Re: Why anonymous may be different (for AcksTo and elsewhere)

David Hull wrote on 02/21/2006 02:15:17 AM:
> 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

To me the last message above is the key thing - I don't believe
C can response back to B1 with A's ack.  When using the anon URI
there has to be some other correlation involved. In the normal
request/response case I believe the correlation is thru the
use of the current connection - in other words, a request is
expecting its response on "this" connection's backchannel.  When
dealing with Acks there are two different correlations that can
be used - the AckRequested header and the Sequence header.  IMO,
Acks can only be sent back on responses using the anon URI _when_
the request message referred to the sequence in question - either
thru the use of the AckRequested header or a Sequence header.
When either of these headers are in the incoming/request message
then the server side (RMDestination) is then free the use the
response flow to piggy-back Acks for the referenced sequence.

Without that kind of correlation you get into the kinds of problems
you're talking about it - and assuming that all clients that use the
anon URI are somehow linked together and can share state (whether it
be RM or WSA state) is not something people should assume.

thanks,
-Doug


> 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.