to='Client@example.com'>
            <describe xmlns='jabber:iq:joap'>
              <desc xml:lang='en-US'>
                This server provides classes for managing a virtual
                remote train set.
              </desc>
              <attributeDescription writable='true'>
                <name>logLevel</name>
                <type>i4</type>
                <desc xml:lang='en-US'>Verbosity level for access logging.</desc>
              </attributeDescription>
              <methodDescription>
                <name>startLogging</name>
                <returnType>boolean</returnType>
                <desc xml:lang='en-US'>Start logging activity on this
                  server. Returns true for success and false for an
                  error.</desc>
              </methodDescription>
              <methodDescription>
                <name>stopLogging</name>
                <returnType>boolean</returnType>
                <desc xml:lang='en-US'>Stop logging activity on this
                  server. Returns true for success and false for an
                  error.</desc>
              </methodDescription>
              <class>Train@trainset.example.com</class>
              <class>Car@trainset.example.com</class>
              <class>Caboose@trainset.example.com</class>
              <class>Engine@trainset.example.com</class>
              <class>Boxcar@trainset.example.com</class>
              <class>PassengerCar@trainset.example.com</class>
              <class>Building@trainset.example.com</class>
              <class>TrackSegment@trainset.example.com</class>
              <class>Switch@trainset.example.com</class>
              <class>Station@trainset.example.com</class>
              <timestamp>2003-01-07T20:08:13Z</timestamp>
          </describe>
        </iq>]]></example>
        <p>To describe the <tt>Car@trainset.example.com</tt> class,
          the JOAP client sends this stanza to the class for
          boxcars.</p>
        <example caption='Describing a Class'><![CDATA[
          <iq type='get'
              id='joap_describe_2'
              from='Client@example.com'
              to='Boxcar@trainset.example.com' >
            <describe xmlns='jabber:iq:joap' />
          </iq>]]></example>
        <p>The class returns this stanza to the JOAP client.</p>
        <example caption='Description of a Class'><![CDATA[
          <iq type='result'
              id='joap_describe_2'
              from='Boxcar@trainset.example.com'
              to='Client@example.com'>
            <describe xmlns='jabber:iq:joap'>
              <desc xml:lang='en-US'>
                A Car in the trainset that can be used to ship cargo.
              </desc>
              <attributeDescription writable='false' required='true'>
                <name>trackingNumber</name>
                <type>i4</type>
                <desc xml:lang='en-US'>Tracking number for this car.</desc>
              </attributeDescription>
              <attributeDescription writable='true' required='true'>
                <name>contents</name>
                <type>string</type>
                <desc xml:lang='en-US'>Contents of the boxcar.</desc>
              </attributeDescription>
              <methodDescription allocation='class'>
                <name>nextTrackingNumber</name>
                <returnType>i4</returnType>
                <desc xml:lang='en-US'>The next available tracking number.</desc>
              </methodDescription>
              <superclass>Car@trainset.example.com</superclass>
              <timestamp>2003-01-07T20:08:13Z</timestamp>
           </describe>
         </iq>]]></example>
        <p>To describe an instance, the JOAP client sends this stanza
          to a particular track segment.</p>
        <example caption='Describing an Instance'><![CDATA[
          <iq type='get'
              id='joap_describe_3'
              from='Client@example.com'
              to='TrackSegment@trainset.example.com/134' >
            <describe xmlns='jabber:iq:joap' />
          </iq>
          ]]>
        </example>
        <p>The instance returns this stanza to the JOAP client.</p>
        <example caption='Description of an Instance'><![CDATA[
          <iq type='result'
              from='TrackSegment@trainset.example.com/134'
              to='Client@example.com'
              id='joap_describe_3'>
            <describe xmlns='jabber:iq:joap'>
              <desc xml:lang='en-US'>
                A length of track in the trainset which can be
                connected to a previous and next length of track.
              </desc>
              <attributeDescription>
                <name>previous</name>
                <type>TrackSegment@trainset.example.com</type>
                <desc>Previous segment of track.</desc>
              </attributeDescription>
              <attributeDescription>
                <name>next</name>
                <type>TrackSegment@trainset.example.com</type>
                <desc>Next segment of track.</desc>
              </attributeDescription>
              <timestamp>2003-01-07T20:08:13Z</timestamp>
            </describe>
          </iq>]]>
        </example>
      </section3>
    </section2>
    <section2 topic='&lt;read&gt;'>
      <p>The &lt;read&gt; verb allows clients to retrieve the values
        of attributes of an object server, class, or instance. The
        client can specify which attributes to return; if no
        attributes are specified, then all attributes are
        returned. <note>This allows clients to cheaply retrieve
          meta-information about an instance that may have exceptionally
          large data, such as bin64-encoded file data.</note></p>
      <p>The &lt;read&gt; verb uses the "get" IQ type.</p>
      <section3 topic='Timestamps'>
        <p>A timestamp, in ISO 8601 format, UTC, can be added to the
          results of a &lt;read&gt;. The timestamp indicates the last
          time any of an object's attribute values have changed (not
          just the requested ones). The timestamp can be used, for
          example, to implement object caching on the client side.</p>
      </section3>
      <section3 topic='Error Codes'>
        <p>The following are some common error codes may be generated
          in response to a &lt;read&gt; verb.</p>
        <ul>
          <li><strong>404 (Not Found)</strong>: The object addressed
            does not exists.</li>
          <li><strong>403 (Forbidden)</strong>: The user is not
            authorized to read attributes of this object, or not
            authorized to read the specified attributes of this
            object.</li>
          <li><strong>406 (Not Acceptable)</strong>: The client sent
            an &lt;read&gt; verb specifying attributes that are not
            defined for the class.</li>
        </ul>
      </section3>
      <section3 topic='Examples'>
        <p>This section gives some examples of using the &lt;read&gt;
          verb.</p>
        <p>A client would send the following stanza to an instance to
          read its attributes:</p>
        <example caption='Reading the Attributes of an
        Instance'><![CDATA[
          <iq type='get'
              id='joap_read_1'
              from='Client@example.com'
              to='Station@trainset.example.com/Paddington'>
            <read xmlns='jabber:iq:joap' />
          </iq>
