Here are some reasons why I would
prefer to use ebMS headers for routing rather than WS-Addressing Reference
Parameters:
Conformance to v3.0 Core
The ebMS 3.0 Part 1 Core does not specify the use of WS-Addressing.
Avoid yet another standard to
support
Requiring support for WS-Addressing in a multihop context adds a burden
to implement ebMS 3.0 in general and the multihop profile in particular.It is
also true that the counterproposal requires adding ebMS headers in some
situations where v3.0 Core does not require them, but any ebMS 3.0
implementation needs to know how to construct or handle ebMS 3.0 headers anyway,
so this is just a reuse of that general functionality.
Inconsistency
If we start using WS-Addressing reference parameters in the multihop
context, the question comes up why we did not use them in ebMS 3.0 Core. There
are already elements in the ebMS header that are also in WS-Addressing, such as
MessageId. Nevertheless, v3.0 Core does not use WS-Addressing even for these
header fields, let alone use reference parameters for the information types that
are in an ebMS 3.0 header but are not regular WS-Addressing field. I think the
ebMS 3.0 header is richer than WS-Addressing header, so that this decision was
right at the time. But even if it were not, I think a multihop profile should
stick to the approach taken in v3.0 Core.
Limited reuse
It might look as if using WS-Addressing is an instance of reusing a more
general specification instead of defining ebMS specific mechanisms. But unlike
the reuse of WS-Security or WS-Reliability/WS-ReliableMessaging, this reuse is
only a syntactic issue. We would just be adding information to some defined
extension points in the schema. Any processing of that information would require
ebMS-specific processing, rather than using generic WS-Addressing functionality.
Any intermediary that provides WS-Addressing functionality and wants to conform
to this profile would still need to have ebMS-specific
functionality.
Duplication in message
structure
Having to store or retrieve the same type of information in two different
locations usually smells of missed generalizations, and I think that is true
here too. We looked at putting the equivalent of an ebMS header in a URL (the
earlier HTTP URL query parameters), I don’t find putting it in WS-Addressing
parameters a lot more elegant. It also raises issue like how to handle any
inconsistencies in situations where there is both a WS-Addressing structure and
an ebMS header. Also, the WS-Addressing specification says that using reference
parameters to store information that would otherwise be in other SOAP headers
could
cause “unintended or erroneous semantics in the resultant SOAP message".
Generalized routing
This is related to duplication. When using just ebMS
headers, assuming routing is done
on destination PartyId, any
message (user, signal, WS-* helper, bundles, piggy backs) can be routed based on
the single following XPath expression:
S:Envelope/S:Header/eb3:Messaging/eb3:UserMessage[1]/eb3:PartyInfo/eb3:To/eb3:PartyId
From: Moberg Dale [mailto:]
Sent: 07 May 2008 23:14
To: Pim van der Eijk;
Subject: RE: [ebxml-msg] MOU on
intermediate support by conformance profile and other auxiliary documents for
ebMS 3.0
Yes, I agee that adding
metadata to ebMS signals or to other messages used in WS protocols is what
is agreed upon as a MOU item.
The way or ways to do
this is apparently TBD J
I think that since we
have several ways to attach the metadata, we probably want to prune these down
into a simple convention to do it in a standard
way.
(Incidentally, would we
then provide CPPA support for these messages? Would they be treated as msh level
messages from the default packaging and delivery channel
standpoint?)
From: Pim van
der Eijk [mailto:]
Sent: Wednesday, May 07, 2008 1:04
PM
To: Moberg Dale;
Subject: RE: [ebxml-msg] MOU on
intermediate support by conformance profile and other auxiliary documents for
ebMS 3.0
We agreed to add ebMS
metadata also to signals and helper messages. And we also looked in some
detail at one option for adding this metadata, by adding it to a WSA
header. Imo we did not fully compare this option to using a an
eb:Messaging header (containing an eb:UserMessage but one that has
no eb:PayloadInfo) to convey the ebMS metadata. I want to fully
understand both options and compare their pros and
cons. Unfortunately, due to travel I will not be able to attend next
week's meeting.
Pim
From: Moberg
Dale [mailto:]
Sent: 07 May 2008 19:42
To:
Subject: [ebxml-msg] MOU on intermediate
support by conformance profile and other auxiliary documents for ebMS
3.0
Goals
Transparency where feasible
Signature intact where feasible (important aspect of
transparency)
WSI Conformance where feasible (especially with respect
to WSI RSP policy that WS-RX Reliable Messaging headers be signed, including
WS-Addressing elements where used within
ReliableMessaging)
Spoke configuration to services within an I-Cloud should
have simplicity of configuration change management.
Specifically, rerouting to services (within the I-Cloud
) can be accomplished without spoke configuration
changes”)
Hub service destinations and spoke clients have
“vanilla” ebMS 3 conformance where feasible. (Some specializations of general
functionality and of extension points is allowed.)
Additional
Implicit Constraints
The internal addresses within an I-Cloud can have
private IP addresses and DNS names that are not publicly reachable or resolvable
in the Internet.
Proposed
Rerouting by intermediaries can be based on ebMS
“metadata” both in forward and return paths.
This functionality is referred to as a “table” mapping
ebMS metadata to next hop URLs so that the HTTP POST (or other?) command can be
rewritten.
TBD: This map may be augmented for return
paths by keeping a map involving Message-Id and Relates-To values.
For ebMS user messages, no special treatment is needed
(so far anyway).
For ebMS signal messages and for WS clerical messages
(such as CreateSequence, CreateSequenceResponse, etc.) several approaches are
under consideration
1. Define a ebMS Reference Parameter that allows
the wsa attribute “isReferenceParameter” applied and that contains the metadata
needed for routing.
2. Allow use of WS-Addressing headers such as
From or ReplyTo or FaultTo that have an EndpointReference model (and include
ReferenceParameter within the EPR).
3. For return path, if WS-Addressing Messaging-Id
was present in incoming message, require use of RelatesTo in response
message.
4. For using the request connection for a
response, some read-only access to ReplyTo is needed to check for the
“anonymous” URL value.
In this case, all
intermediaries would be expected to hold the connection open (?) to allow the
final service to use the HTTP backchannel for its
response.
For Pull MEP-binding, allow first intermediary to handle
MPC requests and let internal I-Cloud arrangements for this option be
implementation specific.
