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No, I am arguing that the practical capacity/performance limitations are
different to the theoretical data model limitations and that we need to
express both. The capacity limitations of significant systems are not
transitory, they hang around for a long time and are based on SLAs
established for those systems on well defined hardware. SLAs that cost
money when they are not met.
The majority of XML standards activity or B2? XML application
development is not aimed at things that are likely to get processed on
my son's desktop machine or on my laptop, it is aimed at larger servers
and higher transaction rates. What I am saying is that the majority of
XML standardisation activity or bespoke code is not dealing with HTML or
RSS and so <p> elements or <item> elements are not the best examples,
better examples would be invoice line items, participant addresses or
criminal charges, things that may (and usually do) have strong
performance criteria associated with them. That said, lets run with
your example for a minute.
Sure, the performance of internet client hardware and software rises
over time, but usability guidelines for webpages still specify an upper
limit on page size which is not changing at the rate that Moores law
would suggest (the US is still around 50% dial-up internet access, I am
extrapolating from comments by Neilsen last year) and in any case
internet client hardware is not really where the majority of business
XML processing takes place.
When we design websites we (should) identify a target page size and
communicate that to our developers and that then places an implicit
limit on the number of elements in the document (the Microsoft homepage
is about 20k of HTML). The cost associated with web page access is
largely network transit time so physical page size is a good measure of
performance for most pages, and in any case if a page doesn't load fast
all it does is annoy the users. The processing cost associated with XML
documents in a business context is more likely to be tied to the
elements in the document (who cares, within limits, how long an invoice
takes to arrive? but businesses do care about getting through their 50k
invoices per day) and taking too long to process them will cost someone
money . So why are we putting the page size/document element count
outside of the system? With HTML we have no choice and in any case, the
page size is under our control because we construct it and send it out,
so we design and test our own pages. With XML we need to tell our
partners what is acceptable so that we can process what they send to us
as quickly as we say we will (see the example I gave of the EDI
profile). Those limits will not go away. Major corporations will not
change their behaviour. The question is whether there will be tool
support for those constraints and whether I have to write code to
structurally validate something that already has its structure partially
specified by a schema.
If we don't have tool support for this, and I am thinking of structural
validation, then we will have the same problems that occur with EDI.
Lots of standards that are modified by their users in small ways that do
not have consistent tool support, which ultimately reduces the usability
and interoperability of those standard data structures.
The question from my earlier email is still there:
If we don't put these limits in the schema, they just have to go
somewhere else, somewhere less visible, less maintainable, and with less
tool support. How SHOULD we do this if we aren't using the schema for
validation of these constraints?
Dare Obasanjo wrote:
>You see to be arguing that transient system limitations (memory constraints, CPU processing power, etc) should end up making their way into schemas of formal specifications. Using that argument what should have been the limit on <p> elements in HTML or <item> elements in RSS based on the typical machine's processing power from the 1990s? After putting those limitations in their schemas would we have had to rev them every 18 months to account for Moore's law?