          ]]>
        </example>
        <p>In return, the instance would send this stanza to the
          client:</p>
        <example caption='Attributes of an Instance'><![CDATA[
          <iq type='result'
              id='joap_read_1'
              from='Station@trainset.example.com/Paddington'
              to='Client@example.com'>
            <read xmlns='jabber:iq:joap'>             
              <attribute>
                <name>name</name>
                <value>Paddington Station</value>
              </attribute>
              <attribute>
                <name>size</name>
                <value>
                  <struct>
                    <member>
                      <name>length</name>
                      <value><i4>4</i4></value>
                    </member>
                    <member>
                      <name>width</name>
                      <value><i4>3</i4></value>
                    </member>
                  </struct>
                </value>
              </attribute>
              <attribute>
                <name>previous</name>
                <value>TrackSegment@trainset.example.com/334</value>
              </attribute>
              <attribute>
                <name>next</name>
                <value>TrackSegment@trainset.example.com/271</value>
              </attribute>
            </read>
          </iq>]]>
        </example>
        <p>To read only specified attributes of an instance, the
          client would send this stanza:</p>
        <example caption='Reading Limited Attributes'><![CDATA[
          <iq type='get'
              id='joap_read_2'
              from='Client@example.com'
              to='Train@trainset.example.com/38'>
            <read xmlns='jabber:iq:joap'>
              <name>location</name>
              <name>cars</name>
            </read>
          </iq>
          ]]>
        </example>
        <p>In return, the instance would send this stanza to the
          client:</p>
        <example caption='Limited Attributes'><![CDATA[
          <iq type='result'
              id='joap_read_2'
              from='Train@trainset.example.com/38'
              to='Client@example.com'>
            <read xmlns='jabber:iq:joap'>             
              <attribute>
                <name>location</name>
                <value>Station@trainset.example.com/Paddington</value>
              </attribute>
              <attribute>
                <name>cars</name>
                <value>
                  <array>
                    <data>
                      <value>Engine@trainset.example.com/14</value>
                      <value>PassengerCar@trainset.example.com/112</value>
                      <value>PassengerCar@trainset.example.com/309</value>
                      <value>BoxCar@trainset.example.com/212</value>
                      <value>Caboose@trainset.example.com/9</value>
                    </data>
                  </array>    
                </value>
              </attribute>
            </read>
          </iq>]]></example>
      </section3>
    </section2>
    <section2 topic='&lt;add&gt;'>
      <p>The &lt;add&gt; verb is used to create a new instance of a
        JOAP class. The verb is sent to the JOAP class, which returns
        the address of the newly-created instance.</p>
      <p>Within each &lt;add&gt; verb the client must include
        attribute values for each required, writable attribute of the
        class.</p>
      <p>The IQ is of type "set".</p>
      <section3 topic='Error Codes'>
        <p>The following are some common error codes may be generated
          in response to an &lt;add&gt; verb.</p>
        <ul>
          <li><strong>404 (Not Found)</strong>: The class for which an
            instance is to be instantiated does not exists.</li>
          <li><strong>403 (Forbidden)</strong>: The user is not
            authorized to instantiate an instance of this class.</li>
          <li><strong>405 (Not Allowed)</strong>: The client sent an
            &lt;add&gt; verb to something that isn't a class.</li>
          <li><strong>406 (Not Acceptable)</strong>: The client sent an
            &lt;add&gt; verb containing attributes that are not
            writable, or without all required, writable attributes,
            or with attributes that are not defined for the class, or
            with attribute values that are of the wrong type.</li>
        </ul>
      </section3>
      <section3 topic='Examples'>
        <p>To create a new PassengerCar, the client would send the
          following stanza to the PassengerCar class:</p>
        <example caption='Adding a New Instance'><![CDATA[
          <iq type='set'
              id='joap_add_1'
              to='PassengerCar@trainset.example.com'
              from='Client@example.com'>
            <add xmlns='jabber:iq:joap'>
              <attribute>
                <name>passengers</name>
                <value><i4>38</i4></value>
              </attribute>
            </add>
          </iq>]]></example>
        <p>The class would return the following response:</p>
        <example caption='A New Instance'><![CDATA[
          <iq type='result'
              id='joap_add_1'
              from='PassengerCar@trainset.example.com'
              to='Client@example.com'>
            <add xmlns='jabber:iq:joap'>
              <newAddress>PassengerCar@trainset.example.com/866</newAddress>
            </add>
          </iq>
          ]]></example>
        <p>Note that the class created a new instance identifier, 866,
          for the new instance. Further communications from the client
          would use the full instance address returned.</p>
      </section3>
    </section2>
    <section2 topic='&lt;edit&gt;'>
      <p>The &lt;edit&gt; verb is used to update the attributes of an
        object. The name and new value of each attribute that is to be
        updated is listed in the &lt;edit&gt; verb.</p>
      <p>The IQ is of type "set".</p>
      <p>Leaving a given attribute out of an &lt;edit&gt; verb does
        not indicate that the attribute should be set to an undefined
        or default value. The new values of attributes that are left
        out is implementation-dependent; in general, though, they
        should remain unchanged, if possible.</p>
      <section3 topic='Content in &lt;edit&gt; Results'>
        <p>If the results of an &lt;edit&gt; verb have content, it
          will contain the new address of the instance that was
          updated. The new address should be used henceforth by the
          client. <note>This is to allow updates that alter the unique
            key or attribute of an instance that determine its instance
            identifier.</note></p>
      </section3>
      <section3 topic='Error Codes'>
        <p>The following error codes may be generated in response to a
          &lt;edit&gt; verb.</p>
        <ul>
          <li><strong>404 (Not Found)</strong>: The object to be
            edited does not exists.</li>
          <li><strong>403 (Forbidden)</strong>: The user is not
            authorized to edit this object, or to change one of the
            attributes specified in the &lt;edit&gt; request.</li>
          <li><strong>406 (Not Acceptable)</strong>: The client sent
            an &lt;edit&gt; verb containing attributes that are not
            defined for the class, or with attribute values that are
