From requirements to specification
• Specification is a refinement of requirements
• Can be included together as Software
Requirements Specifications (SRS)
• Elaborate on the WHAT? the software system
is supposed to do
– As opposed to HOW it will do it
– Should not be very constraining
– Leave the details to the design
Software modeling
• Modeling techniques – visual, executable,
general, readable, …..
• Modeling
– Behavior: Finite state machine (FSM), Petri net
(PN), Sequence diagram, Activity diagram, and
other UML diagrams
– Data: Entity relationship diagram (ERD)
– Process: Data flow diagram (DFD), Decision table,
decision tree
The Finite State Machine Model
(FSM)
• Used to describe:
– State-based behavior of a process within a software
systems.
– Whole system as one process.
• FSM is used when the number of states the process
can be in is small, and the next state of a process is
determined by its current state and the inputs
applied to the process.
• Typically, the process behavior is be described by the
next process state and the output the process
delivers after accepting an input.
Algebraic definition of an FSM
Properties of an FSM model
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Completeness
Determinism
Fully connected
Strongly connected
Minimal
• Can be verified
Complete FSM
• An FSM is said to be complete or completely
specified, if and only if, at each state, there
exists a transition for each input i  SI.
• In other words, the FSM should react to every
input at every state, and this reaction should
be captured and specified in the model.
Complete FSM
• To produce a robust software system, it is
desirable to specify what to do when both
valid (good) and invalid (bad or unexpected)
inputs are applied at each state of the system.
• Specification completeness would allow the
implementation of a robust process, a
process capable of dealing with both correct
and wrong inputs.
Deterministic FSM
• An FSM is said to be deterministic, if and only
if, at any state, the reaction to an input is
unique and unambiguously determined.
• Uniqueness means that for the same input at
the same state, the FSM should move to the
same next state and delivers the same
output, any time it executes.
Example
• Describe the FSM in both the algebraic and tabular
forms.
• Can you describe in English what does this FSM specify?
• Add the necessary transitions and states to the FSM
diagram. Is the FSM complete and deterministic? why? if
incomplete, can you suggest a modification to complete
it.
Limitations of the FSM
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Conditional behavior specification
Time specification
Data manipulation specification
Concurrency specification
• The Extended FSM – EFSM
Timing and Timeout Specification
• We want to specify that if at a state S no input is present within t
time units, a transition should take place and an output should be
delivered.
– This situation would occur in many real-time processes, and in
user-oriented interfaces and systems.
• We would like to have certain delayed transitions, in which a time
delay is specified instead of an input symbol.
• Presence of an input symbol or time delay is mutually exclusive.
Conditional Transitions
• To specify that, at a given state S, in addition to the
availability of an input symbol, certain predicate(s)
should evaluate to True
– Predicated or conditional transitions.
– Transition takes place only if the predicate evaluates to
true.
• Modified definition: EFSM is said to be
deterministic, if at the same state, for the same
input and the same predicate, the same transition
(next state and output) must occur.
Is this deterministic?
Variable Manipulation
• If at given state of the FSM, the reaction to the
next input symbol must change a certain variable
value needed in future transitions (i.e., in a
predicate evaluation).
• For example, in a user interface driven process, at
the logon state, the reaction to a bad login input
may depend on the number of times a bad login
input was performed earlier.
– A variable holding a counter of the number of bad
logins is needed.
Concurrency Specification
• Due to its inherently sequential nature, a basic
finite state machine cannot model concurrent
behaviors that may be needed to describe
complex concurrent applications.
• Extensions to the FSM were added in the UML
statechart diagram to model concurrent
execution by specifying concurrent substates.
Example
• Suppose that at any state (except the initial
idle state) if we do not receive an input
within 10 time units, we want the system to
be blocked for 5 time units and then move to
the locked state again. Add the necessary
transitions and states to the FSM diagram.
Example
• Partial behavior of ATM:
– “First the user inserts the bank card in the ATM’s card
reader.
– Then, the ATM expects the user to enter the PIN within 20
seconds, otherwise the ATM will beep and eject the card.
– If the PIN is valid, the user is allowed to proceed with a
choice of banking transactions.
– Otherwise if the PIN is invalid, the user can retry for a
maximum of two more trials.
– After 3 wrong trials, the card is confiscated by the ATM,
and the user has to contact the main branch.”