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Subject: Re: [ws-caf] ACTION for optimization of registration
Greg Pavlik wrote: > Apologies for multiple replies. Inline comments: > > Green, Alastair J. wrote: > >> Greg, >> >> Amalgamating responses to your point and Mark's earlier ones. >> >> I think the idea of context backflow is the wrong way to think of (the >> first part of) this discussion. >> It isn't a context or a context reply, it's a vector of enlistment >> messages. >> >> > No argument here; that was a mistake on my part. Actually I think that we have two different ways of approaching the same problem. (i) a WS-Context context that flows back on the response; none of our specifications actually restrict context flows on "outbound" messages, since we're in a one-way domain, and this is conceptually appealing as it continues to tie together message exchanges within the same activity because the messages are all contextualised. Put it another way: all one-way messages are contextualised and we are simply providing another specific element within the extension capability we already support, to address this. The precedent for this is the OTS and various commercial implementations. (ii) a "boxcar" (a la BTP) of enlistment messages that accompany the one-way "response" message. The precedent for this is BTP. Now it seems to me that (ii) could be dealt with by the underlying SOAP infrastructure - that's always been the argument against putting boxcarring into the protocol. Whereas (i) is a specific approach for a very specific problem. Pros and cons both ways. > >> (And to repeat, because it may have been buried: I see no reason why an >> implementation should be free to refuse to attempt to process such a >> vector. If it is accepted that there is no need for a message >> tns:iDontDoThis in >> response to a vectorized attempt, then there is no need to argue about >> whether tns = wsacid or wstxm, which is the acid-specific reference I >> objected to in the proposed text.) >> >> >> > I don't see any obvious reason to allow refusal either. Agreed. If it's an optimisation that MAY be used, then that means that endpoints don't have to accept it. > >> You can obviously handle that vectorization in WS-CF. >> >> >> > Yep. > >> The moral equivalent of "prepared" (carrying the implicit semantic of >> WS-CF enlistment, participant-addition or whatever) is not able to be >> represented in WS-CF. As I said, this is a more relevant optimization in >> actuality. This means that a statement must be made in WS-TXM about the >> WS-CF implications of a WS-TXM message. >> >> > Right. Seems as though this is a distinct optimization and not > generally applicable. Why not provide both, at both levels (CF, TXM)? I agree and think it is a separate discussion item. Let's try to keep this discussion on the subject of the original issue which was accepted at the face-to-face and address other optimisations separately. > >> The idea of using a context reply (a la BTP) as a punctuation mark, to >> indicate to the app: "all enlistments done", "checked" is a useful but >> separate idea. It is a concept that belongs in WS-CF, I agree. > I'm not certain it belongs in WS-CF. "checked" is something that is up to the Referencing Specification to define, not the general coordination/registration framework we have. As an example, the OTS describes several ways of doing checked transactions and does not mandate any specific implementation: it simply recommends the X/Open XA version. >> I imagine >> in WS-CAF terms this would be an augmented version of the original >> context? >> >> Its interpretation with respect to transactional behaviour is not a >> WS-CF concept. > Agreed. >> Then there is the issue: what about the uber-optimization of enclosing >> or concatenating the application protocol message "checked" with the >> vectorized protocol messages wscf:enlist and wstxm:prepared (I know >> they're not really called that, just for expository purposes)? >> >> > Another issue IMO. Let's keep this particular discussion in the realms of the original issue please. >> > What is "checked"? Don't you have that AI ;-)? Mark. > >> Alastair >> >> >> Alastair J. Green >> CEO and CTO >> Choreology Ltd >> 68 Lombard Street >> London EC3V 9LJ >> www.choreology.com >> >> +44 870 739 0050 >> +44 870 739 0051 (fax) >> +44 795 841 2107 (mobile) >> >> >> -----Original Message----- >> From: Greg Pavlik [mailto:greg.pavlik@oracle.com] Sent: 12 July 2005 >> 16:29 >> To: Green, Alastair J. >> Cc: Mark Little; ws-caf >> Subject: Re: [ws-caf] ACTION for optimization of registration >> >> It occurred to me that enlistment via context "backflow" ought best >> to be addressed in WS-CF. >> >> Greg >> >> Green, Alastair J. wrote: >> >> >> >>> Two or three hasty comments >>> >>> 1. The ability to group enlistments simply requires that a vector be >>> sent rather than a scalar. There is no inherent need to involve a >>> "context reply" (as is done in BTP). There is no reason for the >>> coordinating entity to refuse such vectorized enlistments. >>> The purpose of a context reply is distinct: if enrolments/enlistments >>> arrive in a context reply then the application is "checked": it can >>> assume that all enrolments required by the service side are those >>> contained in the context reply. The issue of vectorization and checking >>> are separate, and both useful. But checking can also be accomplished by >>> counting and/identification of the participants. >>> 2. In BTP there is a useful combination: ENROL & PREPARED. This avoids >>> unnecessary enlistment traffic: if I am prepared then I exist. This >>> could in turn be vectorized. As it often occurs that only one >>> participant (a sub-coordinator) will be enlisted, this optimization is >>> in practice perhaps more important than the one suggested. The semantic >>> ENROL can of course be implied by the semantic PREPARED (whatever the >>> actual names chosen). >>> >>> 3. If the facility is intended to be tx protocol independent, then it >>> should be defined as such, and its implications (if any) for the >>> coordinator's actions should be separately dealt with in each tx spec. >>> In the ACID case a failed enlistment does imply a rollback, but this is >>> not true of non-atomic (non-uniform outcome) protocols. Therefore, I >>> think