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Subject: T2, Proposed solution for ... Re: SyncReply and ReliableMessagingMethod in QualityOfServiceInfo
Arvola/David F I have some ideas for a few changes that might solve the problems raised in this thread so the rest of this email contains: 1. Extracts from a number of recent emails from you both and analysis of the issues/requirements they raise 2. Some additional further analysis of the issues to identify a few more requirements 3. A proposed solution to meeting the identified requirements which hopefully will solve the problem ... and who knows perhaps bring this email thread to an end :-) So please let me know if: a) I've missed a requirement b) You (or anyone else) think the proposed solutiuon is wrong or could be improved ... but then I know you will anyway ;) Details below David ****************************** EMAIL THREAD and ISSUES/REQUIREMENTS ==================================== Arvola Chan: Thu 8/9/2001 3:28 PM --------------------------------- >>I am of the opinion that the DeliveryReceipt element should be used by the ToParty MSH to inform the FromParty MSH that a reliable message has been received. ... It is the non-arrival of a DeliveryReceipt that requries the MSH to notify the application. In this case, it must rely on persistent information to determine the application service that must be notified.<< >>I think end to end acknowledgement is always needed to support reliable messaging, so it is unnecessary to explicitly set deliveryReceiptRequested to true, it should always be implied by deliverySemantics of OnceAndOnlyOnce. On the other hand, if intermediary acks are optional, then ackRequested needs to be an explicit attribute at the QOS level and it should apply to all intermediaries.<< [DavidB]This highlights a number of issues/requirements: Req 1. A positive Ack sent back to the sender of a message by the ultimate destination is the only sure way the sender can be certain the message was delivered. If no Delivery Receipt is sent, then the sender cannot be so certain. Req 2. If end-to-end Delivery Receipts are always required for the sender to be certain, then it would be simpler if there was a rule that deliverySemantics of OnceAndOnlyOnce implies a Delivery Receipt is sent. The only remaining issue is whether or not the Delivery Receipt should be signed. David Fischer: Thu 8/9/2001 7:31 PM ----------------------------------- >>Question 1 ... the end-to-end MUST be able to do RM as if it does not know (which it probably doesn't) what the IM(s) are doing. The end-to-end probably doesn't care if the IM(s) are doing RM at all (even though each IM might care very much)<< >>On the issue of Sender time outs and retries, their are two kinds: 1) a timeout to the first IM and 2) a timeout getting to the end. The first is easy and obvious so we don't need to discuss it. The second is a timeframe that is usually contractually guaranteed ...<< >>While the Sender/Receiver may not have any idea what path or IM transport the message is taking (and they really don't need to) they must have an idea about delivery times to the end. We MUST generate some kind of an end-to-end Ack allowing the ends to do RM.<< >>Proposal: 1) We need to update section 10 with end-to-end RM (deliveryReceipt or something new that is similar to Acknowledgement). 2) We need to put in the spec somewhere that ALL MessageHeaders (including Via) MUST be passed to the next hop, including the end.<< >>Question 2 ... I agree that it doesn't matter if ackRequested gets changed because an Ack gets sent based upon DeliverySemantics (This was my second solution to Question 2). Why then do we have ackRequested? The only way it would change is if there was some kind of local CPA overriding ackRequested. If RM is requested and an IM can do RM then it MUST, right? Then why have this parameter? (I see from your comments that you are considering this.) << [DavidB]This highlights a number of issues/requirements: Req 3. The sender should not know nor care if an IM is being used and whether or not they are doing RM with the next IM. Req 4. There are two different types of "acks" that are useful: a) It's been accepted by the postal system (i.e. the next MSH has received, this is the Acknowledgement Message) b) It's been received by the final destination (i.e. the To Party has received it, this is the Delivery Receipt) Req 5. Even if the initial Ack (i.e. Acknowledgment Message) is received there needs to be some method of automated retry if the Delivery Receipt is not received within the expected timeframe. Req 6. If you a doing reliable messaging between two hops, then you do not need ackReqeusted as an acknowledgment must be sent if the ebXML RM protocol is being sent and not needed if it isn't. David Fischer: Thu 8/9/2001 8:02 PM ----------------------------------- >>I am concerned that end-to-end RM is taking a back seat to IM RM. This is the opposite of how it should be. Most transactions will be single-hop. Many other cases will be single IM where the sender or receiver may not even know there is an IM so it still appears to be single-hop. The ends should not even have to know about the IMs. Ends will always do automatic retries. RM should work for the ends in exactly the same manner whether or not there are IMs.<< [DavidB]This really just provides further support for issues/requirements numbered 3 and 5 above. Arvola Chan: Fri 8/10/2001 12:23 AM ----------------------------------- >>Even if you have a channel that calls for the use of synchronous reply mode, the syncReply attribute still has to be set. In other words, it is still necessary to use the Via element if the syncReply attribute is present only there, but this constradicts the assumption that the Via element is only used when intermediaries are involved.