Re: R: Issue 6692 (re-send for the list)


On 26 Mar 2009, at 23:06, Gilbert Pilz wrote:

> Taking this logic a step further, it seems that the only reason WS- 
> Eventing might need a "Mode" switch is if the Subscriber wanted the  
> Notifications transmitted via some mechanism that was not  
> representable by an EPR; in other words a mechanism that fell  
> outside the boundary of "web services" (to use that term loosely).  
> Since such a mechanism would probably be out of scope for the WS-RA  
> WG, it seems that Dave's quest for a use-case that justifies the  
> need for a Mode switch is doomed to failure (which is not to say you  
> can't support such cases through the use of existing extension  
> mechanisms - only that WS-Eventing can't say anything about them  
> other than they may exist).
> - gp
> Doug Davis wrote:
>> In thinking about this discussion I'd like to compare this to a  
>> hypothetical situation...
>> Let's say that when WS-Addressing was first created, instead of a  
>> single wsa:ReplyTo EPR/Header the authors created two of them - one  
>> for async responses and one for sync responses.  After all, they  
>> are very different message exchange patterns - in one case you need  
>> a real (addressable) URI, in the other you don't, you'll always use  
>> the current transport's back-channel.  So, let's say we had this:
>> <wsa:AsyncResponse>
>>   <wsa:Address> xs:anyURI </wsa:Address>
>>   <wsa:Ref-Params...>
>>   ...
>> </wsa:AsyncResponse>
>> and
>> <wsa:SyncResponse>
>>   ...
>> </wsa:SyncResponse>
>> If we had this situation and then someone suggested that we combine  
>> them I could hear a lot of the same comments being made.  We're  
>> overloading too many things. How can I tell what's going on?  The  
>> semantics of these two things are so different, you can't possibly  
>> combine them.  The application must know how the messages are being  
>> sent.
>> And yet we do have these two very different message transport  
>> mechanisms combined into one.  The concept of an EPR handles both  
>> of these situations very well.  It even deals with other messaging  
>> patterns.  For example, in WSDD we see it dealing with  
>> broadcasting.  Composing in with MC we allow for polling-style  
>> delivery.  Making the uri something like "java:myClassName" (in  
>> some soap stacks) will invoke local services and never hit the  
>> wire.  And there are probably others.  The point in this is that  
>> the EPR was designed to convey the notion of not just where to send  
>> a message but how to send a message.  And this single entity  
>> manages to do this well for all other WS-* specs and for all types  
>> of messages being sent (including WS-Notification).  This entity  
>> allows for the application (assuming it wants to) to remain totally  
>> unaware of the details of the target service- all it needs to do is  
>> pass around an EPR and some lower-level piece of code (the WS- 
>> Addressing layer) will automagically deal with the details of how  
>> to actually interact with the target service.
>> So, I still come back to the same questions I had on this week's  
>> call:
>> - why is WS-Eventing so special that it can't reuse the EPR as is?   
>> Why does it need a "mode" attribute when no one else seems to?
>> - what is fundamentally different between NotifyTo and ReplyTo ?   
>> Any message exchange pattern that I would want to do with one of  
>> them I can imagine a case for it being used in the other.
>> Wu,
>>   And I'd ask you to look at the AcksTo EPR in WS-ReliableMessaging  
>> [1].  It is not an EPR where "Responses" go. Its meant to be used  
>> for new (one-way) messages being sent by a service - basically the  
>> same thing as PushMode+NotifyTo.  And in that specification that  
>> had to deal with sync, async, addressable and  non-addressable  
>> destinations - the same way WS-Eventing does.  They managed to  
>> solve it by reusing the EPR concept - as is.
>> [1]
>> thanks
>> -Doug
>> ______________________________________________________
>> STSM |  Standards Architect  |  IBM Software Group
>> (919) 254-6905  |  IBM 444-6905  |
>> The more I'm around some people, the more I like my dog.
>> Bob Freund <>
>> Sent by:
>> 03/25/2009 05:55 PM
>> To
>> "Chou, Wu (Wu)" <>
>> cc
>> Subject
>> Re: R: Issue 6692 (re-send for the list)
>> Wu,
>> Yes, I am guessing that you meant 6432, no worries.
>> Here is some of what I think might be blocking a consensus on this  
>> issue.  There may be more, but this is my guess.
>> 1)
>> It is my understanding that in order to receive a notification, one  
>> must subscribe.
>> That would make all notifications solicited, doesn't it?  Are you  
>> anticipating out-of-band communication of EPRs to which the  
>> Subscription Manager might send notifications?
>> 2)
>> The definition of Push Mode
>> "A delivery mechanism where the source sends event messages to the  
>> sink as individual, unsolicited, asynchronous SOAP messages."
>> There are two issues with this definition I think.
>> a) unsolicited: well, see above, there exists a subscription, so  
>> the notification have been solicited.
>> b) Asynchronous:  Asynchronous with respect to what?  The  
>> underlying protocol back channel? Other activity? It is a very  
>> difficult term and needs MUCH clarification before it has meaning.
>> So when the above (in the minds of many who have been through the  
>> WS-Addressing debates and other protocols based on WS-Addressing,  
>> the definition of push mode becomes:
>> "A delivery mechanism where the source sends event messages to the  
>> sink as individual, SOAP messages."
>> Now this might be contrasted with a hypothetical "Pull Mode"  
>> definition, which is not defined, but I will make an attempt:
>> Pull Mode
>> "A delivery mechanism where a client retrieves a notification by  
>> querying the Subscription Manager"
>> However, the spec has not defined a message that might retrieve a  
>> notification, but if it did, then the replyTo in that message might  
