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Subject: RE: [ebxml-msg] Semantics of AS4 Receipt
Comments inline.
Pim
From: Moberg Dale [mailto:dmoberg@axway.com] Sent: 31 January 2012 16:51 To: Pim van der Eijk; ebxml-msg@lists.oasis-open.org Subject: RE: [ebxml-msg] Semantics of AS4 Receipt Hi
Pim I
have been responding to what I thought had been some questions about the
semantics of NRR or of simple receipt. That does not involve product feature
details. Also there are differences in what can be done when an ack of any sort
is returned on a separate connection than on the same
connection. If you need to say something about delivery, then the original sender’s message has been delivered when the recipient has received it. If you want to say something about what subsequent operations of the delivered message have completed, then that is more a question of disposition, which for AS4 purposes need to be semantically aligned with the concept of “processed”
The ebMS 3.0 Core
specification makes a difference between RM-Deliver and MSH-Deliver and between
receiving a message and delivering it. I'm saying that I think AS4 Receipts should be
MSH-Deliver rather than RM-Deliver, and that MSH-Deliver is closer to (without
perhaps going as far as) EDIINT concepts. Please have a look at the
definitions of RM-Deliver and MSH-Deliver and explain if you think an AS4
receipt should signal RM-Deliver or
MSH-Deliver.
In essence, MSH-Deliver seems to require that
only the first of any subsequent steps (the step of getting the message and its
metadata out of the MSH) succeeds. It does not say anything about further
processing steps. But unlike RM-Deliver it seems to rule out a situation
where an MSH sends an acknowledgment, but then fails in some
other SOAP module above the reliability layer and/or subsequently
fails to transfer responsibility to the first component outside the
MSH. As I mentioned, it is difficult to sum up what to say about that next step after processed. There may not actually be a next step (because the data had been placed on a file system during processing, and the next step would have been to place the data on the filesystem. Since that has been done, it is not necessary to do it again.)
Exactly, writing the
message to a file system is a way of transferring a message out of the MSH,
if that's the selected configuration. Whether any other application
or user actually does anything with that message is not its responsibility and
perhaps even invisible to the MSH. But if
the disk was full, or the partition not mounted, or some other error happened,
MSH-Delivery has failed though RM-Deliver has succeeded. The MSH can
detect this failure.
Or queues. Or another transfer over the same or different protocols, and so on.
Your little pseudo code snippet abstracts over a lot of very different cases. Many involve delays not accompanied by a timeout value. Those would pose problems.
We can impose or recommend that the deliver() operation has a timeout value if you want. The most commonly used delivery mechanisms used by B2B gateway products (write to disk, JMS or other message queues, HTTP posts, FTP) support timeouts. The MSH can itself wrap an arbitrary delivery operation in a timeout and decide how a timeout is to be interpreted. But that would be out of scope for the AS4 protocol. Retry, resend, resubmit options are available. Resend means that while the message had been sent (as indicated by a 2xx status for HTTP if that is involved), no expected ack (NRR or simple MDN) has been received. If the ack was lost, “resending” (which viewed from the sending perspective is a re-deliver) will result in generating another ack, as I think it should.
The point I was making was that
in some
products, the receiving MSH
(not the sending MSH as in your description) has a (re)deliver
option. This does not require the message to be
retried/resent/resubmitted by/to the sending MSH. It is an internal
function in the MSH that allows previously received messages to be delivered
again (e.g. writing once more to a queue, or to file system, or ..). An
AS4 product that associates a Receipt with receive() rather than deliver()
would not generate a new receipt. I mentioned one product that does
generate a new Acknowledgment for redelivered ebMS 2.0
messages.
Backend
failure is possible. NRR does not intend to magically solve difficulties by
removing the possibility of backend failures. It does demarcate a kind of
“responsibility” to some degree. The distinction between
RM-Deliver and MSH-Delivery is a subtle one and indeed relates to where the
boundary of responsibility is placed. But neither RM-Deliver not
MSH-Deliver depend on what happens within backends or enterprise business
processes.
Customizations or implementation choices of product are quite another matter. I had thought the question you were raising was what the standard means or specifies.
You are right that we are
specifying a standard protocol, not implementations. The questions are
about how the Receive(), Acknowledge() and Deliver()
operations that are defined in the ebMS and Web Services
specifications relate to each other.
