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Re: Issue: Synch/Asynch Web services

From: Jon Dart <jdart@tibco.com>
Date: Mon, 04 Aug 2003 09:58:59 -0700
Message-ID: <3F2E90D3.4030809@tibco.com>
To: "Cutler Roger (RogerCutler)" <RogerCutler@ChevronTexaco.com>
CC: www-ws-arch@w3.org, www-wsa-comments@w3.org

Very nice summary. I agree about the importance of the topic, and the 
unsatisfactory nature of the current glossary entries.

FYI I prefer definition #1 to #2. #1 is pretty good about being specific 
enough to be meaningful, and usefully vague in the areas it should be 
vague about. #2 is not really bad either, but it talks about a 
"running/active state", which to me kind of implies a state model that 
WSA doesn't have. The glossary does talk about states, but doesn't 
define any specific states.

IMHO #1 or some combination of #1 and #2 would be an improvement over 
what is there now.


Cutler, Roger (RogerCutler) wrote:
> ISSUE:  The current Web services architecture has technically 
> substandard definitions of synchronous and asynchronous, and almost no 
> discussion or use of these concepts in the architecture itself.  The WG 
> needs to come up with adequate definitions of these terms and address 
> how they relate to various aspects of the architecture.
> This note will discuss in some detail the following four assertions.  
> The first assertion, which is essentially “why is this important”, will 
> be discussed last so that it may draw on material developed in the other 
> discussions. The fourth may be viewed as an attempt to suggest 
> sub-issues, follow-ons or consequences of the main issue.
> 1 – There is widespread public interest in and understanding of the 
> concepts of synchronous and asynchronous in the context of Web services, 
> and there is a need to develop these concepts further in the context of 
> the Web architecture.
> 2 – The current discussion of s/as in the WSA is the result of an 
> aberrant process and is technically substandard.
> 3 – The current WSA does not satisfy the WSA requirements because of the 
> lack of treatment of these concepts.
> 4 – There are a number of questions and issues involving s/as that could 
> and in some cases should be addressed in the WSA.
> --- Process History and Current Treatment ---
> To my recollection, the discussion of s/sa involved many lengthy threads 
> and competing definitions, culminating in a more formal process 
> facilitated by David Booth involving collecting candidate definitions, 
> straw polls and discussion of the poll results.  The upshot of this on a 
> particular telcon (date not recalled) was fairly widespread agreement, 
> and perhaps even “unrecognized consensus” on the definition,
> <definition>
> http//lists.w3.org/Archives/Public/www-ws-arch/2003Mar/0146.html
> A request/response interaction is said to be asynchronous when the 
> request and response are chronologically and procedurally decoupled. In 
> other words, the client agent can process the response at some 
> indeterminate point in the future when its existence is discovered, for 
> example, by polling, notification by receipt of another message, etc.
> A request/response interaction is said to be synchronous when the client 
> agent must be available to receive and process the response message from 
> the time it issues the initial request until it is actually received or 
> some failure condition is determined. The exact meaning of "available to 
> receive the message" depends on the characteristics of the client agent 
> (including the transfer protocol it uses); it may, but does not 
> necessarily, imply tight time synchronization, blocking a thread, etc.
> </definition>
> There was, however, at least one person who remained particularly 
> attached to the following definition, which you will note has many 
> features in common with the first.  One clearly different component is 
> the explicit mention of “one-way synchronous messaging”.  To my 
> knowledge, this concept has never been fully explained in these WG 
> discussions.  I believe that a substantial segment of the 
> public/industry thinks that such a thing is not defined (e.g. 
> _http://lists.w3.org/Archives/Public/xml-dist-app/2000Dec/0089.html_) or 
> exists only in contexts other than distributed systems (e.g. 
> _http://www.cs.nott.ac.uk/~bsl/G52CON/Slides/Remote-Invocation.pdf_).  
> Although it does not seem impossible to me, and perhaps it is indeed 
> worthwhile, to give this idea a clear conceptual foundation in the 
> context of Web services, it is frankly difficult for me to find any 
> evidence of a groundswell of industry interest in this specific idea as 
> opposed to the more usual sense in which the term synchronous is used.
