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Pete Cordell writes: > ----- Original Message From: "Costello, Roger L." > > > To minimize the impact of new versions on clients, the web service > > publishes a new URL for each new version. Accordingly, clients can > > update to a new version of the web service when they have thedesire or > > need. > > FWIW, as I understand it, this is the strategy used by Microsoft COM > interfaces. Well, it's not quite the whole story on COM interfaces. A new COM interface, which does indeed get a new unique identifier, is often explicitly defined as an extension of another interface. Indeed, the only exception is the interface known as IUnknown, which is the ultimate base interface for all others. So, the full COM model is that each COM model implements one or more interfaces >each of which< inherits from one or more interfaces and each of which has IUnknown as its ultimate base interface. If you'll excuse the ASCII art, the model of a typical COM object (using IU as a shorthand for IUnknown) is: IU IU IU | | | | | | IA IX IP | | | | | | IB IY | \ | / \ | / \ | / \ | / ------------------- | COM OBJECT | ------------------- If you happen to have a handle on the object cast as IB, then you can using normal C++ conventions (if you're in C++) cast it to IA, since the binary representation of each interface is mapped to C++ vtable forms. If you have that same IB handle then you can from that do ibp->QueryInterface(IY, &ixp). That will fill in ixp with a handle on the same object, but specifically to its implementation of IY. I mention all this, because in addition to the completely decouple versioning that happens when you implement a parallel interface such as IP in the example above, you can also extend interfaces (e.g. IB extends IA). Granting that each interace has a unique name, that sort of extension implies very coupled notions of versioning between IA and IB. Even asking for an IB also gets you an IA. BTW: for anyone interested in comparing this with Java, the key difference is that in COM the same object can provide implementations of more than one method with the same name. For example, if an object for playing card games implements ICardDeck and IGraphics, then the same object can implement ICardDeck::draw() (draw a card off the pile) and IGraphics::draw() ("draw" the card on the screen.) In Java, you can implement lots of interfaces on an object, but if more than one such interace refers to a named method such as "draw", then it has to be the same method in all of them. Noah -------------------------------------- Noah Mendelsohn IBM Corporation One Rogers Street Cambridge, MA 02142 1-617-693-4036 -------------------------------------- "Pete Cordell" <petexmldev@t...> 01/09/2008 07:03 AM To: "Costello, Roger L." <costello@m...>, <xml-dev@l...> cc: (bcc: Noah Mendelsohn/Cambridge/IBM) Subject: Re: RE: Caution using XML Schema backward- or forward-compatibility as a versioning strategy for data exchange ----- Original Message From: "Costello, Roger L." > To minimize the impact of new versions on clients, the web service > publishes a new URL for each new version. Accordingly, clients can > update to a new version of the web service when they have the desire or > need. FWIW, as I understand it, this is the strategy used by Microsoft COM interfaces. Pete Cordell Codalogic Visit http://www.codalogic.com/lmx/ for XML C++ data binding ----- Original Message ----- From: "Costello, Roger L." <costello@m...> To: <xml-dev@l...> Sent: Friday, January 04, 2008 1:26 PM Subject: RE: RE: Caution using XML Schema backward- or forward-compatibility as a versioning strategy for data exchange Hi Fraser, > what approaches we can take to a) identify impacts of > specific types of changes made to the data and/or behavioral > aspects of processing In the scenario that I have been promoting (a web service is deployed and is available to anyone) it is impossible for the web service to know what data changes will impact clients, since the clients are unknown and what they do with the data is unknown. Consequently, the web service operates in its own self-interest: when there is a business need, a new version of the data is created. To minimize the impact of new versions on clients, the web service publishes a new URL for each new version. Accordingly, clients can update to a new version of the web service when they have the desire or need. To be responsive to client wishes and to identify new business opportunities, the web service makes available a feedback web page to its clients. Advantages: 1. The web service is completely decoupled from the clients. The web service needs no knowledge of the clients or their processing. 2. There is no need for the web service to try to "identify impacts of specific types of changes." 3. Versioning is based on business requirements, not on (XML) data validation limitations. 4. Clients are not impacted by version changes, unless they want to be. 5. It's simple. Disadvantage: 1. The web service needs to maintain multiple versions. Thoughts? /Roger -----Original Message----- From: Fraser Goffin [mailto:goffinf@g...] Sent: Friday, January 04, 2008 7:44 AM To: xml-dev@l... Subject: Re: RE: Caution using XML Schema backward- or forward-compatibility as a versioning strategy for data exchange Roger, as Noah mentioned above (copied below) it might be useful to look at some specific approaches for dealing with versioning for the '4 shades' of validation processing mentioned. In particular, although there has been much discussion about the *problems* of versioning i.r.o syntactic and semantic understanding, the thing that probably most of us want to get to is what approaches we can take to a) identify impacts of specific types of changes made to the data and/or behavioral aspects of processing, and b) how to minimise the problem (i.e best practices for designing artefacts that encourage the required level of compatibility). Fraser noah_mendelsohn@u... wrote: > Anyway, I'd say there are at least four shades of grey to consider: > > * Content validation that can be implemented in your schema language (the > element name is legal, and the content is an integer) > * Content validation that your schema language can't handle (the number is > prime) > * Business validation (that looks like a credit card number, but our > records show that the card was stolen, so it's not "valid" for use in a > purchasing transaction) > * Semantic incompatibility (we used to use the field for an account > number, but in Version 2 of the language it identifies a particular credit > card) On 04/01/2008, Costello, Roger L. <costello@m...