>> be wsa:anonymous, in which case the response would be expected on  
>> the transport back-channel, or maybe if replyTo were a full  
>> addressable uri, then the message would be returned to that  
>> address.  This is what a lot of folks think of as an asynchronous  
>> message, it is asynchronous with respect to the connection that  
>> contained the request, or as sometimes described, asynchronous with  
>> respect to the underlying transport.
>> But, we have not defined it, but note that it does change the  
>> behavior and add constraints and demands to the subscription  
>> manager.  It is conceivable that queueing behavior of this  
>> hypothetical pull mode might be implementation specific with no  
>> protocol defined quality of service defined to allow for the  
>> broadest possible scope of implementation.
>> At this point Eventing has really defined only one mode. which is  
>> labeled push, and nowhere is there defined anything (yet)  
>> concerning queueing behavior.
>> Now suppose we deal with the issue concerning a non-addressable  
>> notification destination and suppose that we might try to achieve  
>> that by composition with MC.  MC usually is implemented in  
>> conjunction with WS-Addressing.  Usually the higher level protocol  
>> is unaware of the use of MC, it just sits on top of it. much the  
>> way that the higher level protocol is unaware that it is sitting on  
>> on any particular implementation of tcp/ip or http for that  
>> matter.  It is just a form of transport that has the effect of  
>> making communication with non-addressable endpoints possible.  It  
>> is true that it works by polling, but it does it by polling MC, not  
>> the higher-level protocol.  There is no quality of service  
>> guarantee and yes, MC does require a fifo implementation within it,  
>> but size and other characteristics are implementation specific.
>> The client protocol would see a notification just as if it were  
>> receiving it without MC, it is just that MC polled for it.
>> So in this regard, there is no difference between "Push" mode to an  
>> addressable endpoint and this method of dealing with non- 
>> addressable endpoint with MC.
>> I have a few observations concerning the debate:
>> A) Li is correct, that the delivery would not be instantaneous, but  
>> it is not instantaneous with http over tcp/ip either since the  
>> speed of transport, the distance latency or even the number of re- 
>> tries are unknown.  I can imagine deep-space notifications where  
>> the time would be on the order of months
>> B) polling is like sampling, you have to poll at a rate high enough  
>> to keep up with the anticipated message rate.  There is no way  
>> around this unless several notifications are sent in one message.   
>> One possible difficulty with this is that eventing today states  
>> that there are no constraints on notification messages.  What would  
>> happen if successive messages had different mime types? http allows  
>> for only one.  So in that case, unless constraints are imposed,  
>> messages need to be sent one by one anyway.
>> C) Anything other than the one form of notification delivery we  
>> have defined will probably require additional protocol messages and  
>> indeed extend the protocol in a behavioral sense.
>> D) There are very few ways to send messages to a non-addressable  
>> endpoint.   The two main techniques are polling and tunneling.  It  
>> might be most convenient that we allow users to pick whichever they  
>> choose.
>> To accommodate current implementations that have a push mode  
>> defined, I might suggest that we add an extensibility point in  
>> place of that attribute to allow for use of it, but I don't think  
>> the protocol cares.
>> I hope that this is helpful
>> -bob
>> On Mar 25, 2009, at 3:26 PM, Chou, Wu (Wu) wrote:
>> Bob,
>> I would like to share my thoughts and clarify the point I made in  
>> the WG call regarding issue 6692 (removing "mode") and the use  
>> cases of non-push mode event delivery,
>> Issue 6542 (makeConnect) is a concrete use case of non-push mode  
>> event delivery. Since it won't fit in WS-E push mode definition, it  
>> proposes/needs to change WS-E push mode definition/semantics. As  
>> discussed extensively by Doug, Li, Geoff, etc. as well as the  
>> question you raised, makeConnect needs to pull event from source  
>> and it is not WS-E push mode, e.g. it is not an "unsolicited" event  
>> delivery that controlled solely by the event source as specified by  
>> WS-E push mode specification.
>> "Push" and "pull" are two distinct concepts with well defined  
>> semantics. This is even more so in WS-E with its definitive push  
>> mode semantics specified in the specs (Section 2.2 Terminology),  
>> e.g. it must be [unsolicited], [individual], [soap binding],  
>> [asynchronous], [notification] to send event from source to the sink.
>> I am sure there are many other cases that won't fit into WS-E push  
>> mode. The point I would like to make is: "redundancy" issues should  
>> be dealt with after we finish all proposals with new semantics to  
>> WS-E, since redundancy won't hurt the validity/applications of the  
>> specs. We need to see the whole tree before trimming a branch off,  
>> because "redundancy" may change as new semantics being added.
>> Moreover, we should deal "redundancy" with the sensitivity that WS- 
>> E has been widely implemented. It influences many standards from  
>> various standard bodies. Unless it is absolutely painful and  
>> necessary, I would hope to avoid minor redundancy changes which  
>> have no impact to application semantics.
>> Many thanks,
>> - Wu Chou.
>> Wu Chou, IEEE Fellow, Ph.D. | Director |Avaya Labs Research | AVAYA  
>> | 233 Mt. Airy Road| Rm. 2D48 | Basking Ridge, NJ 07920 | Voice/ 
>> Fax: 908-696-5198 / 908-696-5401 |

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Received on Friday, 27 March 2009 07:45:53 UTC