there should be a "separation of powers" (correct layering). >>> Taking this a little further in a different direction: there is no >>> inherent difference between "direct enlistment failure" and "failure by >>> proxy". >>> 4. The last point again raises the need to identify >>> inferiors/participants. >>> >>> Alastair >>> -----Original Message----- >>> From: Mark Little [mailto:mark.little@arjuna.com] Sent: 07 July 2005 >>> 15:34 >>> To: ws-caf >>> Subject: [ws-caf] ACTION for optimization of registration >>> >>> ACTION: Mark to write a proposal for the group's consideration to >>> >> >> boxcar >> >> >>> registrations, given the room's consensus that it appears to be a >>> potentially useful feature. >>> >>> To summarise the issue >>> (http://services.arjuna.com/wscaf-issues/show_bug.cgi?id=55) and the >>> text I am proposing: consider the case of an exporting transaction >>> domain (e.g., one where the transaction coordinator resides or where it >>> >> >> >> >> >>> was started) making a transactional invocation (one where the context >>> >> >> is >> >> >>> associated with an application call) to an importing transaction >>> >> >> domain. >> >> >>> The importing domain will do work that must ultimately be controlled by >>> >> >> >> >> >>> the transaction outcome and hence needs to register participants (in >>> WS-CF speak that means enlisting participants in the associated >>> >> >> activity >> >> >>> group). >>> >>> Note, this is not specific to WS-ACID and may be applicable to all >>> of the transaction models we wish to support. >>> >>> In some distributed transaction environments, registration of >>> participants with a remote coordiantor does not occur immediately >>> >> >> within >> >> >>> the importing domain. Rather, the information about the participants >>> that should have been enlisted is retained within the importing >>> domain until the response to the invocation is sent. In that case, a >>> context >>> >> >> is >> >> >>> also propagated back with the response and encoded within this >>> context are the EPRs for the participants. The receiver (the >>> original exporting >>> >> >> >> >> >>> domain) is then responsible for enlisting the participants with the >>> coordinator. >>> >>> The advantage of this approach is that it can improve performance, even >>> >> >> >> >> >>> if interposition is used: no cross-domain registration invocations >>> are necessary (at least from the importing domain). >>> >>> The disadvantage is the fact that the registration may fail and in >>> >> >> which >> >> >>> case (assuming failure isn't a transient that can be masked by retries) >>> >> >> >> >> >>> the exporting domain must either rollback the transaction or somehow >>> communicate with the original importing domain and instruct it to >>> rollback its work. In general, it's safer to require that the entire >>> transaction is rolled back, or at least marked for ultimate roll back. >>> >>> If the response fails to be delivered then the sender may retry or >>> automatically rollback the work that was done. If the sender retries >>> (presumably because it did not receive the response), then the work >>> (including registering of participants) will happen again and unless >>> >> >> the >> >> >>> importing domain can guarantee idempotent operations (or something akin >>> >> >> >> >> >>> to retained results), it's safer to require rollback and retry. >>> >>> The important things to remember are that: >>> >>> a) the importing domain (the one that did the work) has participants >>> that have not prepared yet and so, given presumed abort, they can >>> roll back autonomously. >>> >>> b) we don't want split-brain scenario, where the upstream nodes (the >>> exporting domain) takes a different course of action to the >>> downstream nodes in the transaction tree. Although as I've indicated >>> above there are "optimizations" that could be taken in order to >>> negate the >>> >> >> necessity >> >> >>> of rolling back the entire transaction in the case of registration >>> failure (I'll group failure to deliver the response into this >>> >> >> category), >> >> >>> it places more work on the implementation/application and can result in >>> >> >> >> >> >>> non-interoperable/non-portable behaviour. Hence my preference to simply >>> >> >> >> >> >>> err on the safe side and force roll back. >>> >>> So, here's some proposed text: >>> >>> "A service that receives a transaction context on an application >>> >> >> request >> >> >>> SHOULD enlist participants with the corresponding activity group. >>> The transaction service implementation associated with the service >>> MAY decide to use the following protocol to optimize remote >>> registration invocations: this optimization MAY occur transparently >>> to the service. >>> >>> Rather than register participants directly, the EPRs of the >>> >> >> participants >> >> >>> (and the protocols they should be enlisted with) are retained by the >>> service-side transaction service component until the response to the >>> original invocation is sent; in which case a context containing >>> these EPRs and associated data is propagated back with the response. >>> >>> If a receiver of a response obtains this context it MUST either be able >>> >> >> >> >> >>> to perform the enlistment itself or, if it does not support this >>> optimization, send back a wsacid:CannotOptimizeEnlistment fault code >>> message and mark the transaction so that its only outcome is to >>> rollback. If enlistment fails then the transaction MUST be rolled back; >>> >> >> >> >> >>> an implementation MAY retry transient registration failures. >>> >>> If the sender of the response receives an indication that there was >>> a non-recoverable failure (e.g., in delivery of the message or >>> registration of the participant EPRs) then it MUST rollback the work it >>> >> >> >> >> >>> has performed in the scope of that transaction. >>> >>> When using this optimization, the service MUST still see a >>> wscf:participantAdded message in order to be compliant with WS-CF. >>> However, this message SHOULD be generated by the service-side >>> transaction component for this optimization to work." >>> >>> Mark. >>> >>> >>> >>> >> >> >> >> >> > -- Mark Little Chief Architect Arjuna Technologies Ltd (www.arjuna.com)
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