> Here is the alternative definition discussed in the telcon:
> <definition>
> _http://lists.w3.org/Archives/Public/www-ws-arch/2003Feb/0437.html_
> Synchronous message exchange (applies to oneway as well as 
> request/response) requires that both sender and receiver, or initiator 
> and respondant, processes are running/active at the same time as the 
> exchange takes place. In the case of request/response, the exchange is 
> synchronous if both sender and receiver remain in the running/active 
> state for both the request and response.
> Asynchronous message exchange (also applies to oneway or request 
> response) does not require, but does not preclude, that both sender and 
> receiver, or initiator and respondant, processes are running/active at 
> the same time as the exchange takes place. It typically requires some 
> form of mediation between the sender and receiver such as a message queue.
> </definition>
> The telcon ended without clear resolution of this issue and, to my 
> recollection, a subteam was given the mandate of reconciling these two 
> definitions.  This subteam produced, after a rather long time, the 
> current definition which may be viewed at 
> _http://www.w3.org/TR/2003/WD-ws-gloss-20030514/#synchronous_. This 
> definition is clearly not in any way a reconciliation of the first two 
> definition alternatives, and so I think arguably was not consistent with 
> the “charter” of the subteam.  It is, in fact, actually less of a 
> “definition” than it is a “refusal to define”.  It states that “The 
> terms ‘synchronous’ and ‘asynchronous’ are descriptive, and do not 
> correspond precisely to properties of MEPs,” and deprecates the use of 
> these terms in any normative documents whatsoever: “In view of the 
> informal way that the terms are used, it is recommended that documents 
> should not rely upon the use of ‘synchronous’ or ‘asynchronous’ in any 
> normative specification.”
> This “definition” was then included in the glossary with very little 
> discussion and, in my view, no substantive demonstration of consensus.  
> I hasten to add that I do not think this was matter of some sort of 
> conspiracy or ill-motivated behavior, but mostly an understandable 
> matter of fatigue with the subject.  Unfortunately, however, since that 
> time most attempts to discuss issues involving s/sa or to include 
> verbiage in the document involving those concepts have met with strong 
> resistance within the WG, in large part presumably because this entry in 
> the Glossary seems to contend that the terms have no objective meaning 
> and deprecates their use in this and all other normative documents.
> After the process described above, which I consider aberrant (in the 
> sense that the process agreed on by the WG was not in fact followed and 
> there was no real demonstration of consensus), I took the time and 
> energy personally to explore these concepts, concentrating primarily on 
> the concept of synchronous.  With the assistance of some people with 
> more Comp Sci background than mine, I engaged in an off-line 
> correspondence on this subject.  I kept it off-line because of my 
> perception (based on considerable evidence) that the WG was extremely 
> tired of hearing about this subject, but I cc-ed the chair on all 
> correspondence (until it started wandering way off-topic).  Somewhere in 
> this correspondence (the on-topic portion) I was told that I really 
> should read the classic paper by Lamport on synchronizing systems of 
> logical clocks in distributed systems 
> (_http://courseweb.sp.cs.cmu.edu/~cs612/papers/lamport.pdf_) because I 
> was trying to re-derive his analysis.  This was not strictly true, but I 
> was certainly wading in the same waters and Lamprey’s analysis is at 
> least germane background material.  I earnestly recommend this article 
> to anyone reading this note, even those who have read the paper a long 
> time ago.  It is really worth reading or re-reading simply because it is 
> thoroughly excellent.
> At any rate, I have convinced myself personally that it is by no means 
> impossible to understand these concepts in the context of Web services 
> and distributed systems.  I also convinced myself that I personally can 
> understand the concept of synchronous in a slightly more general context 
> and that the term most definitely has an objective meaning.  I have no 
> desire to impose my current understanding on the WG nor to propose my 
> own definitions of the concepts, but I would be glad to explain my 
> conceptual understanding if it would be useful (although it would take a 
> bit of time).  My “bottom line” conclusion, however, is that there is 
> really nothing fundamentally wrong with a number of the candidate 
> definitions that were examined in the process described above, and that 
> in particular a (true) reconciliation of the two definitions quoted 
> above would probably be perfectly satisfactory and usable.
