Proposal for messaging over connectionless protocols.

(email, http(s), ftp, wap etc.)

For messaging in the internet somtimes CORBA may be
too rigid (firewalls and other reasons).

It should be possible, for example to tunnel firewalls
with http, send messages with emails or using ftp.

So this proposal specifies a xmlBlaster message format
to be used with those protocols.

Here is an example, how it could be typed into an email
body and send to xmlBlaster. I will discuss this with
xml directly, since DTD (xmlBlaster.dtd) is not expressive enough
and XML Shemas (xmlBlaster.xsd) are not yet supported.


<?xml version="1.0"?>
<xmlBlaster method="publish" sessionId="12aa3z45X" xmlns="http://www.xmlBlaster.org">

   <key oid="myMessageId" contentType="text/plain">
      <!-- Some user specified meta information for this content
           which is queryable with XPATH -->
   </key>

   <content link="" xmlns="">
      <!<CDATA[ Hello world ]]>
   </content>

   <qos>
      <!-- Some quality of service tags, to control xmlBlaster -->
      <persistent />
   </qos>
</xmlBlaster>



Here a discussion about this approach

1. The 'xmlBlaster' root element encapsulates the xmlBlaster message.

   The attribute 'method' tells xmlBlaster which method to invoke:
   one of the CORBA IDL specified methods like 'publish' 'subscribe' etc.

   The attribute 'sessionId' is the identifier of the sender,
   it was retrieved by a login with a password message (see below)
   Another variant could always send the 'loginName' and 'passwd' attibutes
   with each message.

   The 'xmlns' specifies the xmlBlaster.org namespace as default, so we
   don't need to prefix each tag explicitly.

2. The 'key' element you know already from the xmlBlaster CORBA invocations.
   It may contain some arbitrary user defined meta data about the
   content (not shown here).

   Here it specifies the unique message identifier with the 'oid' attribute.

   The 'contentMime' attribute sets the MIME format of the message content.

3. The 'content' element supplies the message content in the MIME
   format as specified in the attribute 'contentMime' of the key tag.
   Here it is plain text "text/plain", which is protected by a CDATA section.
   Not that the token "]]>" in the plain text would make the message unparseable.

   The xmlns="" attribute resets the namespace to nothing,
   it may be replaced by a message specific namespace if there is one.

   The link="" attribute allows to specify a location where the content
   is delivered. For example a binary content could be in the attachment
   of an email. The empty string tells us that the content is directly
   embeded.

   Note that for example a 'subscribe' method invocation does not need
   to specifiy the content.

4. The 'qos' element you know already from the xmlBlaster CORBA invocations.
   It allows a fine grained control of the xmlBlaster MOM behaviour.


Open questions with this apporach:

a. How can we specifiy to send BLOB (binary data) contents with
   http (how to set the link attribute)?
   Use some sort of multipart request?

b. Is it possible to express this approach exactly with the coming
   XML Schema language?

c. How to do a login?
   What about this approach (only interesting tags shown):

   <xmlBlaster method="login" loginName="ben" passwd="sunshine"
               callbackProtocol="EMAIL" callback="xmlBlaster@marcelruff.info">
      <qos>
      </qos>
   </xmlBlaster>

   This message could respond with the 'sessionId' for a synchronous http
   invocation (allowing to recognize the client again on following
   invocations), but it would not be very smart to use with asynchronous emails.
   With emails sending loginName and passwd with each invocation
   may be more convenient.

   The 'callback' attribute allows to specify where xmlBlaster shall
   send its asynchronous callback messages (updates).
   It may be an email address, an URL to a HttpServlet, a CORBA IOR
   or whatever protocol xmlBlaster will support in future.

   Note that the sender and the callback protocol may be different.

Please comment on this, since there are many out there who know
more about the different technologies.

More informations on RPC and XML you find on our homepage under
'Internet Resources'.

If i find time at the end of the week,
i will implement a little demo with http,
allowing xmlBlaster messages to be tunneled through firewalls.

I believe SUN's XmlRpcServlet may be a nice foundation to
implement it very quickly, see
    http://developer.sun.java.com/developer/products/xml/examples/rpc

cu,

Marcel
xmlBlaster@marcelruff.info