Information Technology Reference
In-Depth Information
Chapter 7
Nigel stepped back from the whiteboard. \Well, at least we agree we need
to have users and sessions in the specication." Claire pursed her lips. \I'm
not sure we mean the same thing, though," she said. \In my computational
model a user has a name and credentials, but in your engineering model it's
just anyone sitting at a browser. My sessions can be suspended, but yours
map directly to network connections, if I understand this constraint here."
Neither of them wanted to change the way they worked and they looked
defiantly at one another, each searching for an opening. This was going to be
dicult.
\Wait a minute, people," said Alex, \this is quite usual, and it is exactly
why I said correspondences are not just a matter of matching names. It seems
clear to me that the correspondences you need here establish a subtype rela-
tionship." They all relaxed slightly. \But how does that cover the dierence in
session lifetimes?" asked Claire. \Well, look at your session states. If a session
exists, it can either be active or suspended. So Nigel's sessions correspond to
your active sessions. We can handle that by putting an extra constraint on the
correspondence between the states." \I think I see," said Nigel, \so that means
we have a subtype relation between the session types and a constrained equiv-
alence relation between the states, but not a one-to-one relation between the
instances." \Yes, that's right. The instance relation is one to many." Claire
smiled. \Actually it's going to be many to many, isn't it? My model has sign
out and sign in operations that can be used in a single communication session,
so that it can carry more than one user session, one after the other."
Nigel walked over to the window and gazed out at the roofline opposite
for a moment; the sun picked out the brilliant orange of the rings of lichen
on the slates. This was another change the cleaner air was allowing back into
the docklands, and he wondered if the moths were changing back to their old
colour to compensate. \What I don't see," he said, turning back to face the
others, \is how changing these constraints works through into implementation
changes in the running system." \Well," Alex began, \there could be a number
of routes, but they all depend on the tools picking up the constraints and
deducing that they imply the choice of suitable engineering templates. The
simplest way is probably to make communications session reuse one of the
selection criteria for picking a binder template.
If there is a computational
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