From: vixie@decwrl.UUCP (Paul A Vixie)
Subject: A parable about generality in architecture
Date: 27 Jun 90 23:30:08 GMT
 
-From:	TLE::DIEWALD "Means, Motive, and Opportunity  14-Jun-1990 0958"   
 
Once upon a time, in a kingdom not far from here, a king summoned two of his
advisors for a test.  He showed them both a shiny metal box with two slots in
the top, a control knob, and a lever.  "What do you think this is?"
 
One advisor, an engineer, answered first. "It is a toaster," he said.  The
king asked, "How would you design an embedded computer for it?" The engineer
replied, "Using a four-bit microcontroller, I would write a simple program
that reads the darkness knob and quantizes its position to one of 16 shades of
darkness, from snow white to coal black. The program would use that darkness
level as the index to a 16-element table of initial timer values.  Then it
would turn on the heating elements and start the timer with the initial value
selected from the table.  At the end of the time delay, it would turn off the
heat and pop up the toast.  Come back next week, and I'll show you a working
prototype."
 
The second advisor, a computer scientist, immediately recognized the danger of
such short-sighted thinking.  He said, "Toasters don't just turn bread into
toast, they are also used to warm frozen waffles.  What you see before you is
really a breakfast food cooker.  As the subjects of your kingdom become more
sophisticated, they will demand more capabilities.  They will need a breakfast
food cooker that can also cook sausage, fry bacon, and make scrambled eggs.  A
toaster that only makes toast will soon be obsolete.  If we don't look to the
future, we will have to completely redesign the toaster in just a few years."
 
"With this in mind, we can formulate a more intelligent solution to the
problem.  First, create a class of breakfast foods. Specialize this class into
subclasses: grains, pork, and poultry.  The specialization process should be
repeated with grains divided into toast, muffins, pancakes, and waffles; pork
divided into sausage, links, and bacon; and poultry divided into scrambled
eggs, hard-boiled eggs, poached eggs, fried eggs, and various omelet classes."
 
"The ham and cheese omelet class is worth special attention because it must
inherit characteristics from the pork, dairy, and poultry classes.  Thus, we
see that the problem cannot be properly solved without multiple inheritance.
At run time, the program must create the proper object and send a message to
the object that says, 'Cook yourself.' The semantics of this message depend,
of course, on the kind of object, so they have a different meaning to a piece
of toast than to scrambled eggs."
 
"Reviewing the process so far, we see that the analysis phase has revealed
that the primary requirement is to cook any kind of breakfast food.  In the
design phase, we have discovered some derived requirements.  Specifically, we
need an object-oriented language with multiple inheritance.  Of course, users
don't want the eggs to get cold while the bacon is frying, so concurrent
processing is required, too."
 
"We must not forget the user interface.  The lever that lowers the food lacks
versatility, and the darkness knob is confusing.  Users won't buy the product
unless it has a user-friendly, graphical interface.  When the breakfast cooker
is plugged in, users should see a cowboy boot on the screen.  Users click on
it, and the message 'Booting UNIX v. 8.3' appears on the screen. (UNIX 8.3
should be out by the time the product gets to the market.)  Users can pull
down a menu and click on the foods they want to cook."
 
"Having made the wise decision of specifying the software first in the design
phase, all that remains is to pick an adequate hardware platform for the
implementation phase.  An Intel 80386 with 8MB of memory, a 30MB hard disk,
and a VGA monitor should be sufficient.  If you select a multitasking, object
oriented language that supports multiple inheritance and has a built-in GUI,
writing the program will be a snap.  (Imagine the difficulty we would have had
if we had foolishly allowed a hardware-first design strategy to lock us into a
four-bit microcontroller!)."
 
The king had the computer scientist thrown in the moat, and they all lived
happily ever after.