W3C home > Mailing lists > Public > public-ws-addressing@w3.org > February 2006

Why anonymous may be different (for AcksTo and elsewhere)

From: David Hull <dmh@tibco.com>
Date: Tue, 21 Feb 2006 02:15:17 -0500
To: "public-ws-addressing@w3.org" <public-ws-addressing@w3.org>
Message-id: <43FABE05.5060503@tibco.com>
The more I ponder the problem of anonymous AcksTo, the more I think that
anonymous can't quite be treated as just another EPR address.  I'm
posting this here because I think the situation is not unique to AcksTo
or WS-RX.  Rather, WS-RX happens to have presented the use case that
flushes the issue out.  Also, while I may have got the WS-RX use case
wrong here, I believe the overall point is still of interest to WSA. 
Finally, there are clearly enough WS-RX folks in the audience here that
if this is in fact relevant to WS-RX, I'm sure the news will find its
way there.

Here's the scenario:

Suppose that A and B both want to use a reliable sequence to send
messages to C.  Each uses its own sequence.  As I understand it, this is
the normal case (as opposed to, say, A and B sharing the same
sequence).  Now further suppose that A and B both use anonymous for
AcksTo.  Given that acks for a given message could come back with a
later message, the following could happen:

    * A sends C message A1
    * C sends back a response with no acks
    * B sends C message B1
    * C sends back a response with an ack to message A1

For that matter, if C isn't careful, it could send acks to some
completely unrelated process that happened to be sending to it without
even using WS-RX.

So, in order for this to work, C has to be careful only to use anonymous
AcksTo for replies to request messages that are part of the same
sequence (by the way, how does C ack the last message of such a sequence
if it doesn't know it's the last?).

By contrast, it doesn't seem C would have to be so careful in the case
where the AcksTo EPRs are not anonymous.  Normally (and again I'm not a
WS-RXpert) I would expect that A and B would use separate AcksTo
addresses, and C would just send acks to the addresses it was told to
use.  Even if A and B used the same AcksTo address and used the sequence
number on the acks to sort out whose acks they were, they could do so
without any help from C because the acks would at least always end up at
the same destination.

Another way to look at this is that using the same AcksTo for two
different sequences ties the two sequences together and whoever's
getting the acks has to know about both sequences.  This would apply
equally to anonymous and non-anonymous.  I believe this handles most of
the problem, but not all.  First, this would require every sender that
wants to use anonymous AcksTo with a given destination to know about
every other one.  Second, by itself it doesn't deal with senders that
aren't even using WS-RX.  I'm not sure the second problem is serious,
but the first one seems like it might be.

It also seems worth considering the case where A uses anonymous and B
doesn't.  If B is expecting acks at non-anonymous D, then it wouldn't
normally be checking for acks in its responses, and neither could it be
expected to know what to do with A's acks if it did notice them.  C can
send B's acks freely to D, but it has to be careful to send A's acks
back only on responses to messages from A.

Even if there turn out to be no serious issues here for WS-RX, it seems
worth noting that trying to extend anonymous beyond the context of a
single request-response message exchange effectively ties together all
senders to a given destination into one virtual receiver for messages to
anonymous.  That is, a message sent to anonymous in such a scenario
could arrive back at /any/ sender.
Received on Tuesday, 21 February 2006 07:15:24 UTC

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