From: Pim van der Eijk
[mailto:pvde@sonnenglanz.net] Dale, "For example, the
payload might be placed on a queue, but whether the payload was taken off the
queue is an open question as far as the signal is concerned.)" As I am
writing for the third time now, I am just asking for the success of the
"enqueue" operation to be checked. This does not require any new
coordination protocols, it just imposes some ordering in the structure
of your application code. Instead of something
like ....
acknowledge(message) ....
deliver(message) You
would write something like .... if
deliver(message) == Success: .... .... acknowledge(message) .... else: .... .... errormessage(message,'Delivery
Failure') In at
least one Axway product, ebMS 2.0 acknowledgments seem to be triggered by
delivery. I know this because if a system administrator manually
re-delivers a message from this product (which is an option that is useful
in some exceptional situations), an acknowledgment is sent to the trading
partner). So assuming the AS4 implementation will follow
this pattern, your company's product already does what I'm asking
for. Pim From: Moberg Dale
[mailto:dmoberg@axway.com] I
think that AS4 NRR is now roughly the same as AS1-3 NRR. My concern was
that we not proceed beyond secured, acknowledgement of receipt (and, when
security features are OK, an indication that the next processing step was
engaged). I am uncertain about “successful transfer” – the MSH did its step in
“processing,” but whether the next processing step was successful is not
assured. ([Many requests from “end user communities” have emerged over time to
add on various assurances about further processing. I don’t think we should go
there with data exchange protocols. That does not prevent embedding data
exchange protocols in richer coordination protocols that might add on various
notification services. But I would not want to get into these additions within
AS4. I think the additional protocols should be developed as applications of
existing IETF protocols, maybe as an EDIINT-NG or B2B-NG interest group. I don’t
think it should be an OASIS effort. Just my opinion (or using the new catch
phrase), just sayin’] From: Pim van der Eijk
[mailto:pvde@sonnenglanz.net] Thanks
for the response and background information. The proposal I made was
quite modest: to link the sending of an AS4 receipt to the ebMS 3.0 concept of
"MSH-Delivery". If ebMS 3.0 and WS-Reliability optionally support
this concept, it would be inconsistent for AS4 to commit to less.
AS2 (http://www.ietf.org/rfc/rfc4130.txt),
section 7.1, relates an MDN to delivery (my emphasis): "The
signed MDN, when received by the sender of the EDI Interchange, can be used
by the sender [... as] an acknowledgement that the EDI Interchange sent
was delivered and
acknowledged by the receiving trading partner [..]". MSH-Delivery
seems in fact to correspond exactly to what you call successful transfer
to the first "receiving
agent" that
is indeed often "just the first step
of a pipeline".
The defining requirement is that the message has been transferred
successfully out of the AS4 MSH. This is the case for all
the examples in my message. The increase in latency is minimal, the
impact on implementations limited, and fewer false receipts are produced.
The functionality better matches the expectations of end-users and is
supported in commonly used B2B gateway products for other
protocols. Pim From:
ebxml-msg@lists.oasis-open.org [mailto:ebxml-msg@lists.oasis-open.org] On
Behalf Of Moberg Dale The
first observation is that the requirements internet-draft was never published as
an informational RFC. If
it had been, some of the tradeoffs involved in being an “applicability
statement” and in seeking to make AS1, AS2 and AS3
as much alike as possible would have necessitated some revisions to the original
draft. The original draft also included several ideas that were not
implemented. Going
back over all the tradeoffs would be tedious. I think the main one relevant here
is that the MDN was reused as the message type for
receipts. In
an email context, a MDN is more associated with what the a UA does upon seeing
what the final MTA has come up with. So POP3 and IMAP based UA access can also
generate MDNs. EDIINT, however, only makes use of a success code from the MDNs
of “Processed”. Here is where the semantics that got implemented got
defined. RFC
2298 was I think in scope when the EDIINT work started, but I don’t think all
the subsequent “obsoleting or updating” RFCs in this area change the
situation. "processed"
The message has been processed in some manner (i.e., by some sort of rules or
server) without being displayed to the user. The user may or may not see
the message later, or there may not even be a human user associated with the
mailbox. The
“processed” code then indicates the semantics of any kind of
receipt/delivery/disposition on the receiver side. So the overly ambitious
requirement of the draft were aligned with the constraints of reuse, and what
already existed. The reasons for the tradeoff were many and varied. Mainly
however they all reflected the unpredictable latencies between
MTA/MSH/WebServer/FTPserver reading of data and the acquisition by the next
processing step(s). Would it be a POP3 UA or was there an automated pipeline
pushing to a UA feeding a translator,JMS, MFT, ESB, or any of the integration
technologies in use? Or for HTTP, would a servlet, appserver, CGI push to the
next app, and would a success from that app be relevant to count as
“disposition”. The ultimate “backend application” is only in simple
middleware cases just one hop from reception; the receiving agent is just the
first step of a pipeline (where there can be branching or forking process
flows). How is all that to be summed up in one code? In
general the A2A visibility problem remains unsolved for general deployment
situations by any current standard. It will take some significant effort and
cost to make A2A visibility commonplace. AS4 will not be doing it IMO. The
backflow of status notifications down the integration pipeline is just too
varied to smooth over, without considerable thought given to lightweight
notification technologies, with built in subscription, expiration, digest, and
other capabilities. Hello, I was working on an
AS4-related topic over the weekend and have some questions on AS4 non
repudiation receipts. The AS4 profile is
presented as designed to meet EDIINT requirements.