            of the wrong type, or with attribute values that are
            outside the range for the attribute.</li>
        </ul>
      </section3>
      <section3 topic='Examples'>
        <p>To change the number of passengers in a PassengerCar, the
          client would send the following stanza to the instance:</p>
        <example caption='Editing an Instance'><![CDATA[
          <iq type='set'
              id='joap_edit_1'
              from='Client@example.com'
              to='PassengerCar@trainset.example.com/199'>
            <edit xmlns='jabber:iq:joap'>
              <attribute>
                <name>passengers</name>
                <value><i4>31</i4></value>
              </attribute>
            </edit>
          </iq>
          ]]></example>
        <p>The client would return the following stanza:</p>
        <example caption='Results of Editing an Instance'><![CDATA[
          <iq type='result'
              id='joap_edit_1'
              to='Client@example.com'
              from='PassengerCar@trainset.example.com/199'>
            <edit xmlns='jabber:iq:joap' />
          </iq>
          ]]></example>
        <p>If a client wanted to change the name of a Building, it
          would send the following stanza to the instance:</p>
        <example caption='Editing an Instance'><![CDATA[
          <iq type='set'
              id='joap_edit_2'
              from='Client@example.com'
              to='Building@trainset.example.com/JonesFamilyHome'>
            <edit xmlns='jabber:iq:joap'>
              <attribute>
                <name>name</name>
                <value>Smith Family Home</value>
              </attribute>
            </edit>
          </iq>
          ]]></example>
        <p>The results would be as follows:</p>
        <example caption='Results of Editing an Instance'><![CDATA[
          <iq type='result'
              id='joap_edit_2'
              to='Client@example.com'
              from='Building@trainset.example.com/JonesFamilyHome'>
            <edit xmlns='jabber:iq:joap'>
              <newAddress>Building@trainset.example.com/SmithFamilyHome</newAddress>
            </edit>
          </iq>
          ]]></example>
        <p>Note that the instance indentifier, and thus the instance
          address, of the instance has changed. The <tt>from</tt> part
          of the IQ, however, contains the old address.</p>
      </section3>
    </section2>
    <section2 topic='&lt;delete&gt;'>
      <p>The &lt;delete&gt; verb is used to delete an instance. The IQ
        is of type "set". The &lt;delete&gt; stanza has no
        sub-elements.</p>
      <p>Only instances can be deleted. Classes and object servers
        cannot be deleted. After an instance is deleted, it is no
        longer addressable.</p>
      <p>A given user may not be able to delete a particular
        instance.</p>
      <section3 topic='Error Codes'>
        <p>The following error codes may be generated in response to a
          &lt;delete&gt; verb.</p>
        <ul>
          <li><strong>404 (Not Found)</strong>: The instance to be
            deleted does not exists.</li>
          <li><strong>403 (Forbidden)</strong>: The user is not
            authorized to delete this instance.</li>
          <li><strong>405 (Not Allowed)</strong>: The client sent a
            &lt;delete&gt; verb to an object server or class.</li>
        </ul>
      </section3>
      <section3 topic='Examples'>
        <p>To delete an instance, a client would send the following
          stanza:</p>
        <example caption='Deleting an Instance'><![CDATA[
          <iq type='set'
              id='joap_delete_1'
              from='Client@example.com'
              to='Building@trainset.example.com/Courthouse'>
            <delete xmlns='jabber:iq:joap' />
          </iq>]]></example>
        <p>The instance would return this stanza:</p>
        <example caption='A Deleted Instance'><![CDATA[
          <iq type='result'
              id='joap_delete_1'
              to='Client@example.com'
              from='Building@trainset.example.com/Courthouse'>
            <delete xmlns='jabber:iq:joap' />
          </iq>]]></example>
        <p>If the user is not authorized to delete the instance, it
          would return this error:</p>
        <example caption='Error on Unauthorized Deletion'><![CDATA[
          <iq type='error'
              id='joap_delete_1'
              to='Client@example.com'
              from='Building@trainset.example.com/Courthouse'>
            <delete xmlns='jabber:iq:joap' />
            <error code='403'>
              You are not authorized to delete this instance.
            </error>
          </iq>]]></example>
      </section3>
    </section2>
    <section2 topic='&lt;search&gt;'>
      <p>The &lt;search&gt; verb allows rudimentary searching and
        listing of instances in a class. The IQ is of type
        "get".</p>
      <p>The client sends a &lt;search&gt; verb to the class,
        specifying the attributes that are search criteria and values
        to search for. The class returns a list of the addresses of
        matching instances.</p>
      <p>Multiple attributes are logically AND'd; that
        is, resulting instances must match <em>all</em> of the
        attribute values.</p>
      <section3 topic='Value Matching'>
        <p>How attribute values are specified for matching depends on
          the type of the attribute.</p>
        <ul>
          <li>For numeric types (&lt;int&gt;, &lt;double&gt;),
            &lt;boolean&gt;, and &lt;dateTime.iso8601&gt;, values match
            if they are exactly equal.</li>
          <li>For &lt;string&gt; types, a search value matches an
            attribute value if it is a case-dependent substring of
            that value. For example, "hat" will match
            "hat", "that", and "real-time
            chat server".</li>
          <li>For the &lt;base64&gt; type, a search value matches
            an attribute value if the base64-decoded value of the
            search value is an 8-bit clean substring of the
            base64-decoded attribute value. For example,
            "aGF0Cg==" ("hat") will match
            "cmVhbC10aW1lIGNoYXQK" ("real-time
            chat").</li>
          <li>For instance addresses, a search value matches an
            attribute value if they are exactly equal.</li>
          <li>For &lt;struct&gt; types, a search value matches an
            attribute value if each of its named members matches the
            corresponding named members in the attribute value, and
            has the same type.</li>
          <li>For &lt;array&gt; types, a search value matches an
            attribute value if each of its members matches the
            corresponding members in the attribute value, in order,
            and has the same type.</li>
        </ul>
      </section3>
      <section3 topic='Instances of Subclasses'>
        <p>Classes should return all instances of the class that are
          on the same object server (that is, which have the same
          domain identifier in their address) and that match the search
          criteria. This includes instances of subclasses of the
          class.</p>
        <p>Whether a class returns instances of subclasses that reside
          on other object servers is implementation-dependent.
