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Subject: Re: SyncReplyMode as defined in .91 is a misnomer


The CPA/ebXMLHeader based solution *is* a deterministic solution as it is clearly identified in the DeliveryChannel specification and is applicable to the entire message path (all node pairs) including intermediaries uniformly.

With the "ebxmlresponse=synchronous" HTTP level parameter, how do you expect it to be propagated to all the nodes? Plus with the way HTTP transport bindings are specified, there is no clear distinguishing factor between HTTP level MIME headers and ebXML message level MIME headers. These could easily get into the ebXML message level (MIME) headers and things would be confusing. Special handling would be needed by MSHs.

With the HTTP level "ebxmlresponse=synchronous" kind of approach, this needs to specified for each and every transport and needs to be revisited when a new transport support is added or each time transport level bindings are touched.

With the CPA (DeliveryChannel) based approach, we get to use a uniform and simple way specifying the synchronous response requirement, rather than, one way for HTTP, a different way for some other protocol etc.

The HTTP (communication protocol) binding level approach is not going to scale..

Please see follows below in <PY> also.


"Burdett, David" wrote:

 PrasadI think there is a disconnect between what we are each thinking on this topic as you seem to be thinking that I'm trying to defeat the purpose of reliable messaging (which I'm not), and I think that you are making the specification too non-deterministic which will add unnecessarily to MSH complexity, as well as requiring the addition of header attributes/elements that could be in conflict with other elements that have already been defined.See more comments inline.David
-----Original Message-----
From: Prasad Yendluri [mailto:pyendluri@webmethods.com]
Sent: Friday, January 05, 2001 12:41 PM
To: Burdett, David
Cc: ebXML Transport (E-mail)
Subject: SyncReplyMode as defined in .91 is a misnomer

The way the syncReplyMode attribute is defined in .91 version of the spec is very confusing.  Section reads:

The syncReplyMode is an optional attribute that indicates whether a response to a message must be returned at the same time as any acknowledgments. It has two values:
This is basically defining if a message should be processed before "any" response is sent back or not. This combined with the deliverReciptRequested parameter, simply boils down to whether the acks and response are all sent together or if acks are sent first and then the response.

1.  Since the ebXML message structure is now flexible enough to permit the acks to be sent either in separate ebXML messages or in the same ebXML message as the response, why impose the requirement that all of them should be sent together, as a general case? This defeats the purpose of reliable messaging IMHO, where you have the mechanism of Acks to inform the sender that the message had been received (by the receiver), prior to a response being available. If the receiver was able to process the message within the timeout constraints of sending an acknowledgment, they might be able to send them all in the same message. However, the sender should not be able to mandate it. 

<DB>This means that a sender of a message will not know if the response they get will be an ack, or an ack and a response together. This is not normally an issue unless you are using a synchronous transport - see more below.

           <PY>The will know what all they expect to comeback. They will not know if they all comeback in the same message or not. But that does not matter since, they will know when they get the message what all is in the message. The message formats permits the flexibility, very desirable IMHO. Lets keep it that way. </PY>
I don't think it defeats the purpose of reliable messaging. It's really a question of how tightly/loosely you control what goes into a response to a message.</DB> 
           <PY>If you mandate sending acks and responses together, it does defeat the purpose since, we are not using the ack mechanism driven retries effectively to accomplish reliable messaging on asynchronous connections</PY>
2. This is NOT addressing  the issue of sending response (and Acks) "synchronously" on the same communication protocol level connection as the request. Hence the use of the term "Sync" in the "SyncReplyMode" is totally misleading. 

<DB>I agree. However, I think that returning a response on the same communication protocol level connection is a communication protocol issue, and therefore information that only applies to a particular protocol should not be in the Message Header. In version 0.91 there an HTTP header "ebxmlresponse=synchronous" that is used to indicate whether or not a message sent in reply to an earlier message must come on the HTTP response.

<PY> It is a communication protocol issue but, after analysis turns out it is better to specify it in the DeliveryChannel level in the CPA and in the ebXMLHeader than at the communication protocol level parameters. We get to use a uniform and simple way specifying it rather than, one way for HTTP, a different way for some other protocol etc.  See my response in the beginning also. </PY>
What I am uncomfortable with is the idea that the sequence of messages that are returned will be different in a non-deterministic way depending on whether or not a synchronous transport protocol is being used. For example if we follow your approach and use AcksAndResponse for a message sent over HTTP then the acknowledgement and response would come back together. But if you sent the same message over SMTP, then they might come together but then they might not. This seems to add unneccessary additional complexity IMO to the behavior of a MSH.

I can understand how you might want to be allow a recipient of a message to decide whether or not reply with an ack or an ack and a response, but I also think that the CPA should enable that a response is always in a particular format.

<PY> I think CPA requiring the reply "format" or packaging together acks and response for asynchronous exchanges also, goes against the principle and flexibility of asynchronous reliable messaging and also the flexibility offered by the current ebXML message header. Why does one want them all together? What is the advantage? Only in the synchronous exchanges case this becomes a factor. The reason I called for parameters as follows is so that, only the ack can comeback synchronously so that basic mechanism of reliable messaging is still in place.</PY>
So how about the following parameter values"
I am also uncomfortable with having two parameters that imply essentially the same thing. For example if deliveryReceiptRequested or immediateAckRequested are both set to NONE, then a syncReplyMode of AcksOnly or AcksAndResponse would be inconsistent. We really ought be able to avoid this sort of problem with the correct choice of parameters.
<PY> If the message builder does not ask for acks and sets SyncReplyMode to return acks synchronously that is an error and results in error or warning  message as with any other errors in the header.</PY>

3. The only situation where you would want the sender to indicate that both Acks and Responses  should be sent together is, when we have the case of "synchronous exchanges/transactions". Here the response comes back on the same communication protocol level connection and since typically the communication protocol level session ends after the reply comes back, we need to provide for a way to specify, if only the Ack needs to be returned synchronously; only response (no acks); or both acks and response need to be returned synchronously. In my proposal, I call for this to be part of the CPA with headers simply echoing  the CPA for the convenience of processing by the MSHs. I am attaching my proposal for immediate reference here.

4. Since the way you defined the SyncReplyMode in .91 version is not addressing how RPC like responses can be delivered on  HTTP / MessageQueues / LU6.2 etc. connections synchronously. Use of the same attribute name "SyncReplyMode" that I used in my proposal that addresses this case, is really a misnomer and confusing the issue.

<DB>I disagree, the HTTP parameter allows you to get a RPC like response. I agree though that a name change might apply, would replyMode be a reasonable term?</DB>

5. If there still is a need for the senders to specify that both acks and responses should be sent together irrespective of "synchronous" exchanges, I think we need to use a different name for this attribute. This makes no sense to me however, as this defeats the purpose of reliable messaging. 

Regards, Prasad


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