inline:
From: Sander Fieten
[mailto:]
Sent: Wednesday, December 05, 2007 3:16
PM
To:
Subject: Re:
[ebxml-msg] Reliable messaging and intermediaries
My remarks as just made on the call:
* Why use RM on each leg of the route? I don't see any advantage because a
=
missing ebMS ACK will lead to a re-send of the message on the ebMS level.
(=
Assuming a At-Least-Once contract). At the moment I don't see what the RM
o=
n each leg gives us.
<JD> In my opinion, the ebMS synchronization signals cannot be
expected to provide the same level of QoS as protocol-level RM. I would not for
instance expect a synchronization exchange - back and forth - to occur for each
message sent, or at every second, as the overhead of processing these is quite
significant, certainly higher than sequence-based RM signals. I would not expect
either to implement duplicate check at ebMS level (that would put a significant
burden on endpoint MSHs that lets not forget, should ideally not need implement
more than core V3 features), and rely instead on the one in RM (meaning indeed
that with per-segment RM, if a duplicate is generated during the transfer
through an intermediary, we wouldn't detect it). But at least using
segment-level RM provides some RM QoS, that would take care of 50% of message
losses/duplication. The only "end-to-end" feature that 2-tier reliability (as
previously defined) provides, is message loss awareness and possible remedy. But
the more of these losses are taken care of in protocl-level RM, the
better.
There is no question that 2-tier
reliability as defined here is not as good as "flat" end-to-end RM at
protocol level.
* If considering special reliability
behavior on the intermediaries as is t=
he case with the last one in the
2-tiered solution described by Jacques, co=
uld it be possible to use WS-RM
on each leg with the ACK on the current leg=
postponed till the the
intermediary receives an ACK on the next leg? I did=
n't have time to look
into, just an idea.
<JD> we wish that were possible, but this Ack semantics is not the
one specified in WS-RM. It is very unlikely that RM built-in available SOAP
stacks will allow this...
Regards
Sander
Fieten
----
Sander
Fieten
IT Architect
FIETEN IT
e:
t: +31 6
51507886
On Dec 5, 2007, at 3:16 , Durand, Jacques R. wrote:
So we could call this option (already alluded to by
Ian in a previous
call in November) the 2-tiered reliability, the main
advantage of which
is that it does not require to route anything other than
ebMS messages:
Level 1: WS-RM/R only needs be used between pairs of
MSHs (so each path
segment is "independently reliable"), in other words the
reliability
headers are changing across an intermediary, but not the ebMS
header.
Level 2: additional ebMS signals allow for end-to-end
doublecheck, by
synchronizing the status on both sides. E.g. sender will
periodically
notify a list of "should-have-received" ebMS IDs to
receiver, and a
symmetric synchronization signal sent by receiver MSH,
would notify of
what has NOT been received/delivered. Or conversely the
receiver will
notify a list of "have-received". Remedial action need not be
prescribed
(P-Mode will control) - could range from just notifying
the application
layer on either/both sender or receiver side, to re-issuing
the missing
messages (from the ebMS layer, in order to avoid creating
RM-level
duplicates that could be eliminated at next RM
endpoint).
Some interesting aspects about this solution:
- the
status synchronization is a useful feature in itself as we seem to
agree
(awareness of loss by a receiver MSH allows better diagnostics on
failed
choreographies, etc).
- the ultimate receiving MSH may not have to support
this feature in
order to have end-to-end reliability: the last intermediary
has to.
Indeed, when getting the "should-have-received" signal, the
intermediary
could be required to respond with only those messageIDs that
it has
actually successfully forwarded, removing the last possibility
of
"intermediary loss". E.g. it could be required to get the RM Ack
of
ultimate receiver, before even sending the "have-received" signal.
SO
that mode of reliability does not require an ultimate MSH to
support
more than core V3 features. This is becoming in the CDC network
case,
where MSH hubs serve clusters of endpoint MSHs. Even if last MSH is
not
supporting RM, this enables "end-to-previous-to-end" reliability
!
I am starting to like it.
So there seems to be two general
modes of reliable ebMS multi-hop:
(1) The "flat reliability" mode,
supported end-to-end by the RM layer.
Here, only the use of WS-Reliability
really makes sense, as the routing
is supposed to be controlled by ebMS
header info, even for establishing
RM sequences.
(2) two-tier
reliability, where the RM mechanism from WS protocol is
only used per
segment. Here both WS-Reliability and WS-ReliableMessaging
can be used
between MSH pairs.
Jacques
-----Original
Message-----
From: David RR Webber (XML) [mailto:]
Sent: Wednesday,
November 28, 2007 6:46 AM
To: Durand, Jacques R.
Cc: Pim van der Eijk;
;
Moberg Dale
Subject: RE: [ebxml-msg] Pim on routing
intermediaries,
WS-ReliableMessaging
Jacques,
Yes - I think
the ability to do status checks definately beckons
eventually.
I
like what you suggest - good lightweight logical next step.
During the
BCM work and then ebBP - we identified the need in extended
collaborations
to have a "linking and switching" status tracking
service.
This is
way more than what's needed for message delivery of course - but
then again
- the types of mechanisms could definately be interchangeable
/ reusable
here.