          <note>This caveat is to allow different types of subclassing
            policies. Classes that define a well-known, standard
            interface -- for example, a class defined by a standards
            organization -- would probably not be "aware" of
            all instances of that class. However, it is conceivable to
            have a multi-tier business application where the object
            servers did know about other servers, their classes, and
            their instances.</note></p>
        <p>Classes cannot be searched on attributes that are defined
          only in subclasses; for example, a search for the attribute
          "contents" sent to the
          <tt>Car@trainset.example.com</tt> class should result in a
          <tt>406 (Not Acceptable)</tt> error.</p>
      </section3>
      <section3 topic='Empty &lt;search&gt;'>
        <p>The semantics of an empty &lt;search&gt; verb is to request
          <em>all</em> instances of a class. This provides a listing
          or browsing functionality.</p>
      </section3>
      <section3 topic='Error Codes'>
        <p>The following error codes may be generated in response to a
          &lt;search&gt; verb.</p>
        <ul>
          <li><strong>404 (Not Found)</strong>: The class to be
            searched does not exists.</li>
          <li><strong>403 (Forbidden)</strong>: The user is not
            authorized to search this class.</li>
          <li><strong>405 (Not Allowed)</strong>: The client sent a
            &lt;search&gt; verb to an object server or instance.</li>
          <li><strong>406 (Not Acceptable)</strong>: The client sent
            an &lt;search&gt; verb containing attributes that are not
            defined for the class, or with attribute values that are
            of the wrong type.</li>
        </ul>
      </section3>
      <section3 topic='Examples'>
        <p>To search for Boxcar instances carrying coal, the client
          would send the following stanza to the Boxcar class:</p>
        <example caption='Searching for Instances'><![CDATA[
          <iq type='get'
              id='joap_search_1'
              from='Client@example.com'
              to='Boxcar@trainset.example.com'>
            <search xmlns='jabber:iq:joap'>
              <attribute>
                <name>contents</name>
                <value><string>coal</string></value>
              </attribute>
            </search>
          </iq>]]></example>
        <p>The Boxcar class would return a list of all matching
          instances:</p>
        <example caption='Search Results'><![CDATA[
          <iq type='result'
              id='joap_search_1'
              from='Boxcar@trainset.example.com'
              to='Client@example.com'>
            <search xmlns='jabber:iq:joap'>
              <item>Boxcar@trainset.example.com/195</item>
              <item>Boxcar@trainset.example.com/35</item>
              <item>Boxcar@trainset.example.com/681</item>
            </search>
          </iq>]]></example>
        <p>To get a list of all Building instances, the client would
          send an empty &lt;search&gt; verb, as follows:</p>
        <example caption='Listing All Instances of a Class'><![CDATA[
          <iq type='get'
              id='joap_search_2'
              from='Client@example.com'
              to='Building@trainset.example.com'>
            <search xmlns='jabber:iq:joap' />
          </iq>]]></example>
        <p>The Building class would return the following stanza:</p>
        <example caption='List Results'><![CDATA[
          <iq type='result'
              id='joap_search_2'
              from='Building@trainset.example.com'
              to='Client@example.com'>
            <search xmlns='jabber:iq:joap'>
              <item>Building@trainset.example.com/Courthouse</item>
              <item>Station@trainset.example.com/Paddington</item>
              <item>Station@trainset.example.com/GareDeLyon</item>
              <item>Building@trainset.example.com/SmithFamilyHome</item>
            </search>
          </iq>]]></example>
        <p>Note that the class returns instances of subclasses, as
          well as direct instances of the class.</p>
      </section3>
    </section2>
    <section2 topic='Method Calls'>
      <p>Method calls in JOAP are simply XML-RPC calls, as defined in
        XEP-0009.<note>XEP-0009 leaves some open questions as to use
        of widely-defined extensions to the XML-RPC standard, such as
        the &lt;nil&gt; type.</note> To call a method
        on an object, the client simply sends an XML-RPC message to that
        object. Method calls must match the parameters as defined in
        the method definition returned by the &lt;describe&gt;
        verb.</p>
      <p>Method names must be the exact method name as returned by
        &lt;describe&gt;. No class or instance identifier prefix (with
        "." or ":") is used.</p>
      <p>Note, also, that the addressee of the method call, that is,
        the object that defines the method, is not specified as a
        parameter of the method, as it is in some programming
        languages. The addressee of the method is implicit in the
        address to which the method was sent.</p>
      <section3 topic='Examples'>
        <p>To start the event log on the train set server, the client
          would send the following stanza:</p>
        <example caption='Method Call on an Object Server'><![CDATA[
          <iq type='set'
              id='joap_xmlrpc_1'
              from='Client@example.com'
              to='trainset.example.com'>
            <query xmlns='jabber:iq:rpc'>
              <methodCall>
                <methodName>startLogging</methodName>
              </methodCall>
            </query>
          </iq>]]></example>
        <p>The object server would respond with the following results:</p>
        <example caption='Method Call on an Object Server'><![CDATA[
          <iq type='result'
              id='joap_xmlrpc_1'
              to='Client@example.com'
              from='trainset.example.com'>
            <query xmlns='jabber:iq:rpc'>
              <methodResponse>
                <params>
                  <param>
                    <value><boolean>1</boolean></value>
                  </param>
                </params>
              </methodResponse>
            </query>
          </iq>]]></example>
        <p>To retrieve the next available Car tracking number, the
          client would send the following stanza to the Car class:</p>
        <example caption='Method Call on a Class'><![CDATA[
          <iq type='set'
              id='joap_xmlrpc_2'
              from='Client@example.com'
              to='Car@trainset.example.com'>
            <query xmlns='jabber:iq:rpc'>