<< [DavidB]This highlights a number of issues/requirements: Req 7. The syncReply needs to be set at the message level whether or not an intermediary is being used Req 8. There is a contradiction in the spec (which therefore needs to be removed) that the Via element is only for intermediaries when it is actually also needed for non-intermediaries. DAVID BURDETT's COMMENTS ======================== Before proposing a resolution to all these requirements I'd like to make a few comments and identify an additional couple of requirements. Firstly on Requirement 6 above (you don't need ackRequested). There could be benefit in gettingan acknowledgement element back even if you are using a reliable messaging protocol such as MQ Series as you then have evidence (especially if it is signed) that the next MSH has received the message. I'm not convinced though that this is a huge benefit. Secondly if we put syncReply at the message level then there is an additional requirement ... Req 9. The next recipient of a message needs to know whether or not to return an acknowledgement message synchronously or not. Now for the proposed solution. PROPOSED SOLUTION ================= The solution dsecribed below refers to the requirements identified above ... Change 1 -------- Summary: Rename the Via element as "Next Actor Data" or similar Rationale: There can always be "intermediaries" in a message transfer even if you are going point-to-point. For example consider the two example message flows that I recently posted (also posted here) that cover the following use cases: 1. A genuine intermediary who is a third party that is running a MSH and forwards messages to the final destination. 2. A Party which has a MSH that acts as a "mailroom" that forwards the message internally using ebXML RM to another MSH that then forwards it to the application. The "mailroom" MSH ia an intermediary. I think we need to support both use cases. By renaming the Via element as "Next Actor Data" we are simply saying that the data contained within the element is for the Next Actor **only** and does not need to be forwarded. The Next Actor recreates the data as they need to. If we think of the data in the Via as being for the "Next Actor" then we are more closely aligned with SOAP. It also removes the problem of treating intermediaries as "something special" and allows an internal MSH to forward the message to another MSH without the original sender needing to know and without having to re-create the complete message. This change addresses Req 3 and 8. Change 2 -------- Summary: Data in the Next Actor Data (Via) element is for the Next Actor only Rationale: What Change 1 means is that we must carefully review the existing elements in the header and check to see whether they are needed by the ultimate destination/endpoint or the next actor. I think that this will require the following changes: 1. CPAId. The CPAId in the Message Header identifies parameters that apply "end-to-end", e.g. business process level stuff, whereas the CPAId in the NAD/Via element applies to the transport over a single hop, e.g. transport level stuff. I realise this will require changes to the CPP/A ... and probably more discussion. 2. Acknowledgment Element. This should be part of the NAD/Via element as acknowledgments are between two MSHs and are not propogated to the original sending party. This change addresses Req 3, 4a Change 3 -------- Summary: Make the return of a Delivery Receipt required if deliverySemantics=OnceAndOnlyOnce Change 4 -------- Summary: Replace deliveryReceiptRequested by deliverReceiptSigned=true or false(the default) Rationale: As the return of a Delivery Receipt is the only sure way that you know a message was delivered suggests that it will be a simpler solution if we make this always the case. Therefore we can make the return of a delivery required if the deliverySemantics are OnceAndOnlyOnce. However you still need to know if the receipt must be signed. These changes address Reqs 1, 2, 4b Change 5 -------- Summary: Make the return of an Acknowledgment element in a message required if the ebXML RM protocol is being used Change 6 -------- Summary: Replace ackRequested by ackSigned=true or false(the default) Rationale: The rationale for doing this is similar to changes 3 and 4. It simply gives the rule that if you are doing ebXML RM then you must include an Acknowledgment element in the response. The response can be synchronous or asynchronous (see change 9 below). Change 7 -------- Summary: Include automated retry by the original sender (From Party) if no Delivery Receipt is returned Change 8 -------- Summary: Include "ResendOfMessageId" element in the Message Header Rationale: There is a need for automated retry by the original sender (from party) if a Delivery Receipt is not received. However, the original sender cannot send the **identical** message as it will be treated as a duplicate and therefore ignored by any intermediate MSH that has previously received it. To solve this problem the from party needs to use a different MessageId. However there is now a need to stop the message being treated as a completely new message. To solve this problem we could add a "ResendOfMessageId" element that identifies which message the new message is a resend of. In this case even if the resend is received first and the original appears some time later, the ToParty will be able to recognize that the message has already been processed and therefore the original should be ignored. This logic needs to be included in section 10 and probably needs a bit more thinking through. These changes addresses Req 5 Change 9 -------- Summary: Include syncReply at both the Message Header and the NAD/Via elements The To Party needs to know whether the From Party wants the Delivery Receipt and Business Payload assembled into one message.The next MSH needs to know whether to send back an Acknowledgment synchronously or wait for the Business Payload before sending it. The Delivery Receipt and Business Payload can be sent asynchronously and the Acknowledgment sent synchronously and vice versa as they are independent of each other. As you can't easily cover both requirements in a single element, they need to be included separately in the header and in the via. This change adresses Reqs 7 and 9. Product Manager, xCBL, XML Standards Solution Strategy, Commerce One 4400 Rosewood Drive, Pleasanton, CA 94588, USA Tel/VMail: +1 (925) 520 4422; Cell: +1 (925) 216 7704 mailto:david.burdett@commerceone.com; Web: http://www.commerceone.com
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