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Subject: Re: [ws-tx] Issue 007 - WS-C: Make Register/RegisterResponse retriable
Christopher B Ferris wrote: >> >> I'm not sure that using ws-a messageId is the easiest... it means that >> impls need to remember messageId >> which can get onerous. > > Well most/all distributed systems I've known since the mid 1980's have done it and it hasn't been a problem. It's also been discussed several times in the WS-A WG as the way to do retries. >> >> The WS-A WG avoided the issue of EPR equivalence mostly because of >> issues related to use of >> EPRs to identify something. IMO, in that spirit, EPR comparison >> becomes one of comparing the >> <Address> element which comes down to URI equivalence issues which can >> go in a number of >> directions... the namespace URI approach (straight string comparison) >> or the approach which normalizes the URI >> first before comparing. > > Not the case. The ReferenceProperties elements as well as the MetaData can be different for each EPR even if the EPR has the same URI. So comparing purely on the URI isn't going to help. Mark. >> >> 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@arjuna.com> wrote on 12/12/2005 03:20:16 PM: >> > > >>> > There are multiple ways of making the operation idempotent. Using WS-A >>> > semantics is one and IMO is probably the easiest way of doing it: it >>> > goes back to traditional Retained Results RPC mechanisms of the late >>> > 1980's, where idempotency was imposed at the comms level. If we try to >>> > do it higher up the stack, within the actual implementation, then we're >>> > going to have to address the issue of EPR comparisons: how can I ensure >>> > this is the same operation if I can't determine that the parameters are >>> > identical? >>> > >>> > So, I think we're agreed that it needs to be idempotent. But >>> > until/unless we address EPR comparisons, I think the WS-A retry route >>> > gets my vote. >>> > >>> > Mark. >>> > >>> > >>> > Christopher B Ferris wrote: >>> > >> >> >>>> > > >>>> > > 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 >>>> > > >>>> > > >>>> > > To >>>> > > ws-tx@lists.oasis-open.org >>>> > > cc >>>> > > >>>> > > 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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