> I emphasize above that I have mostly concentrated my personal efforts on 
> the concept of synchronous, and although I have some hope that 
> asynchronous may come along fairly easily as the complement of 
> synchronous, nonetheless Frank’s introduction of the concept of 
> rendezvous as a necessary component of asynchronous operations gives me 
> a bit of pause.  Is this a requirement that asynchronous messages carry 
> information not required in synchronous messaging in order to support a 
> later rendezvous?  If so such a requirement might well be discussed more 
> explicitly in the WSA.
> --- WSA Requirements ---
> To be honest the assertion that the current WSA fails substantially to 
> meet the requirements because of the treatment of s/sa is not exactly a 
> slam dunk because the requirements do not explicitly mention the 
> concepts of synchronous or asynchronous.  If I had had any idea that 
> this would be a problem I certainly would have drafted something that 
> did, but who would expect this amount of difficulty over such commonly 
> understood and used concepts?
> On the other hand, the requirements document 
> (_http://www.w3.org/TR/2002/WD-wsa-reqs-20021114_) does talk about 
> supporting the Usage Scenarios.  This is discussed in Section 2 as a 
> fundamental part of the requirements process, and section 3.2.2 states 
> that the WSA “architecture should support use cases for all aspects of 
> Web services” and that “the WSA must be validated against WSA use 
> cases”.  The concepts of s/sa occur many times in the usage scenarios 
> document (_http://www.w3.org/TR/2003/WD-ws-arch-scenarios-20030514/_).  
> In particular, there are several atomic usage cases that use these words 
> in the name of the case (“S007 Multiple asynchronous responses”, “S070 
> Asynchronous messaging” and “S071 Asynch/Synchronous specificity”) and 
> there are specific references to the concepts in the texts describing 
> other usage cases (e.g. “S003 Request/Response”, “S300 System 
> Messages”).   In addition, although the terms are not specifically used, 
> having worked on the extended usage scenarios I know that these are part 
> of the conceptual underpinnings of these models.  For example, the 
> editorial comments documenting changes in the Travel Service usage case 
> mention the concept, and having authored the EDI usage scenario I know 
> that this concept, although not explicitly stated, was an important part 
> of the picture.
> It is my best judgment that the current WSA does not meet at least the 
> spirit of the requirements because of the content about s/sa in the 
> current Usage scenarios and the lack of content in the WSA related to 
> these issues.  In my view when the WSA does not support significant 
> content in the usage scenarios it is an indication that there is a real 
> problem.  I might add that in my view the best solution to that problem 
> is not to modify the Usage Scenario working draft to eliminate the 
> content that is not reflected in the WSA.
> --- Questions to address ---
> The following questions are intended as suggestions for issues that 
> might be addressed in the WSA related to the concept of synchronous and 
> asynchronous.  It is not intended to be complete nor is it asserted that 
> all topics are in scope and nontrivial.  To some extent these questions 
> may be a start at developing a “stakeholders viewpoint”, using the 
> current nomenclature of the document, but such an analysis might also 
> reveal more formal elements that need to be included in the concepts and 
> relationships portion of the document.
> - Can some WS’s be classified as appropriate for synchronous usage (e.g. 
> based on response time), some as appropriate for asynchronous usage 
> (e.g. based on handling of the information required to define a 
> rendezvous) and others as both?  If so, should this be standardized in WSDL?
> - Should a WS requestor be able to assert what it expects in terms of 
> synchronous or asynchronous operation?  Or is it inherently a matter of 
> “try it and see”?  Or recognize from description?
> - In the context of late binding, are there potential problems if a 
> requestor looking for a synchronous Web service actually has found one 
> intended for asynchronous use or vice versa?  [The answer to this is 
> obviously “Yes”].  How can such problems be recognized or avoided?
