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Time Paper Description Conceptual Mode of
Sheet Focus Operation
00:00 Gets the problem statement handed over. Expresses some
difficulties in understanding the specififcation on
available hardware.
00:02 #1 Draws a sketch of a building with a number of elevators Drawing
and floors. This is a virtuel building with n floors and
m elevators. Afterwards he uses a couple of minutes to
understand the rules under which the elevators have to
move.
00:05 Asks what to understand by the sequentially servicing as Asking
stated in the problem statement. Observer clarifies the
uncertainties.
00:07 Expresses that the task consists of a number of control Independence Reflecting
units namely the elevators. States that the elevator is Communication
to be compared with an independent entity. Outlines some
properties of the future solution as he sees it. He says
that the floors can communicate with the elevator and
that the elevator then can decide to which floors it
wants to visit. Further, he says that the floors will not
communicate with each other. He also states that some of
his knowledge of this problem is not only based upon the
problem statement, but also on his general knowledge of
elevators.
00:10 Gives a first draft outline of his future solution. He is Parallel Drawing
of the opinion that he will solve the task by having only Explaining
one elevator servicing the floors. He considers the
elevators to be non-committal and hence he will be able
to put the elevators together in parallel later on (this
can easily be done in CCS). The people on the floors and
the elevator are then able to influence the behaviour of
the elevator by pushing the buttons. He is not quite sure
if it is okay to make the elevators independent, but he
chooses this solution for the present.
00:11 Starts to specify. Introduces the concept of an elevator Proces Specifying
shaft. He defines a shaft to consist of an elevator and Parallel
all the floors. States that the elevator and each of the Synchronization
floors are non-deterministic final automats which again
could be compared to processes. The task at this point is
to specify these in more detail. Afterwards one can then
put these into parallel. It is, however, also possible to
synchronize them. This is in CCS done by having some
transitions with the same label. Describes the aspect of
synchronization with the opening of doors in a elevator
system where the elevator and the floors synchronize to
achieve this action. According to his opinion this is how
its done in real life and it is easy to model in CCS.
00:15 States that he now has a more general design sketch for
his solution. However, afterwards he starts to discuss
aspects of problem statement where he realizes that
people can push the buttons in a randomized order.
00:17 #2 Starts to specify the entity of an elevator. This first Specifying
shot involves an elevator and a push at a pushbutton. He
outlines what states it goes from and to.
00:18 Realizes quickly that he needs some sort of a medium in Complexity Reflecting
which he can storage the requests from already pushed
buttons. This introduces some difficulties with his form
of notation.
00:20 Asks observer about some uncertainties regarding how
people can register requests and how these request should
be serviced. Agrees with the observer that he might skip
some of the information in the beginning.
00:21 Starts to specify the elevator again. This time he has Synchronization Specifying
introduced a parameter that indicates which floors have
registered requests. Outlines the consequences of pushing
a button on a floor.
00:23 Realizes that the elevator also can turn into a reduced
state when the elevator arrives at a floor registered in
the parameter. States that he now has to look at the
behaviour of a floor.
00:26 Starts to look at the problem statement again to resolve
some uncertainties. This includes aspects of how an
elevator change direction and what happens when an
elevator has no more requests.
00:27 Continues with the specifying. Wants to add some "things" Specifying
the state of an elevator. Expresses that he also has to
incorporate what floor the elevator is currently located
on. Meditates for a while of this problem.
00:29 #3 Specifies a floor in its initel state (a floor without Synchrinization Specifying
any requests). Outlines from this floor that the only
possible actions are pushing up or down. Draws the states
to which the actions lead. The outcome of this is
actually a state transistion diagram. He says that the
state Elevator-Up is a state where a floor is waiting for
an elevator to arrive and that this is elevator is going
up.
00:33 Wants to build in some extra functionality into the Complexity Explaining
elevator. Says that at first he will only modell an
elevator that is going all the way up to the top floor
and then all the way down in order to reduce the
complexity of the solution.