              <methodCall>
                <methodName>nextTrackingNumber</methodName>
              </methodCall>
            </query>
          </iq>]]></example>
        <p>The class would respond with the following results:</p>
        <example caption='Results of a Class Method Call'><![CDATA[
          <iq type='result'
              id='joap_xmlrpc_2'
              to='Client@example.com'
              from='Car@trainset.example.com'>
            <query xmlns='jabber:iq:rpc'>
              <methodResponse>
                <params>
                  <param>
                    <value><i4>909</i4></value>
                  </param>
                </params>
              </methodResponse>
            </query>
          </iq>]]></example>
        <p>To make a Switch change to a different track segment, the
          client would send the following stanza to the instance:</p>
        <example caption='Method Call on an Instance'><![CDATA[
          <iq type='set'
              id='joap_xmlrpc_3'
              from='Client@example.com'
              to='Switch@trainset.example.com/981'>
            <query xmlns='jabber:iq:rpc'>
              <methodCall>
                <methodName>switchTo</methodName>
                <params>
                  <param>
                    <value>TrackSegment@trainset.example.com/119</value>
                  </param>
                </params>
              </methodCall>
            </query>
          </iq>]]></example>
        <p>The instance would respond with the following results:</p>
        <example caption='Results of an Instance Method Call'><![CDATA[
          <iq type='result'
              id='joap_xmlrpc_3'
              from='Switch@trainset.example.com/981'
              to='Client@example.com'>
            <query xmlns='jabber:iq:rpc'>
              <methodResponse>
                <params>
                  <param>
                    <value><boolean>1</boolean></value>
                  </param>
                </params>
              </methodResponse>
            </query>
          </iq>]]></example>
      </section3>
    </section2>
  </section1>
  <section1 topic='Potential Applications'>
    <section2 topic='Application Server'>
      <p>A simple application server can be provided using JOAP. This is merely
        the degenerate case of an object server that provides only methods and
        attributes, with no classes.</p>
    </section2>
    <section2 topic='Relational Database Interface'>
      <p>A more complex example would be an interface to a relational database
        server, such as Oracle, PostgreSQL, or mySQL. The object server would
        represent a single database within the database server. Each table in
        the database would be represented by a class with no class attributes
        or methods. Each row in the database would be an instance of its
        table's class, with attributes but no methods.</p>
    </section2>
    <section2 topic='N-Tier Application'>
      <p>A distributed n-tier application can be built fairly directly
        with JOAP. N-tier applications are usually defined as having
        three main segments:</p>
      <ul>
        <li>A user-interface segment</li>
        <li>A business-object segment, defining objects with business
          rules encoded into their behavior</li>
        <li>A data-storage segment, handling basic storage of
          relatively unintelligent objects</li>
      </ul>
      <p>With JOAP, application developers can create the last two
        segments with a JOAP interface. User-interface clients can use
        JOAP to access and manipulate the business objects in a
        business object server. In turn, the business objects can use
        JOAP to manipulate underlying database objects in the data
        storage layer (perhaps implemented using a relational database
        interface, as defined above).</p>
    </section2>
    <section2 topic='Jabber Component Controller'>
      <p>Jabber protocols typically define a base set of functionality
        for a component to provide. Implementers often want to provide
        specialized, fine-grained control of the component that is not
        part of the core functionality of a component. For example,
        the implementer may wish to allow administrators to get
        metrics on a component, enable or review logs, note error
        situations, or configure the component remotely.<note>Most
        Jabber components currently define Web interfaces, or
        command-line scripts, to perform this kind of
        control.</note></p>
      <p>A component can provide an additional JOAP interface, along
        with its regular protocol-specific interface, to enable this
        kind of control functionality. Implementers can in this way
        provide implementation-specific functionality in an open
        way.</p>
      <p>For example, if <tt>conference.example.com</tt> is
        a MUC component, <tt>control.conference.example.com</tt> might
        be a JOAP component with access to the internal data
        structures of the MUC component. A conference room addressed
        in the MUC component as
        <tt>ModelTrains@conference.example.com</tt> might be addressed
        in the JOAP component as
        <tt>Room@control.conference.example.com/ModelTrains</tt>.</p>
    </section2>
    <section2 topic='Distributed Object System Gateway'>
      <p>There are a number of existing distributed object systems,
        such as SOAP, CORBA, distributed OLE, Enterprise Java Beans,
        etc.</p>
      <p>It would be reasonable to create gateways for these object
        systems or object servers implementing their protocols using
        JOAP. JOAP could also be used to allow disparate object
        systems to communicate through a common protocol.</p>
    </section2>
  </section1>
  <section1 topic='Implementation Notes'>
    <p>To follow.</p>
  </section1>
  <section1 topic='Security Considerations'>
    <p>This section describes some security considerations for
      implementers of JOAP.</p>
    <section2 topic='Authentication'>
      <p>No provision is made for authentication of users to the
        object server. Jabber users authenticate to a login server
        before they are able to send any Jabber stanzas.</p>
    </section2>
    <section2 topic='Authorization'>
      <p>Authorization for users to access and manipulate objects and
        attributes in JOAP is fine-grained; object servers can return
        error codes to indicate a lack of authorization for any given
        attribute, object, or method.</p>
      <p>No provision is made to define a user's authorization for an
        object, attribute, or method. Implicit authorization is
        outlined with the results of the &lt;describe&gt; verb.</p>
