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<section version="5.0"
         xml:id="other-miscellaneous-topics-designing-a-client-server-protocol-in-xml"
         xml:lang="en" xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
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         xmlns:ns="http://docbook.org/ns/docbook"
         xmlns:m="http://www.w3.org/1998/Math/MathML">
    <title>Designing a client/server protocol in XML</title>

    <para>Occasionally, people try to define a custom protocol that allows
    multiple XML requests/responses to be sent over a single transport channel.
    This section discusses the non-trivial interaction between XML and
    sockets, and how you can design a protocol correctly.</para>

    <para>XML1.0 requires a conforming parser to read the entire data till end
    of the stream (because a parser needs to handle documents like
    <literal>&lt;root/&gt;&lt;!-- post root comment --&gt;</literal>). As a result,
    a naive attempt to keep one <literal>OutputStream</literal> open and marshal
    objects multiple times fails.</para>

    <example>
        <title>One easy way to work around this limitation is to design your
        protocol so that the data on the wire will look like the
        following:</title>

        <programlisting language="xml"><![CDATA[<conversation>
  <!-- message 1 -->
  <message>
    ...
  </message>

  <!-- message 2 -->
  <message>
    ...
  </message>

  ...
</conversation>]]></programlisting>
    </example>

    <para>The <literal>&lt;conversation&gt;</literal> start tag is sent immediately after the
    socket is opened. This works as a container to send multiple "messages",
    and this is also an excellent opportunity to do the hand-shaking (e.g.,
    <literal>protocol-version='1.0'</literal> attribute.) Once the
    <literal>&lt;conversation&gt;</literal> tag is written, multiple messages can be marshalled
    as a tree into the channel, possibly with a large time lag in between. You
    can use the JAXB marshaller to produce such message. When the sender wants
    to disconnect the channel, it can do so by sending the
    <literal>&lt;/conversation&gt;</literal> end tag, followed by the socket
    disconnection.</para>

    <para>Of course, you can choose any tag names freely, and each message can
    have different tag names.</para>

    <para>The receiver would use the StAX API and use
    <literal>XMLStreamReader</literal> to read this stream. You'd have to use this
    to process the first <literal>&lt;conversation&gt;</literal> start tag. After that, every
    time you call a JAXB unmarshaller, you'll get the next message.</para>

    <para>For the concrete code, see the <literal>xml-channel</literal> example in
    the JAXB RI distribution.</para>
</section>
