Uncoordinated senders using sequential Message Ids [Was: Detailedproposal to allow Batching of Acks and RM faults using ResponseReply pattern.]

[Tony Graham <Tony.Graham@Sun.COM>]

Tom Rutt <tom@coastin.com> wrote at Fri, 20 Feb 2004 22:44:23 -0500:
> Sunil Kunisetty wrote:
> >Tom Rutt wrote:
...
> >>NOTE: In cases where the underlying transport connection is not
> >>available to return an Acknolwedgement or RM fault, a Receiving RMP MAY
> >>include that Acknowledgement or RM fault with the response to a future
> >>reliable message request with the same GroupID.
> >>“
> >
> > -1.
> >
> > As I mentioned in one of the con. call, 2 different nodes can share the
> > same groupId and be part of the same group. In such case, there is no
> > guarantee that subsequent "acks" will go back to the original and destined
> > Sender.
> >
> This is a difficult scenario to swallow. How could they coordinate the 
> sequential use of sequnce numberin if they are
> not in close coordination for the group?

Sunil's is a reasonable scenario.

Consider a servlet for which the servlet container can instantiate
multiple instances to handle a heavy request load.

To make it a bit more concrete, consider a servlet in the Coffee Break
Server shown at
http://java.sun.com/webservices/docs/1.3/tutorial/doc/CB2.html#wp107729
that uses WS-Reliability when communicating with the Coffee
Distributor Web Service.

If, under heavy load, there are multiple instances of a servlet, they
all still look like one servlet from outside the servlet container,
and they all each behave as if they were the only instance of the
servlet.  Only the servlet container knows how many instances there
are at any one time.

Each servlet instance would use the same Group Id on its sent reliable
messages and would use successive sequence numbers for successive
messages without overlap or gaps in the sequence, but would not
necessarily know what to do with an acknowledgment for any other
message.

This is especially true if the servlet blocks on a Request/Response
operation.  That instance of the servlet isn't going to do anything,
including respond to another instance receiving its Acknowledgment,
until it gets its response.

Consider also the implementation scenario that Sunil has used before
of an RMP implemented as a message handler in a handler chain.
Getting a pool of message handlers to create MessageHeader elements
with the same Group Id and successive sequence numbers is easy enough,
but getting one message handler to communicate Acknowledgments to
another doesn't fit the message handler lifecycle.

Regards,


Tony Graham
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