      <ul>
        <li>For attributes, if a user is unauthorized to &lt;read&gt; the
          attribute, the object server should not return a definition of
          the attribute in the &lt;describe&gt; results.</li>
        <li>If a user is unauthorized to &lt;edit&gt; an attribute, the
          object server should note that the the attribute is not
          writable in the &lt;describe&gt; results.</li>
        <li>If a user is unauthorized to execute a method, the object
          server should not return a definition of the attribute in
          the &lt;describe&gt; results.</li>
        <li>For classes that the user is not allowed to access at all,
          the object server should not return a reference to that
          class in the &lt;describe&gt; results for the object
          server.</li>
        <li>For instances that the user is not allowed to access at
          all, the object server should not return references to that
          instance in &lt;search&gt; results.</li>
      </ul>
    </section2>
    <section2 topic='Privacy and Confidentiality'>
      <p>No provision is made in the JOAP protocol for providing
        privacy and confidentiality in JOAP conversations. This is
        left up to existing, more general Jabber protocols and
        extensions.</p>
      <p>Confidentiality from external, non-Jabber observers can be
        obtained using transport-layer security (TLS) in all legs of
        the Jabber path -- from client to server to (potentially)
        another server to the object server component.</p>
      <p>Maintaining confidentiality against observers in the Jabber
        pathway -- for example, servers relaying JOAP stanzas --
        requires using end-to-end encryption.</p>
      <p>Due to the nature of the JOAP addressing scheme, however,
        perfect confidentiality cannot be preserved. Even if the
        contents of an IQ packet are encrypted, the address of the
        object the packet is sent to -- e.g.,
        <tt>Tips@whistleblower.example.org/NuclearRegulatoryInfractions</tt>
        -- will reveal some information about the JOAP
        conversation which could be harmful to the user.</p>
    </section2>
  </section1>
  <section1 topic='IANA Considerations'>
    <p>This document requires no interaction with the IANA.</p>
  </section1>
  <section1 topic='XMPP Registrar Considerations'>
    <p>This protocol defines one new namespace, 'jabber:iq:joap'.</p>
    <p>Experimental implementations of this protocol should use the
    namespace '<tt>http://www.xmpp.org/extensions/xep-0075.html#0.3</tt>' to
    avoid conflicts with future versions.</p>
  </section1>
  <section1 topic='Future Considerations'>
    <ul>
      <li>The presence mechanism provided by Jabber is currently not
        integrated into the JOAP protocol. It would be relatively
        easy to incorporate an observation mechanism, based on
        presence, that would enable JOAP clients to request state
        updates on JOAP instances. It is currently an open question
        as to whether this would be genuinely useful, or merely a
        "gee-whiz" add-on that needlessly complicates
        implementation of the JOAP protocol.</li>
      <li>More sophisticated distributed object protocol mechanisms,
        such as transactions, are not addressed in this
        specification.</li>
      <li>The JOAP addressing mechanism restricts the type of Jabber
        entity that can act as an object server. A Jabber instant
        messaging client, for example, is normally addressed with
        username and resource, such as
        'AJabberUser@example.com/Home'. This address has no place
        to hang the class and instance identifiers to allow
        JOAP interactions.</li>
      <li>Additionally, the JOAP addressing mechanism inhibits
        letting components that already use the node and resource
        identifier parts of their addresses, such as multi-user
        chat (MUC) services. This restriction can be worked around by
        providing corresponding JOAP components for existing Jabber
        components.</li>
      <li>An additional type for object references may be called
        for.</li>
      <li>The addition of the widely used &lt;nil&gt; XML-RPC data
        type may be called for.</li>
      <li>Searching is fairly rudimentary; a full boolean logic (and,
        or, not) may be necessary to provide rich searching.</li>
      <li>There is no indication in class descriptions of the
        inheritance hierarchy of the class, or which superclass's
        implementation of a method the class may use. This is an
        implementation decision, and it seems counterproductive to
        force any given inheritance method on the implementer.</li>
      <li>Some functionality will be expensive using JOAP. An example
        might be finding all Cars in a Train and adding them to
        another Train. In our example, this would require one IQ to
        get the train's cars, and N many other IQs to update each
        Car. A batching mechanism -- being able to send multiple
        updates in one IQ message -- would make this somewhat less
        chatty. However, this functionality may be better addressed by
        a global Jabber batching mechanism, rather than
        special-purpose batching just for JOAP.</li>
      <li>A more appropriate term than &lt;verb&gt; may be necessary
        to describe the different types of IQs that make up the JOAP
        namespace.</li>
      <li>The names of the verbs come from the so-called basic READ
        data-manipulation functions: read, edit, add, delete. A more
        SQL-oriented set of names might be SELECT, UPDATE, INSERT,
        DELETE. A set of names from C-related object languages would
        be get, set, new, destroy or delete.</li>
      <li>No mechanism is provided to describe the range of an
        attribute. For example, if an attribute can only be an integer
        less than 10, or a string in the set {'open', 'closed'}, there
        is no allowance for describing this in JOAP.</li>
      <li>Currently, <tt>struct</tt> and <tt>array</tt> attributes and
        parameters are described just with the "struct" and
        "array" types, and no definition is given of their
        parts' types, names, or semantics. It may be valuable to
        expand JOAP type descriptions to also describe the members of
        a <tt>struct</tt> or <tt>array</tt>.</li>
    </ul>
  </section1>
  <section1 topic='Appendix A: Glossary'>
    <p>The following glossary collects some definitions of terms used
      in this document.</p>
    <dl>
      <di><dt>Object services</dt><dd>Modelling an object or
        collection of objects, and providing an interface to
