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Subject: Re: [ws-tx] Issue 007 - WS-C: Make Register/RegisterResponse retriable
- From: Christopher B Ferris <chrisfer@us.ibm.com>
- To: Mark Little <mark.little@jboss.com>
- Date: Mon, 12 Dec 2005 13:43:26 -0500
That is one way, the other is to make
the Register message idempotent.
Seems to me that Register SHOULD be
idempotent. It is much simpler to simply process
the Register as if it had never been
received... makes the implementation of the client
a bit simpler.
I also think that the "AlreadyRegistered"
fault is probablematic. It doesn't reflect
back the CoordinationProtocolService
EPR that the RegisterResponse message does.
So, from the perspective of the registrant,
it ISN'T registered if it doesn't receive the
RegisterResponse message since it doesn't
know the CoordinationProtocolService
EPR.
From the perspective of the registration
service, overlaying the previous registered
EPR is effectively an idempotent operation,
and the response can be the same as if
it didn't have the registration beforehand.
IMO, making the operation idempotent
makes the implementation much simpler and
more robust in the long run.
Cheers,
Christopher Ferris
STSM, Emerging e-business Industry Architecture
email: chrisfer@us.ibm.com
blog: http://webpages.charter.net/chrisfer/blog.html
phone: +1 508 377 9295
Mark Little <mark.little@jboss.com>
12/12/2005 11:36 AM
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cc
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Subject
| Re: [ws-tx] Issue 007 - WS-C:
Make Register/RegisterResponse retriable |
|
Actually I'll retract this. As Kevin just reminded
me, we're using
WS-Addressing anyway, so surely lost messages and retries can be coped
with at that level: using the same wsa:MessageID for example, should
sort this.
Mark.
Mark Little wrote:
> I think this makes proposal makes sense.
>
> Mark.
>
>
> Peter Furniss wrote:
>
>> This is hereby declared to be ws-tx Issue 007.
>>
>> Please follow-up to this message or ensure the subject line starts
Issue
>> 007 - (ignoring Re:, [ws-tx] etc)
>>
>> The Related Issues list has been updated to show the issue numbers.
>>
>> Issue name -- WS-C: Make Register/RegisterResponse retriable
>>
>> Owner: Alastair Green [mailto:alastair.green@choreology.com]
>>
>> Target document and draft:
>>
>> Protocol: Coord
>>
>> Artifact: spec
>>
>> Draft: Coord spec working draft uploaded 2005-12-02
>>
>> Link to the document referenced:
>>
>> http://www.oasis-open.org/committees/download.php/15738/WS-Coordination-
>> 2005-11-22.pdf
>>
>> Section and PDF line number:
>>
>> WS-Coordination spec, Section 3.2 "Registration Service"
l. 294
>>
>>
>> Issue type: Design
>>
>>
>> Related issues:
>>
>> Issue 008 - WS-C: Remove fault 4.6 AlreadyRegistered
>> Issue 014 - WS-C: EPR equality comparison is problematic Issue
009 -
>> WS-C/WS-AT: Is request-reply MEP useful?
>>
>>
>> Issue Description:
>>
>> Register/RegisterResponse should be retriable exchange
>>
>>
>> Issue Details:
>>
>> [This issue stems from Choreology Contribution issue TX-20.]
>>
>> Section 9 of WS-AT defines the WS-Coordination exchanges
>>
>> CreateCoordinationContext/CreateCoordinationContextResponse
>> Register/RegisterResponse
>>
>> as request-reply exchanges.
>>
>> (Whether this request reply MEP should be used at all in the WS-TX
>> specs is addressed in a separate issue: see "Issue
009 - WS-C/WS-AT:
>> Is request-reply MEP
>> useful?".)
>>
>> Substantively, it may be particularly misleading to think of the
>> Register/RegisterResponse
>> exchange as a request-reply pattern. The implication of using
this
>> pattern is that there is a simple one message in, one message
out
>> exchange. The presence of a fault
>> (AlreadyRegistered) as a potential response to Register hardens
>> that implication.
>>
>> Current behaviour would lead to service being informed it has
already
>> registered a
>> Participant, when it has in fact simply succeeded in registering
a
>> Participant. Superficially, the
>> AlreadyRegistered fault could simply be
>> viewed as being unnecessarily verbose: the reaction of the service
to
>> the fault at run-time must be to treat
>> it as uninteresting, i.e. as equal in effect to a successful
>> registration.
>>
>> In fact there is a deeper problem. Consider the following scenario:
>>
>> A Coordination Service (CS) creates a Coordinator (C) for a new
>> atomic transaction (AT), and emits a CoordinationContext (CC).
>>
>> The CC is transmitted to an application service (AS). AS (logically)
>> creates a P which sends Register (R) to the Registration Service
(RS)
>> EPR for AT, embedding the EPR for receipt
>> of protocol messages outbound from C to P (CP EPR).
>>
>> The RS, on receiving Register, creates an EPR for inbound protocol
>> messages from P to C (PC EPR), and embeds this in the
>> RegisterResponse (RR), which it sends to P.
>>
>> AS and P crash before the RR message is received by P, or the
RR message
>> drops and is never received by P. Either way, AS (on recovery,
or after
>> waiting) causes P to resends R to RS. RS examines the inbound
Register,
>> and determines that it has come from a known P (see "Related
Issues",
>> "WS-C: EPR equality comparison should
>> not be relied upon"), i.e. that it is a duplicate registration.
>>
>> Currently, RS replies with an AlreadyRegistered fault, sent to
P. P
>> now knows that he is registered with C, but has never received
the PC
>> EPR (/RegisterResponse/CoordinationProtocolService element). Any
>> further retries of P send R to C will result in the same situation.
>>
>> C will never be able to receive messages from P. P will never
become
>> Prepared. The transaction will eventually collapse through timeout.
>>
>> Therefore, the Register/RegisterResponse exchange must tolerate
>> duplicates. If a Register message is delivered more than once
(either
>> by the transport, or through comms-failure- or recovery-induced
>> retry) then the Registration Service should respond on each occasion
>> with a RegisterResponse containing the same PC EPR, to ensure
>> reliable completion of the EPR exchange that permits the subsequent
>> coordination protocol to operate correctly.
>>
>> NOTE.
>>
>> This change brings the R/RR exchange in line with the behaviour
of
>> the CreateCoordinationContext/...Response
>> exchange. There is a difference. R/RR is likely to be implemented
as
>> a true idempotent operation. CCC/CCCR is
>> not: each CCCR embeds a new RS EPR, and a new /Context/Identifier.
>> But each exchange can be harmlessly
>> replayed indefinitely, in the event of failure to receive the
>> response message.
>>
>>
>> Proposed Resolution:
>>
>> Insert the following text in WS-Coordination spec, Section 3.2
>> "Registration Service" immediately following current
l. 294
>>
>> "[New paragraph]The requester MAY send a Register message
for a given
>> Participant more than once, and the underlying transport could
>> deliver the Register message more than once.
>> On receipt of a Register message for a
>> given Participant, which has already been processed succesfully,
the
>> Registration Service MUST send to the
>> requester a RegisterResponse containing the same
>> CoordinationProtocolService element (Endpoint Reference for
>> Participant to Coordinator protocol messages) as that contained
in
>> all previous RegisterResponses generated by
>> the Registration Service which relate to the Participant's request
to
>> register for this activity.
>> [New paragraph]"
>>
>>
>>
>>
>>
>
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