According to section
3.7 of http://tools.ietf.org/id/draft-ietf-ediint-req-09.txt EDIINT
defines the requirements for non repudiation
as: "Receipt by the
sender of a signed receipt, is an implicit According to section
8 of http://docs.oasis-open.org/ebxml-msg/ebms/v3.0/core/ebms_core-3.0-spec.odt the
ebMS 3.0 Core Specification distinguishes between RM-Delivery and
MSH-Delivery. Case 2: The acknowledgment is "on MSH-delivery"
(supported in WS-Reliability). In that case, notification option 1 can be
supported as well as option 2. In order for option 1 to be supported, an RMP
must implement RM-Deliver operation so that it is only considered successful
(worthy of sending an acknowledgment) if the Deliver operation from MSH to
Consumer also succeeds. It is RECOMMENDED that an implementation support this
acknowledgment semantics. In http://www.oasis-open.org/apps/org/workgroup/ebxml-msg/download.php/44888/AS4-profile-v1.0-wd22.odt
section 3.4, the following is stated about AS4: "The Receiving MSH is
taking responsibility for processing this user message. However, no guarantee
can be made that this user message will be ultimately delivered to its Consumer
application (this responsibility lays however now on the Receiver
side)." Now this seems to be
more like the RM-Delivery (only option supported in WS-ReliableMessaging) than
MSH-Delivery ? The EDIINT definition
of "non repudiation of receipt" seems to require MSH-Delivery rather
than RM-Delivery. If we're claiming that AS4 meets EDIINT requirements, is
this enough ? I'm not saying that
AS4 products should support a VAN mailbox interface based on pickup of a
message, and should delay sending a receipt until such a pick-up happened
(possibly hours or days later). Drawbacks of this have been
discussed in the past when we looked at multi-hop reliability models.
But in the AS4
context, it could be defined as: the MSH successfully passed
responsibility for delivering the message to some third entity (system, program,
user). Examples of succesful
or unsuccessful delivery could depend on
implementation: -
Delivery to a mailbox:
Succes: message was added to mailbox.
Failure: quota exceeded, mailbox full.
(No guarantee that any user or system actually processes the
message) -
Delivery to a file system:
Success: the MSH successfully created, wrote data to, and closed
a file representation of the message metadata and contents.
Failure: Permission denied, I/O errors
(There is no guarantee that any user or program actually processes the created
file) -
Delivery to a message queing system
Success: successful enqueue()
Failure: enqueuing failed
(There is no guarentee that any other application dequeue the
item) -
Delivery to another B2B messaging system:
Success: the Submit() operation was invoked
successfully
Failure: Submit() operation failed
(No dependency on any downstream receipts) In the case of
failure, which the AS4 MSH can detect immediately after receiving a
message, I would prefer the AS4 to not send a Receipt but to send an Error
instead. An alternative idea
is discussed in section 8.2.4 of the ebMS 3.0 Core Specification, which has
a discussion on false delivery failures: "False DF - which can
never be completely eliminated - can always be detected outside the reliable
messaging (RM) layer, in a tractable and less urgent way - e.g. the sending
party may synchronize on a daily basis by communicating its list of assumed
delivery failures, for confirmation by receiver. The Status Request feature
(which may be described in a forthcoming Part 2 of the ebMS specification) could
facilitate this function." Pim |
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