Since right now each ebBP / ebMS is responsible for its own
status
tracking - the obvious next logic step is to provide centralized
status
checking services. We've explored this model in BCM - the
linking and
switching Appendix B spec' - which uses the Prolog
assertion/retraction
predicate based approach.
At some point we will
need to get serious about enabling that all -
would be a very cool facility
- incredibly useful for all kinds of
applications...
;
-)
Cheers, DW
-------- Original Message --------
Subject: RE: [ebxml-msg] Pim on routing
intermediaries,
WS-ReliableMessaging
From: "Durand, Jacques R." <>
Date: Wed, November 28, 2007 9:06 am
To: "David RR Webber (XML)" <>, "Moberg
Dale"
<>
Cc: "Pim van der Eijk" <>,
<>
David:
That sounds like re-implementing reliability at ebMS
level - something
I'd prefer to avoid until all other options fail to
deliver :-)...
Besides guaranteed delivery there is also duplicate
detection (based
on sequence numbers or so for the 2 RM specs), so
the redundancy is
starting to build-up.
Now, we may consider a mode of multi-hop reliability
where RM is
per-segment, and in order to catch the cases of loss
during router
forward, a kind of (ebMS) status request where the
initial sender MSH
could notify the ultimate receiver MSH of the list
of message IDs it
is supposed to have received over the last hour or
so, and the
receiver MSH would notify back on what it has not
been able to
deliver. Re-sending by the ebMS layer itself may or
may not be
mandated - could be left to a re-submit at
application level (if no
risk of duplicates). At least that would add
awareness of both sender
and receiver of message loss
(i.e.
non-delivery), something we do not have today
especially with WS-RM.
This awareness by the receiver would in turn allow
the choreography
layer to better diagnose failures.
Jacques
-----Original Message-----
From: David RR Webber (XML) [mailto:]
Sent: Monday, November 19, 2007 6:42
PM
To: Moberg Dale
Cc: Pim van der Eijk; Durand, Jacques R.;
Subject: RE: [ebxml-msg] Pim on routing
intermediaries,
WS-ReliableMessaging
Dale,
I'm not sold on WS-Reliable messaging - for starters
its merely trying
to mimick what ebMS already has for the past five
years!
Then - who knows what oversights they've made in the
rush to get
something in place. Reading their design and such
does not give me
great confidence. My sense is we are well ahead of
them - and they
should be adopting ebMS reliable messaging (and
actually they are
trying to clone most of it!!!) Of people using
WS the majority could
not careless about reliable delivery - in fact their
whole modus
operandi is built assuming unreliable messaging and
instant traffic
exchanges (query/response).
OK - so where does that leave us?
Why can we not look at some kind of orchestration
agent here? Seems
to me that what is needed is a very simple component
that is tasked
with plugging the gap and acting as an ebMS/SOAP
delivery extension
agent.
All it has to be able to do is manage SOAP-based
exchanges at the
basic transport level. If its built very dumb
and simple - it just
acts as a relay and a pass through - with minimal
logic - mostly just
leveraging SOAP acks and errors
themselves.
The ebMS then interacts with one or more of these
SOAP relay agents -
and will keep trying delivery until it gets an
end-to-end SOAP
acknowledgement relayed back to it.
The agent could be designed to use basic WS - and in
fact then any
SOAP server could be integrated as a delivery
service with the simple
addition of the agent component. This seems
much more like what we
want. Coopting delivery via SOAP
servers.
DW
"The way to be is to do" - Confucius (551-472
B.C.)
-------- Original Message
--------
Subject: [ebxml-msg] Pim on routing
intermediaries,
WS-ReliableMessaging
From: "Moberg Dale" <>
Date: Mon, November 19, 2007 3:32
pm
To: "Pim van der Eijk" <>,
"Durand, Jacques
R."
<>,
<>
[Pim explained the conflict between
WS-ReliableMessaging and the
advantages in using routing
intermediaries.]
The main options for dealing with the conflict
are
0. Use WS-Addressing anonymous for reply-to and
acks-to, and have
all intermediaries wait before their HTTP reply is
made.
[The above does not scale well, is brittle, and
utilizes lots of
resources.]
1. Use piggybacking (and presumably 1 message long
sequences?).
[Still lacks routing advantages. RMSes have to
target RMDs, and so
have high lifecycle configurability burdens.
Routing intermediary
topology cannot be changed while leaving RMS
configuration
unchanged.
Probably destroys advantages of ebMS routing
intermediary, as Pim
explained.]
2. Locate RMD at routing intermediary, and check
security at
intermediary also.
[ Loss of end to end
reliability.]
Pim has convincingly explained how
WS-ReliableMessaging is not very
intermediary friendly and, along with Jacques,
reflects a step
backwards in trying to accommodate intermediaries
with end-to-end
acknowledgments.
However, the goal of WS-* technology level
convergence seems to
require that we support WS-ReliableMessaging
because it is the WS-I
sanctioned solution.
Is option 2 above an option that could be
embellished to be
satisfactory? What would it take? Add on an
ebBP/RN style
ReceiptAcknowledgment or
ReceiptAcknowledgmentException (if we
wanted to be optimistic with respect to
WS-ReliableMessaging
terminated at intermediary working most of the
time?)
Just some reactions. Good analysis
Pim!
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