> wrote: > > > What exactly do you mean by validation? > > Hi Noah. By "validation" I mean the use of any XML-based validation > tools, including grammar-based validation (XML Schema, RELAX NG, DTD) > and rule-based validation (Schematron). By "processing" I mean > everything else that a client does after doing validation. > > /Roger > > -----Original Message----- > From: noah_mendelsohn@u... [mailto:noah_mendelsohn@u...] > Sent: Thursday, January 03, 2008 5:56 PM > To: Costello, Roger L. > Cc: xml-dev@l... > Subject: RE: RE: Caution using XML Schema backward- or > forward-compatibility as a versioning strategy for data exchange > > Roger: > > I think this discussion would converge more quickly if you would > rigorously define the terms in the propositions below. What exactly do > > you mean by validation, for example? Let's say I have a purchase order > > document and I: > > * Use XSD to make sure a credit card number element is in the right > place > in the document > * Use Schematron to make sure the expiration date on it is later than > the > order date on some element far away in the same document > * Use the Java language to pull the credit card number out of the XML > DOM > and make sure that some digits in the number properly checksum [1] the > others (You could probably do this in SchemaTron with some work, or in > Schema 1.1 assertions if we allowed them on simple types, but let's > assume > just for the moment that the checksum required computation beyond what > the > schema languages could do, or that you chose not to mess with coding > the > LUHN algorithm in XPath. See [2] for basic information on credit card > number checksums.) > * Use the Java language to open a database of stolen credit card > numbers > to ensure that the card is still "valid" and not stolen > * Use the Java language to place to the order and send a Web Services > message to bill the card > > Which of those steps do you define as "validation", and which as > "processing"7? Unless you quite carefully define what you mean by > processing and what you mean by validation, then it's hard to consider > an > assertion that: > > 1. Validating data is different from processing data. > > Indeed, the assertion may follow from or be contradicted by the > definitions that you choose, I would think. Thanks! > > Noah > > [1] http://en.wikipedia.org/wiki/Luhn_algorithm > [2] http://en.wikipedia.org/wiki/Credit_card_number > > -------------------------------------- > Noah Mendelsohn > IBM Corporation > One Rogers Street > Cambridge, MA 02142 > 1-617-693-4036 > -------------------------------------- > > > > > > > > > "Costello, Roger L." <costello@m...> > 12/28/2007 09:02 AM > > To: <xml-dev@l...> > cc: (bcc: Noah Mendelsohn/Cambridge/IBM) > Subject: RE: RE: Caution using XML Schema > backward- or forward-compatibility as a versioning strategy for data > exchange > > > Hi Folks, > > The discussion has been truly excellent. It has clarified many > concepts for me. Thank you! > > Below is a summary of my understanding of the key concepts that have > emerged from our discussion. Do you agree with them? If not, which > ones do you not agree with? /Roger > > > RELATIONSHIP BETWEEN DATA PROCESSING, DATA VERSIONING, AND DATA > VALIDATION > > 1. Validating data is different from processing data. > > 2. Just because an application can validate some data doesn't mean it > can process the data. > > 2.1 Just because an application can process some data that it validated > doesn't mean that *any* data it validates can be processed. > > 3. A backward-compatible XML Schema means that a new version of the XML > Schema can validate instance documents conforming to an old version of > the XML Schema. Consider an application that is designed to process > the old instance documents, and suppose that it has obtained the new, > backward-compatible XML Schema. Now it can validate both old instance > documents as well as new instance documents. However, just because it > can validate the new instance documents doesn't mean it can process > them. > > 4. A forward-compatible XML Schema means that an old version of the XML > Schema can validate instance documents conforming to a new version of > the XML Schema. Consider an application that is designed to process > the old instance documents. It can validate both old instance > documents as well as new instance documents. However, just because it > can validate the new instance documents doesn't mean it can process > them. > > The following items are targeted at this scenario: a web service has > unknown clients (anyone can use the service); the data it makes > available to clients is described by an XML Schema (identified in a > WSDL document) and some English prose (in a web page); periodically the > data is changed (i.e. new version). See the Amazon web service for an > example. > > 5. Versioning the data made available by the web service based on > backward- or forward-compatible XML Schemas imposes severe restrictions > on the types of changes permitted; these restrictions may not be > consistent with the needs of the business (the "business" is all the > technical, political, and managerial stuff that went into funding, > creating, deploying, and maintaining the web service). > > 6. Don't base your web service data versioning strategy on a data > validation strategy. Decouple your data versioning strategy from your > data validation strategy. > > 7. Base your web service data versioning strategy on business needs. > > > NOTES > > The assertions identify XML Schemas as the validation language, but the > assertions apply to any validation language, such as RELAX NG, DTD, or > Schematron. > > _______________________________________________________________________ > > XML-DEV is a publicly archived, unmoderated list hosted by OASIS > to support XML implementation and development. To minimize > spam in the archives, you must subscribe before posting. > > [Un]Subscribe/change address: http://www.oasis-open.org/mlmanage/ > Or unsubscribe: xml-dev-unsubscribe@l... > subscribe: xml-dev-subscribe@l... > List archive: http://lists.xml.org/archives/xml-dev/ > List Guidelines: http://www.oasis-open.org/maillists/guidelines.php > > > > > _______________________________________________________________________ > > XML-DEV is a publicly archived, unmoderated list hosted by OASIS > to support XML implementation and development. 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