00:34 #4 Starts a new specification of the elevator. Specifying
00:35 #2 Realizes he lacks some information on paper sheet number Complexity Explaining
2. Introduces a limitation where it is only possible to Time aspects
push buttons that have not been pushed yet. Gives an
informal introduction to the purpose and working of CCS.
He stresses that the actions in CCS are of an atomar kind
which implies that the execution of an action takes no
time. He claims the existence of an enhanced version of
CCS where the aspect of time has been incorporated and
the use of this version would be much more applicable
with this problem. However, this will introduce a lot
more complexity and hence he will not introduce in this
solution. He will rather have a more untrustworthy yet
simple solution.
00:37 #4 Returns to the specifying of an up elevator. Outlines the Communication Specifying
actions possible for an elevator going up. He is aware of
the fact that this solution is neither efficient nor
economical sound. The specification further involves the
pushing of not yet pushed buttons.
00:38 Suddenly he realizes that with his current solution the Reflecting
elevators will move up and down all time even without any
requests. He is not quite sure how this can be fixed.
00:40 Continues with the specifying of an up elevator. Specifying
Specifies that the up elevator turns into a down elevator
with it reaches the top floor.
00:42 Stops to reflect upon additional conditions that have to
hold for a floor. Says thát the pushing on buttons is
provided the users.
00:43 #5 Draws an abstract symbol of an elevator with one Synchronization
transparent action push. Assumes that the users will push
the buttons whenever they fell like it. In his solution
he will not modell the users. Draws also an abstract
symbol of a floor and says the these two synchronize upon
the action of opening a door or closing a door.
00:47 #4 Returns to his specification of the up elevator where he Complexity Specifying
wants to incorporate the aspect of opening a door.
Realizes that it is not quite simple to specify which
floors the elevator has to visit. His previously designed
attribute describing all pushed buttons only involve the
elevators pushbuttons and not the floors. Reduces some
complexity by replacing the action open door with the
action sync. Action sync defines that the doors open,
people get out and in, and the doors close again.
00:51 #3 Returns to his state transistion diagram to incorporate Time aspects Specifying
his latest knowledge of the action sync. Stresses once
more that he is not modelling the time aspect. Explains
how the transition diagram should be interpreted.
00:54 #4 Adds some information to his up elevator. Tries to figure Synchronization Specifying
out when the floor is synchronizing with the elevator.
00:58 Talks informally about actions that the world must not Reflecting
see in this kind of systems. Visualizes it with a tall Drawing
building where people enter the building and later leave
the building. The outside world does not know what has
happen in between. He is uncertain whether the sync
action should be unvisible to the outside world.
Expresses difficulties how this can be specified in CCS.
01:00 #3 Adds information to his state transistion diagram about a Specifying
down elevator.
01:04 #6 Starts to draw a state transition diagram for the up
elevator. Seems to be faced with some difficulties and
gradually stops to think aloud during this period. After
a couple of minutes he asks if the observer wants another
CCS expression or maybe a more realistic solution where
the elevators do not have to go all the way up or down.
01:07 Observer says that it could be interesting to see what
synchronization are involved between elevators.
01:09 During this discussion he realizes that he has
misunderstood the functions of the pushbuttons on the
floors. Has some difficulties in understanding that there
is only one set of pushbuttons at each floor.
01:13 States that the elevators have to have more information Communication Explaining
on each other at all times. Claims that it is not very
difficult to get the elevators communicate all they have
to do is to live in the same universe.
01:14 #5 Sketches a possible solution for above outlined problem. Drawing
Here he is using a more informal type of notation. He is
of the opinion that deciding which elevator that has to
serve a request is a trivial task. However, he soon
realizes that more factors influence this decision: which
elevator is nearest, which direction is it going etc. He
changes his opinion of this being a trivial task.
