W3C home > Mailing lists > Public > www-ws-arch@w3.org > December 2002

RE: Different Levels of Reliable Messaging - Intermediaries

From: Ricky Ho <riho@cisco.com>
Date: Sat, 14 Dec 2002 00:12:46 -0800
Message-Id: <4.3.2.7.2.20021213235643.026c8e48@franklin.cisco.com>
To: "Burdett, David" <david.burdett@commerceone.com>, www-ws-arch@w3.org
David, can you verify my understanding of what you mean by Intermediary ACK 
and End-to-end ACK ?

Lets say A sends message M to B (an intermediary), which forwards M to 2 
destinations C and D.
"IACK" stands for intermediary acknowledgement, "EACK" stands for 
end-to-end acknowledgement

The following will describe the sequence of message flow.

1) A --(M)--> B
2) A <--(IACK)-- B
3) B --(M)--> C;  B--(M)--> D
4) B <--(IACK)-- C;  B <--(IACK)-- D
5) B <--(EACK)-- C;  B <--(EACK)-- D
6) A <--(EACK)-- B

At step (2), A get an "IACK" from B that indicates B has received the 
message M.
At step (6), A get an "EACK" from B indicates ALL final destinations (A may 
not know who they are) have received the message M.

Is this what you mean ?

Best regards,
Ricky

At 09:47 PM 12/12/2002 -0800, Burdett, David wrote:

>Ugo.
>
>One idea you suggested was that you could do end-to-end reliable messaging 
>if each of the hops used reliable messaging. Although this is technically 
>true, where it falls down is that the sender of the original message might 
>not always *know* that there are multiple hops as perhaps the path being 
>followed is determined dynamically.Even if they know there are multiple 
>hops they might not know if each hop is reliable. So, although your 
>suggestion can work in some cases, it won't always.
>
>END-TO-END ACKS ARE THE ONLY GUARANTEE
>--------------------------------------
>So to be *sure* that a message has been delivered the Acknowledgement 
>Message MUST come from the ultimate receiver of the message. However 
>identifying the true "ultimate receiver" is not always obvious as you can 
>argue that it should be the (first?) SOAP node at the destination or 
>perhaps the application running behind it - this isssue is discussed later.
>
>However doing end-to-end acks over multiple hops using different transport 
>protocols can be problematical because of the different speeds of the 
>transport protocol could be used in each hop.
>
>DIFFERENT SPEEDS OF TRANSPORT PROTOCOLS
>---------------------------------------
>If you are doing a single hop then, since you can know the *speed* of the 
>transport protocol being used from beginning to end, you can estimate how 
>long it "should" be before the acknowledgement arrives. Then, if the 
>response takes some time longer than this then you can resend the original 
>message following the Level 1 Simple Reliable Messaging Approach described 
>in my earlier email.
>
>As said earlier, if you have multiple hops then this doesn't always work 
>since each hop could involve a transport protocol that works at a 
>different speed. For example the first hop could be HTTP and the next 
>SMTP. What makes it worse, is that the sender of the original message 
>might not even know that there is another hop and that it is going over SMTP.
>
>CALCULATING THE TIMEOUT
>-----------------------
>The simplest way around this problem is to just do an end-to-end ack BUT 
>base the value of the "timeout" to use on when the message expires as 
>indicated by the "expires at". So, for example, if the original message 
>had to be delivered within 1 hour, you might want to do a retry if the 
>acknowledgement message had not been received within 10 minutes giving 6 
>attempts to deliver the message before "giving up".
>
>If you think about, the "expires at" HAS to be set to a value that allows 
>sufficient time for the original message to be sent and the 
>acknowledgement to be received for the message to be sent with any success 
>at all. So this approach is pretty safe.
>
>EARLY DETECTION OF NODE FAILURE
>-------------------------------
>However, what this approach does not do is provide early warning that an 
>intermediate node has failed. For example if you are sending the message 
>by HTTP, then you will not know, using the timeout value described 
>earlier, that transportation of the message was impossible until you had 
>exhausted all the 6 retries.
>
>IF (note the capitals) this is a problem then the solution is to use 
>intermediate acknowledgement messages in addition to the end-to-end ack 
>where the intermediate ack is targeted (e.g. using the SOAP Actor) at the 
>"Next" node in the network.
>
>INTERMEDIATE ACKS
>-----------------
>Since the intermediate ack is targeted at the next node, you usually can 
>use a timeout value that is based on the speed of the transport protocol. 
>For example, if you are using HTTP, then you might want to retry after 1 
>minute. If you are using SMTP, then maybe every 30 minutes. Then work out 
>a reasonable number of retries to do before "giving up" say 3 or 4.
>
>If you do this then you would know, again given the timeout values used 
>above, that the message could not be sent after a much shorter time, e.g. 
>3-4 minutes for HTTP.
>
>However, the complexity does not end there. It's not much use if an 
>intermediate node has detected that it cannot send the original message 
>any further. The sender of the original message needs to know. This means 
>tha the intermediate node that detected the delivery failure HAS to send 
>another message back to the sender of the original message to notify them 
>of the failure.
>
>So what intermediate acks really provide is an optimisation that provides 
>much earlier warning of delivery failure - albeit at a fairly large 
>increase in complexity.
>
>Also remember that if End-to-End acks are used as well as the intermediate 
>acks then, if Level 3 - Reliable Messaging with Recovery, is used, it is 
>quite possible that it could cause another retry at sending the message 
>some time later.
>
>OTHER FACTORS TO CONSIDER
>-------------------------
>However there are some issues with using intermediate acks which you need 
>to think about:
>1. You can get two acks not one - you can now receive more than one ack, 
>the intermediate ack and the end-to-end ack which adds to the complexity
>
>2. The Intermediate Ack *should* also be targeted at the next node in the 
>network otherwise the sender of the original node would get an ack back 
>from each and every intermediate node in the path
>
>3. What do you do if the delivery failure message does not get through? 
>Should you send the delivery failure message reliably ... and what do you 
>do if that doesn't work either.
>
>4. If the nodes in the network that are being targeted are SOAP nodes, 
>then there might be other, non SOAP systems between the SOAP nodes that 
>use different transport protocols at different speeds to connect. This 
>makes calculating the timeout value for intermediate acks harder to do 
>reliably.
>
>ULTIMATE RECEIVER
>-----------------
>The other issue with end-to-end acks is knowing what actually is the end. 
>If the last SOAP node, for example, in the path actually consumes the 
>payload/body of the SOAP message then there isn't much of a problem. But 
>if it does not, then the message could be lost after the SOAP node has 
>passed on the message to the application that actually processes it. This 
>is why the Acknowledgement Message described in the original email in this 
>thread can optionally provide additional information on the validation and 
>passing off of the message to the application.
>
>BOTTOM LINE
>-----------
>I think that doing end-to-end acks where the timeout is based on the 
>expiry time of the message is the best approach to use. Doing intermediate 
>acks, only provides early warning of a problem, it isn't a substitute for 
>end-to-end acks. So I would recommend that intermediate acks are left out 
>of scope ... if this ever becomes an activity.
>
>Thoughts?
>
>David
>
>
>
>-----Original Message-----
>From: Ugo Corda [<mailto:UCorda@SeeBeyond.com>mailto:UCorda@SeeBeyond.com]
>Sent: Thursday, December 12, 2002 5:56 PM
>To: Burdett, David; www-ws-arch@w3.org
>Subject: RE: Different Levels of Reliable Messaging
>
>David,
>
>I am glad you mention the case of intermediaries, because I have been 
>thinking for a while about how that affects Reliable Messaging.
>
>In the case of security, what is usually said in order to justify the need 
>to address it at the SOAP headers level, instead of just using HTTPS, is 
>that you need that level if you want to do end-to-end security. Otherwise 
>intermediate nodes would need to have access to security sensitive 
>information related to the message in order to relay information from hop 
>to hop, and those nodes might not be authorized to do that.
>
>But in the case of reliable messaging, it seems that you should be able to 
>use, for example, SOAP over HTTPR on one hop, and SOAP over JMS on the 
>next hop, and still be able to support reliable messaging end-to-end. (The 
>message goes reliably from A to C iff it goes reliably from A to B and 
>from B to C - for example, B waits until it gets the transport-level ack 
>from C before sending its transport-level ack to A).
>
>In fact, I think this was the rationale when IBM designed HTTPR, so that 
>you could go from Internet to intranet (and vice versa), using SOAP over 
>HTTPR on the Internet, and then switching to SOAP over MQSeries (or other 
>MOM) once inside the intranet.
>
>Your previous message seems to imply that this approach would not be 
>sufficient for end-to-end reliable messaging. Could you please elaborate?
>
>Thank you,
>Ugo
Received on Saturday, 14 December 2002 03:13:16 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Tuesday, 3 July 2007 12:25:11 GMT