Top-down or bottom-up?

David Megginson david at
Thu Jun 17 17:02:32 BST 1999

Len Bullard writes:

 > > That's not what I mean -- creating a data model is tractable, but a
 > > data model is of questionable value if it's not based on a fairly
 > > accurate business model, use cases, etc.  I don't think that any of us
 > > can reasonably draw up a reliable business model that will cover the
 > > Web for the next five years, and even the use cases will be pretty
 > > shakey.  Without good models, bottom-up is our best bet.
 > Umm.. isn't that why we do markup and write DTDs?  They may not
 > last forever, but like mudbricks that cleave, they make a
 > reasonable structure and, well, beat the heck out of fighting bears
 > for caves.

Just so, but imagine if we had all waited to start writing DTDs until
ISO approved a master document architecture that would govern all DTDs
in the SGML and XML document space (kudos to HyTime for trying,

This is the point that I (and others) have been making in this
discussion: a top-down approach (start with the master architecture)
can work for something like a new parts-management system for; a bottom-up approach (start with the components, such as
individual specs and DTDs) is pretty much required in an open and
fast-changing system like the Web.

As Paul Prescod has pointed out, however, in both cases the process is
really iterative: in a bottom-up approach, it's often useful to stop
and throw together a straw-man architecture to see if what we've done
so far makes sense together; in a top-down approach, it's often useful 
to stop and throw together some proof-of-concept components, to see if 
there will be any obvious implementation problems.  The difference
comes simply from which of the two is formalized.

All the best,


David Megginson                 david at

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