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I think its a matter of how the external interface contract is specified.
If you advertise yourself as using the industry standard schema, then
that sort-of resolves the issue. The obligations from the advertisment
go both ways.
If you don't, how do you convey your flavour of the schema to someone
else witout opening yourself up to endless tedious discussion of what
the element relationships are? There are options, other people have
suggested them, but you need to look at costs and benefits and at your
capacity to impose the model.
You could specify things in terms of additional technologies, such as
Rick Jelliffe's suggestion, but if the schema is non-trivial then that
is a large amount of work in a technology that is different to the
standard specification, which might be a hard sell. A test-driven
approach assumes that you know something about the stability of your
clients and that there are not too many of them. This might be the case.
Making your own version of the standard schema would be feasible if you
have significant market power, but there is a lack of tools to maintain
the delta on the standard schema in a consistent way and your partners
may not be all that wild about having to support your variant as well as
the standard. If you de-facto define the standard then that is
different, your market power will encourage or require the other
participants in the market to support your model. There was a
discussion some time ago about profiling schemas in the same way that
EDI messages are profiled, but the idea was unexpectedly novel to a lot
of people here.
Fraser Goffin wrote:
> Thanks Greg, some interesting points to consider.
> I am mostly concerned with B2B. One of the issues I'm wrestling with
> is that :-
> a. the service contract is defined by an external standards body (we
> are but one implementer).
> b. the data model that underpins the service operations are defined
> using XML schema and these reflect the broad business semantics for
> each operation (as agreed by a panel of contributors from our industry
> c. our business rules (in terms of what data content/structural
> constraints that would be acceptable) are less strict than the XML
> schema specifies (for example we may be tolerant of missing data).
> So I guess I was considering whether we should validate according to
> our internal business rules rather than that of the externally defined
> contract, even when this can mean that a message received could be
> schema invalid (according to the industry standard definition) ?
>> From: Greg Hunt <email@example.com>
>> To: Fraser Goffin <firstname.lastname@example.org>, email@example.com
>> Subject: Re: [xml-dev] Validation - Is it worth it ?
>> Date: Sun, 08 Jan 2006 10:31:11 +1100
>> I know that "be liberal in what you accept and strict in what you
>> emit" is firmly embedded in the race memory now, but I am not sure
>> that that applies to the technical aspects of B2B type transactions.
>> Why do you want to process messages that "may" be processable? If
>> you keep in mind a distinction between technical processes and
>> business processes, there is less doubt. Technically acceptable
>> messages should enable the business process. Acceptance of the
>> message should mean that there is a business process that will handle
>> the message. Rejection of the message should indicate technical
>> issues. Anything else is probably not scalable in terms of effort
>> and in terms of guaranteed processing time. It seems to come down to
>> whether the business rules for fixing the message data can be defined
>> and used. We need to be careful that we are not also fixing the
>> semantics of the messages that we receive.
>> If there are business rules that define the changes to data content
>> that are acceptable then there is no doubt about whether the message
>> can be processed, what is in doubt is whether in the end you get a
>> business transaction (money) out of it.
>> If there are technical issues with a message, if the received
>> structure is wrong or unexpected for some reason, then the semantics
>> are also in doubt and passing what is probably machine generated data
>> to a person to resolve an issue involves asking that person to decide
>> whether this is a bug or something unexpected in the mind of a
>> programmer. It is easier and more reliable to ask the originator
>> what is going on than it is to ask a human to interpret a broken XML
>> structure for, for example, a purchase order (meaning that I am not
>> sure that you can always push the message to a person to have them
>> look at it in a scalable, business-meaningful way).
>> If this is B2B type traffic, who pays for getting the semantics of
>> the message wrong? If you receive a value transaction from someone
>> and "fix" it in some technical and non-trivial way, who now pays for
>> the transaction? Is the cost of sending and receiving the message so
>> high that the time and effort of a human and the associated risk of
>> further error is warranted?
>> Fraser Goffin wrote:
>>> Recently I've been involved in building a validation process using a
>>> combination of schema based and rules (schemaTron) and it got me
>>> thinking about how much validation is the right amount.
>>> The 'type' of validation processing I'm talking about is that which
>>> might be performed at a B2B gateway and is perhaps better
>>> categorized as 'technical' validation (ie. basic structural
>>> conformance and some content) rather than business rules (although
>>> the distinction is pretty thin).
>>>> From the business perspective, it is undesirable to reject any
>>>> message and
>>> thus lose an opportunity to complete a transaction. So from this
>>> point of view one might imagine that validation at this stage should
>>> be minimal, perhaps not even full schema (or perhaps a 'more
>>> relaxed' version of the published interface). This might be
>>> justified on the basis that rules, perhaps in a business process
>>> engine or application logic, are better at determining whether a
>>> message is business processable or not. Plus one can always push
>>> messages to a manual process and let a human decide !
>>> On the flip side, we want to protect the integrity of our
>>> operational systems from erroneous data and, perhaps the most
>>> obvious reason, validation can provide an optimization of the
>>> process in the sense that, when the interaction is asynchronous (and
>>> possibly long running), it may be preferable to let a caller know
>>> right away that a message has some 'bad data' rather than for them
>>> to find that out some time later after having received an initial
>>> acknowledgement of receipt.
>>> To me this highlights the conundrum of a desire for strongly typed
>>> [service] interfaces versus the looser coupling and tolerance to
>>> change that we also typically seek. I am trying to find the 'sweet
>>> spot' that allows through messages that 'may' be processable, but
>>> rejects those where even if directed to a manual (human workflow)
>>> process would still not be worth the effort. I sometimes refer to
>>> this as 'compatible' messages versus enforcing strict adherence to a
>>> technical specification.
>>> I also have noted that versioning service interfaces (or even just
>>> XML schemas) can be somewhat problematic and can exacerbate
>>> validation issues, and to some extent mitigates against using them
>>> for validation purposes, particularly if they haven't been designed
>>> with any extensibility mechanisms at all to accommodate 'non
>>> breaking' change (e.g. xs:any/anyAttribute).
>>> Some of you may be thinking 'is there a question here anywhere ?',
>>> sorry I have meandered on somewhat. What I'm really after is finding
>>> out what others have found to be a good approach to message
>>> validation and whether there are views about how to achieve a
>>> balance between optimizing business opportunity and rejecting 'junk
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