        manipulate those objects to other entities.</dd></di>
      <di><dt>Object server</dt><dd>A Jabber component that
        provides object services.</dd></di>
      <di><dt>Class</dt><dd>A category of object
        instances that defines their structure and interface.</dd></di>
      <di><dt>Instance</dt><dd>A collection of
        data with identity (address), state (attributes), and
        behavior (methods).</dd></di>
      <di><dt>Attribute</dt><dd>A unit of state that makes up
        part of an object server, instance, or class.</dd></di>
      <di><dt>Method</dt><dd>A unit of behavior.</dd></di>
      <di><dt>Object</dt><dd>An object server, class, or
        instance.</dd></di>
      <di><dt>User</dt><dd>A person or process that accesses
        object services through JOAP.</dd></di>
      <di><dt>Client</dt><dd>The software or agent a user
        employs to access object services through JOAP.</dd></di>
      <di><dt>Instance address</dt><dd>The full JID of an
        instance, e.g.,
        <tt>Train@trainset.example.com/OrangeBlossomSpecial</tt>.</dd></di>
      <di><dt>Instance identifier</dt><dd>The resource
        identifier part of an instance address. For example, in
        <tt>Train@trainset.example.com/OrangeBlossomSpecial</tt>, the
        instance identifier is <tt>OrangeBlossomSpecial</tt>.</dd></di>
      <di><dt>Class address</dt><dd>The full JID of a class,
        e.g., <tt>Switch@trainset.example.com</tt>.</dd></di>
      <di><dt>Class identifier</dt><dd>The node identifier part
        of a class address. For example, in
        <tt>Switch@trainset.example.com</tt>, the class identifier is
        <tt>Switch</tt></dd></di>
      <di><dt>Authentication</dt><dd>The act of determining
        that a user is who they say they are. In the Jabber world, this
        is done at login time.</dd></di>
      <di><dt>Authorization</dt><dd>The act of determining
        whether a given user has the right to execute a particular
        action.</dd></di>
    </dl>
  </section1>
  <section1 topic='Appendix B: JOAP XML Schema'>
    <p>The following is an XML Schema for JOAP.</p>
    <code><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>
<schema xmlns='http://www.w3.org/2001/XMLSchema'
  xmlns:joap='jabber:iq:joap'
  targetNamespace='jabber:iq:joap'
  elementFormDefault='qualified'
  attributeFormDefault='unqualified'>
  <element name='describe'>
    <complexType>
      <choice>
        <group ref='joap:DescribeRequest' />
        <group ref='joap:DescribeResponse' />
      </choice>
    </complexType>
  </element>
  <element name='read'>
    <complexType>
      <choice>
        <group ref='joap:ReadRequest' />
        <group ref='joap:ReadResponse' />
      </choice>
    </complexType>
  </element>
  <element name='edit'>
    <complexType>
      <choice>
        <group ref='joap:EditRequest' />
        <group ref='joap:EditResponse' />
      </choice>
    </complexType>
  </element>
  <element name='add'>
    <complexType>
      <choice>
        <group ref='joap:AddRequest' />
        <group ref='joap:AddResponse' />
      </choice>
    </complexType>
  </element>
  <element name='delete'>
    <complexType>
      <choice>
        <group ref='joap:DeleteRequest' />
        <group ref='joap:DeleteResponse' />
      </choice>
    </complexType>
  </element>
  <element name='search'>
    <complexType>
      <choice>
        <group ref='joap:SearchRequest' />
        <group ref='joap:SearchResponse' />
      </choice>
    </complexType>
  </element>
  <group name='DescribeRequest'>
    <sequence /> <!-- empty -->
  </group>
  <group name='DescribeResponse'>
    <sequence>
      <element name='desc' type='joap:Description'
        minOccurs='0' maxOccurs='unbounded' />
      <element name='attributeDescription' type='joap:AttributeDescription'
        minOccurs='0' maxOccurs='unbounded' />
      <element name='methodDescription' type='joap:MethodDescription'
        minOccurs='0' maxOccurs='unbounded' />
      <choice>
        <element name='superclass' type='joap:ClassAddress'
          minOccurs='0' maxOccurs='unbounded' />
        <element name='class' type='joap:ClassAddress'
          minOccurs='0' maxOccurs='unbounded' />
      </choice>
      <element name='timestamp' type='joap:Timestamp'
        minOccurs='0' maxOccurs='1' />
    </sequence>
  </group>
  <group name='ReadRequest'>
    <sequence>
      <element name='name' type='joap:JOAPName'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
  </group>
  <group name='ReadResponse'>
    <sequence>
      <element name='attribute' type='joap:Attribute'
        minOccurs='0' maxOccurs='unbounded' />
      <element name='timestamp' type='joap:Timestamp'
        minOccurs='0' maxOccurs='1' />
    </sequence>
  </group>
  <group name='EditRequest'>
    <sequence>
      <element name='attribute' type='joap:Attribute'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
  </group>
  <group name='EditResponse'>
    <sequence>
      <element name='newAddress' type='joap:InstanceAddress'
        minOccurs='0' maxOccurs='1' />
    </sequence>
  </group>
  <group name='AddRequest'>
    <sequence>
      <element name='attribute' type='joap:Attribute'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
  </group>
  <group name='AddResponse'>
    <sequence>
      <element name='newAddress' type='joap:InstanceAddress'
        minOccurs='1' maxOccurs='1' />
    </sequence>
  </group>
  <group name='DeleteRequest'>
    <sequence /> <!-- empty -->
  </group>
  <group name='DeleteResponse'>
    <sequence /> <!-- empty -->
  </group>
  <group name='SearchRequest'>
    <sequence>
      <element name='attribute' type='joap:Attribute'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
  </group>
  <group name='SearchResponse'>
    <sequence>
      <element name='item' type='joap:InstanceAddress'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
  </group>
  <complexType name='Attribute'>
    <sequence>
        <element name='name' type='joap:JOAPName'
        minOccurs='1' maxOccurs='1' />
        <element name='value' type='joap:JOAPValue'
        minOccurs='1' maxOccurs='1' />
    </sequence>
  </complexType>
  <complexType name='AttributeDescription'>
    <!-- XXX: enforce name rules -->
    <sequence>
      <element name='name' type='joap:JOAPName'
        minOccurs='1' maxOccurs='1' />
      <element name='type' type='joap:JOAPType'
        minOccurs='1' maxOccurs='1' />
      <element name='desc' type='joap:Description'
        minOccurs='0' maxOccurs='unbounded' />
    </sequence>
    <attribute name='required' type='boolean' default='false' />
    <attribute name='writable' type='boolean' default='false' />
    <attribute name='allocation' type='joap:Allocation' />
  </complexType>