> -Could/should a management interface expose information to potential 
> users of a WS about past performance metrics and/or expectations of 
> relevance to the issue of s/sa usage?  (e.g. expected/observed response 
> times)?
> -Are there messaging requirements related to information required to 
> define a later rendezvous for asynchronous messaging?  Is there more 
> than one way for an asynchronous WS to affect the rendezvous?  Are these 
> questions addressed, or should they be addressed, by XMLP?
> - Do intermediaries for WS's always need to assume that the interactions 
> are synchronous?  If not, do they need to be able to tell from the 
> nature of the interaction what it is or do the principles need to know 
> something about the intermediaries based on whether they expect 
> synchronous or asynchronous operation?
> - Are there security and/or privacy issues that are unique to 
> synchronous or asynchronous WS's?  If so, are the methods of resolution 
> fundamentally different?
> - Are there timing issues that need to be standardized with respect to 
> synchronous Web services?  Or Web services that are invoked 
> synchronously.  I'm not sure if those are different things.  I'd like 
> the WSA to tell me.
> - Are there management functions specific to synchronous or asynchronous 
> Web services?  Or S/A invocations of WS's, if this is the way to look at it?
> --- Public Interest ---
> There is no lack of discussion of the concepts of synchronous and 
> asynchronous in the context of Web services, and no apparent confusion 
> about what the terms mean.  Examples include: 
> _http://searchwebservices.techtarget.com/tip/1,289483,sid26_gci905098,00.html_ 
> _http://java.sun.com/blueprints/webservices/using/webservbp3.html_
> _http://www-106.ibm.com/developerworks/webservices/library/ws-asynch1.html_
> A number of authors note that asynchronous operations are not explicitly 
> supported by current standards, although there is seldom any comment 
> about whether this is good or bad.  The thrust of many of these 
> treatments is to discuss how asynchronous implementations of Web 
> services may be achieved in various proprietary settings.  I have not, 
> however, seen any discussions anywhere of whether these implementations 
> are interoperable.  This might make one a bit nervous.
> There is, of course, a large body of academic work related to the 
> concepts of synchronous and asynchronous in distributed systems in 
> general.  Aside from calling attention above to the classic paper by 
> Lamport I make no attempt to review these materials.
> There is also the issue of the importance of asynchronous Web services 
> to the business community at large.  In this case I will consider myself 
> to be a reasonable source and hereby make some comments:
> Historically Web services originated from the primarily synchronous 
> background of the RPC model.  Although many or most vendors implementing 
> this style of Web services also offer some sort of asynchronous variant, 
> this is not the thrust of the technique and, as noted above, there is no 
> explicit support in the standards nor do I know of detailed discussions 
> of interoperability.  Recent developments in Web services, however, are 
> directed more at the document exchange model which is central to 
> automating business processes.  Many or most of core business processes 
> are, by their nature, asynchronous because the work flows involve steps 
> that include decisions by humans.  Replacing these steps by automation 
> is not on the front burner of business priorities and at the very least 
> would have to proceed by slow, incremental steps.  To start this 
> process, if one indeed wants to start it at all, one must first 
> establish the asynchronous operations.
> There is a considerable distrust among business end users of the idea of 
> using Web services for core business processes.  Although the primary 
> sources of that uneasiness are probably security and reliability, the 
> ability to handle asynchronous processes is also an often-unstated part 
> of the equation and many business people have pigeon-holed Web services, 
> perhaps implicitly, as “suitable only for synchronous operations”.  Web 
> services are simply not widely known as handling asynchronous 
> processes.  Do the Web services standards form an adequate basis for 
> asynchronous implementations?   An essential refusal to address the 
> concepts would send a very negative message to the business community.  
> It would certainly affect the recommendations I would give to my company 
> with respect to strategic positioning of alternative technologies.
> There is no point whatsoever in worrying about how to choreograph or 
> manage Web services if you don’t trust them enough to use them in the 
> first place.
Received on Monday, 4 August 2003 13:02:45 UTC

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