01:17 States that it is possible to design a more decentralized Decentralizatio
solution in CCS. This would imply that elevators would n Broadcast
hear a request from a floor and then they should decide
which elevator to service the request autonomously. But
he stresses that this is not easily solved. Claims that
the use of broadcast should be avoided.
01:19 Observer says that it would be interesting to see how the
avoidence of having all elevators service a request from
a floor. The observer also asks whether the solution
avoids starvation whereto he replies that since the
elevators move all the way up and all the way down no
floor will suffer from starvation in his solution.
Observer and designer discusses the potential content of
the rest of the experiment and agree upon looking at the
synchronization between elevators.
01:22 #7 Starts to specify an action for receiving requests from Communication Specifying
elevators. Says that when this action has received a
request it has to go into another state where the request
is been processed.
01:23 Suddenly stops the specifying and states that you get Communication Explaining
some properties for free in CCS. He continues, when a
floor has a request then it is only one elevator that are
able to "hear" this fact. This is defined by the
operational semantics. Illustrates this fact on middle of
paper sheet number 7. So when using CCS a request from a
floor is only serviced by one elevator.
01:28 #4 States that the just specified action on paper sheet
number 7 could be incorporated into the up elevator
action. However, he realizes that the chosen elevator for
a request may likely not be the optimal one.
01:30 #7 Continues with the specifying. The spcification is the Synchronization Specifying
final version of the up elevator where all possible
actions are included. Part of this action is already
outlined at paper sheet number 4. The actions includes
the movement of the elevator in an upward direction, the
receiving of requests from all the floors, and the
synchronizing with the floors when people wants to get on
or off. After the specifying, he compares the actions
with the state transition diagram illustrated at paper
sheet number 3.
01:34 States that the solution so far could be put into a Parallel
parallel composition consisting of the floors and the
elevators.
01:35 Starts to simulate a scenario in order to test his Synchronization Experimenting
solution. Adds some information to his state transistion
diagram for the elevator. Asks himself when the action
sync can be performed. Says that this can be performed
when a request has been registered either in the elevator
or on the floor. Thereby you can perform the action sync
and let the floor and the elevator synchronize. Stresses
once more that his current solution is certainly not the
most optimal one. When a request has be given at one of
the floors this request will be heard by one of the
elevator. However, as he points out you do not know which
elevator will hear this request.
01:39 He claims that he would be nice to have a medium in CCS Communication
where you could exchange information which could be heard
be all entities involved. Then he would like to have some
intelligent unit that could tell the elevators which to
service the requests. But this cannot be done in pure CCS
where only two entities can communicate at a time. He is,
however, of the opinion that this could be solved by
introducing a communication function.
01:41 #8 Draws a sketch of some elevators in order to illustrate Communication Drawing
how the above communication function could be designed.
This sketch is of a very informal nature with respect to
CCS.
01:45 States that the deciding of the closest elevator to Complexity Explaining
service a request is like hell in CCS. He would rather
describe such aspects in another language e.g. Pascal. He
further says that at first the algorithm looked quite
simple but he later realized that this was not the case.
01:46 Says that CCS can be used when you are designing a system Parallel
with a lot of components. For each of these components Synchronization
you can define a state machine. The semantics for letting
them act in parallel has been given by the paradigm. And
yo can further let them synchronize on certain actions.
01:49 States that the cores of the CCS are the aspects of Concurrency Reflecting
concurrency and communication. CCS solves the problem of Communication
describing state machines living in parallel. This would Parallel
be very complicated in e.g. Pascal wher you don not have
the same medium for expressing these things. So finally,
he is of the opinion that CCS is well-suited for
specification but not that well-suited for actual
implementation.
01:52 Says that the concept of proces is blurred in CCS. One Proces Explaining
can consider the higher level entities as processes but
it is also possible to consider the lower level entities
as processes. He has difficulties in determine more
abstract processes in his solution.
01:55 Observer and designer discusses some aspects of processes
and concurrency informally.