In a recent ebXML MS phone meeting, we talked about how to deal with
attributes and properties of messages that appear to be specified both
in the CPA and in the message itself. I said that I would try to put
together a list of these.
I assume that every ebXML MS conversation is governed by a
pre-agreement on parameters, whether specified in an actual CPA
document or by some other means ("virtual CPA").
If values are pre-agreed then why bother to reiterate them in the
message itself? We discussed two possible answers: (1) to let the
sender control the attribute on a per-message basis, and (2) so that
intermediaries, who may not be privy to the pre-agreement, can see the
values. If, for some attribute, neither of these is a concern, then
there would not seem to be any reason for the attribute to be
reiterated in the message.
One obvious exception: the message header should contain whatever
fields are necessary to identify the message, especially whatever is
necessary to establish which pre-agreeement applies to the message.
Thus there's nothing wrong with MessageHeader subelements From, To,
CPAId, ConversationId, Service, Action, and MessageId.
Here is a list of thing that *might* constitute attributes that are
specified in both the CPA and the message. In cases where I'm not
sure, I err on the side of inclusion.
Section 7.4 says "This parameter information can be specified in the
CPA or in the MessageHeader", but some of the parameters listed among
the subsections of 7.4 do not appear to be in the MessageHeader, or
indeed in the message at all: Retries, RetryInterval, PersistDuration.
Perhaps the wording in section 7.4 proper needs a small change.
Taking all this into account, there actually don't seem to be very
many conflicts. The ones I can see that deserve scrutiny are:
(1) syncReply
The message has MessageHeader/QualifyOfServiceInfo/@syncReply
(3.1.7.1) with values true and false. The CPA has
CPA/PartyInfo/DeliveryChannel/Characteristics/@syncReplyMode
(7.5.11.1) with values "signalsOnly", "resonseOnly",
"signalsAndResponse", and "none".
The CPA certainly appears to be talking about BPSS "signals" and BPSS
"Business-response Messages", whereas the message header seems to be
talking about MS-level acknowledgement. There has been a lot of
discussion of this one already and I won't attempt to recap it here.
(2) duplicateElimination / idempotency
The message has attribute
MessageHeader/QualifyOfServiceInfo/@duplicateElimination (3.1.7.2)
with values true or false, while the CPA has attribute
CPA/PartyInfo/DocExchange/ebXMLBinding/ReliableMessaging/@idempotency
(7.6.4.2) with values true or false. These really do seem to mean
the same thing.
(3) request for acknowledgement / deliverySemantics
The message has DeliveryReceiptRequested (6.1.1) with "signed"
attribute that can be either true or false, and AckRequested (7.3.1)
with the same "signed" attribute and an "actor" attribute. The CPA
has the attribute
CPA/PartyInfo/DocExchange/ebXMLBinding/ReliableMessaging/@deliverySemantics
with possible values "OnceAndOnlyOnce" and "BestEffort".
These do not mean exactly the same things, but they seem to at least
overlap.
The CPA "deliverySemantics" attribute has only two possible values,
rather than expressing all four possibilities the way the Message
Specification currently does. The four possibilities are:
Name Retry/ack? Dup elimination?
BestEffort No No
AtLeastOnce Yes No
AtMostOnce No Yes
OnceAndOnlyOnce Yes Yes
In fact, it seems to me that it's not altogether clear what would be
meant by setting deliverySemantics to OnceAndOnlyOnce and setting
idempotency to true. If you know that a message is idempotent then
"only once" is not important, and effort spent preventing duplicates
may not be worth the cost.