  <complexType name='MethodDescription'>
    <sequence>
      <element name='name' type='joap:JOAPName'
        minOccurs='1' maxOccurs='1' />
      <element name='returnType' type='joap:JOAPType'
        minOccurs='1' maxOccurs='1'/>
      <element name='params' minOccurs='0' maxOccurs='1'>
        <complexType>
          <sequence>
            <element name='param' minOccurs='0' maxOccurs='unbounded' >
              <complexType>
                <sequence>
                  <element name='name' type='joap:JOAPName'
                    minOccurs='1' maxOccurs='1' />
                  <element name='type' type='joap:JOAPType'
                    minOccurs='1' maxOccurs='1' />
                  <element name='desc' type='joap:Description'
                    minOccurs='0' maxOccurs='unbounded' />
                </sequence>                  
              </complexType>
            </element>
          </sequence>
        </complexType>
      </element>
      <element name='desc' type='joap:Description'
         minOccurs='0' maxOccurs='unbounded' />
    </sequence>
    <attribute name='allocation' type='joap:Allocation' />
  </complexType>
  <simpleType name='ClassAddress'>
    <restriction base='string'>
      <pattern value='[^@]+@[a-zA-Z0-9\.]+' />
    </restriction>
  </simpleType>
  <simpleType name='InstanceAddress'>
    <restriction base='string'>
      <pattern value='[^@]+@[a-zA-Z0-9\.]+/.+' />
    </restriction>
  </simpleType>
  <simpleType name='XMLRPCType'>
    <restriction base='string'>
      <enumeration value='int' />
      <enumeration value='i4' />
      <enumeration value='double' />
      <enumeration value='boolean' />
      <enumeration value='string' />
      <enumeration value='array' />
      <enumeration value='struct' />
    </restriction>
  </simpleType>
  <simpleType name='JOAPType'>
    <union memberTypes='ClassAddress XMLRPCType' />
  </simpleType>
  <simpleType name='JOAPName'>
    <restriction base='string'>
      <pattern value='[a-zA-Z_][0-9a-zA-Z_]*'/>
    </restriction>
  </simpleType>
  <simpleType name='Boolean'>
    <restriction base='unsignedByte'>
      <enumeration value='0' />
      <enumeration value='1' />
    </restriction>
  </simpleType>
  <!-- FIXME: figure out how to do this without using mixed
  content, which allows stuff we don't want. -->
  <complexType name='JOAPValue' mixed='true'>
    <choice minOccurs='0'>
      <element name='i4' type='integer' />
      <element name='int' type='integer' />
      <element name='boolean' type='joap:Boolean' />
      <element name='string' type='string' />
      <element name='double' type='decimal' />
      <element name='datetime.iso8601' type='dateTime' />
      <element name='base64' type='base64Binary' />
      <element name='struct' type='joap:XMLRPCStruct' />
      <element name='array' type='joap:XMLRPCArray' />
    </choice>
  </complexType>
  <complexType name='XMLRPCStruct'>
    <sequence>
      <element name='member' minOccurs='1' maxOccurs='unbounded'>
        <complexType>
          <sequence>
            <element name='name' minOccurs='1' maxOccurs='1'
              type='string' />
           <element name='value' minOccurs='1' maxOccurs='1'
              type='joap:JOAPValue' />
          </sequence>
        </complexType>
      </element>
    </sequence>
  </complexType>
  <complexType name='XMLRPCArray'>
    <sequence>
      <element name='data' minOccurs='1' maxOccurs='1'>
        <complexType>
          <sequence>
           <element name='value' type='joap:JOAPValue'
              minOccurs='0' maxOccurs='unbounded' />
          </sequence>
        </complexType>
      </element>
    </sequence>
  </complexType>
  <simpleType name='Allocation'>
    <restriction base='string'>
      <enumeration value='instance' />
      <enumeration value='class' />
    </restriction>
  </simpleType>
  <simpleType name='Description'>
    <restriction base='string' />
  </simpleType>
  <simpleType name='Timestamp'>
    <restriction base='dateTime' />
  </simpleType>
</schema>
]]>
    </code>
  </section1>
  <section1 topic='Appendix C: JOAP DTD'>
    <p>The following is a document-type description (DTD) for JOAP.</p>
    <code><![CDATA[
<!ELEMENT name (#PCDATA)>
<!ELEMENT type (#PCDATA)>
<!ELEMENT desc (#PCDATA)>
<!ATTLIST desc
          xml:lang NMTOKEN #IMPLIED>
<!ELEMENT i4 (#PCDATA)>
<!ELEMENT int (#PCDATA)>
<!ELEMENT string (#PCDATA)>
<!ELEMENT double (#PCDATA)>
<!ELEMENT datetime.iso8601 (#PCDATA)>
<!ELEMENT base64 (#PCDATA)>
<!ELEMENT class (#PCDATA)>
<!ELEMENT superclass (#PCDATA)>
<!ELEMENT item (#PCDATA)>
<!ELEMENT returnType (#PCDATA)>
<!ELEMENT member (name, value)>
<!ELEMENT struct (member+)>
<!ELEMENT data (value+)>
<!ELEMENT array (data)>
<!ELEMENT value (#PCDATA|i4|int|string|double|datetime.iso8601|base64|struct|array)*>
<!ELEMENT attribute (name, value)>
<!ELEMENT timestamp (#PCDATA)>
<!ELEMENT newAddress (#PCDATA)>
<!ELEMENT attributeDescription (name, type, desc*)>
<!ATTLIST attributeDescription
          required (true|false|0|1) "false"
          writable (true|false|0|1) "false"
          allocation (class|instance) "instance">
<!ELEMENT params (param+)>
<!ELEMENT param (name, type, desc*)>
<!ELEMENT methodDescription (name, returnType, params?, desc*)>
<!ATTLIST methodDescription
          allocation (class|instance) "instance">
<!ELEMENT describe (desc*, attributeDescription*, methodDescription*,
  (class*|superclass*), timestamp?)>
<!ELEMENT read (name*|(attribute*, timestamp?))>
<!ELEMENT edit (attribute*|newAddress?)>
<!ELEMENT add (attribute*|newAddress)>
<!ELEMENT search (attribute*|item*)>
<!ELEMENT delete EMPTY>
]]>
    </code>
  </section1>
  <section1 topic='Appendix D: Objects in Extended Example'>
    <p>Because JOAP requires some significant examples to define the
      protocol, an example domain was developed to provide
      consistency. Readers familiar with UML may find the following diagram
      useful to illustrate some of the fine points of JOAP listed
      above.</p>
    <example caption='Object Diagram'><![CDATA[
      Train:
      number: i4
      name: string
      location: TrackSegment
      cars: Car[]
      void forward()
      void back()
      void insertCar(Car car, Car before)

      Car:
      trackingNumber: i4
      i4 nextTrackingNumber() {class}

      Caboose: Car

      Engine: Car
      canPull: i4

      Boxcar: Car
      contents: string

      PassengerCar: Car
      passengers: i4
      
      Building:
      name: string
      size: struct (length: i4, width: i4)

      TrackSegment:
      previous: TrackSegment
      next: TrackSegment

      Switch: 
      in: TrackSegment
      out: TrackSegment[]
      boolean switchTo(TrackSegment)

      Station: TrackSegment, Building           
      ]]></example>
  </section1>
</xep>