Information Technology Reference
In-Depth Information
Chapter 6
Technology Viewpoint
\The King is dead; long live the King," muttered Alex as he stared at
the huge notched axe in its glass case. He had insisted on coming here when
Trevor and Trudy had admitted that neither of them had ever been inside the
White Castle, even though they worked only three blocks away. It was well
worth a Saturday morning to see their reaction to the scale of the place.
\I never did understand that saying," said Trevor. \Surely either he's dead
or he isn't. He can't be both." \It's making a deliberate contrast between
two views of kingship," said Alex. \The oce of king continues, with the heir
taking over instantaneously when the old king dies. So one instantiation of
the king terminates, but there is still a king, even though it's not the same
individual." Trudy grinned. \It sounds more like a Hydra in this case," she
said. \But it seems a bit of a funny way of looking at it."
\Do you think so? Think about the specication of a call centre operator
we were reviewing on Friday. The operator deals with one customer at a
time. A particular dialogue is terminated and the customer description in the
operator's state is destroyed. The immediately following action responds to a
new customer by instantiating a new description. In our specification, it was
still a customer, but a dierent instance."
\But hang on," said Trevor, turning from an array of crossbows to join the
conversation, \aren't both the operator and the customer just roles?" \Yes
they are, but my point is that rules for filling them are different, because one
operator deals with many customers over a session, but there is a level of
continuity in the model. Even if we bind a specific employee to the operator
role, there is still a steady flow of different customers, and their identities are
chosen by the environment."
They walked through into the banqueting hall, its dark vaulted roof hung
with banners. \If you don't like that one, think about the system congu-
rations we have been describing. The elements in the description have quite
different lifecycles and ownership. The main servers are fully determined, and
we know all about them, right down to their serial numbers and the colour of
the cabinets. The local networks are under our management, but are recon-
figured and upgraded all the time; we try to track the changes, but they are
largely self-configuring at the detailed level, so we concentrate on capturing
major properties, like VLAN structure and security domains. The customer
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