Specification

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Specification
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Outline

• Discussion of the term "specification"
• Types of specifications
• operational
• Data Flow Diagrams
• (Some) UML diagrams
• Finite State Machines
• Petri Nets
• descriptive
• Entity Relationship Diagrams
• Logic-based notations
• Algebraic notations
• Languages for modular specifications
• Statecharts
• Z

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Specification

• A broad term that means definition
• Used at different stages of software development
  for different purposes
• Generally, a statement of agreement (contract)
  between
• producer and consumer of a service
• implementer and user
• All desirable qualities must be specified

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Uses of specification

• Statement of user requirements
• major failures occur because of misunderstandings
  between the producer and the user
• Lack of knowledge of computer
• In most cases the owner does not exactly know
  what he/she wants to be developed
• "The hardest single part of building a software
  system is deciding precisely what to build"
  (F. Brooks)

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Uses of specification (cont.)

• Statement of the interface between the machine
  and the controlled environment
• serious undesirable effects can result due to
  misunderstandings between software engineers and
  domain experts about the phenomena affecting the
  control function to be implemented by software

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Uses of specification (cont.)

• Statement of requirements for implementation
• design process is a chain of specification (i.e.,
  definition)implementationverification steps
• requirements specification refers to definition
  of external behavior
• design specification must be verified against it
• design specification refers to definition of the
  software architecture
• code must be verified against it

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Uses of specification (cont.)

• A reference point during maintenance
• corrective maintenance only changes
  implementation
• adaptive and perfective maintenance occur because
  of requirements changes
• requirements specification must change accordingly

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Specification qualities

• Precise, clear, unambiguous
• Consistent
• Complete
• internal completeness
• external completeness
• Incremental

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Clear, unambiguous, understandable

• Example specification fragment for a
  word-processor

Selecting is the process of designating areas of
the document that you want to work on. Most
editing and formatting actions require two
steps first you select what you want to work
on, such as text or graphics then you initiate
the appropriate action.
can an area be scattered?
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Precise, unambiguous, clear

• Another example (from a real safety-critical
  system)

The message must be triplicated. The three copies
must be forwarded through three different
physical channels. The receiver accepts the
message on the basis of a two-out-of-three
voting policy.
can a message be accepted as soon as we receive 2
out of 3 identical copies of message or do we
need to wait for receipt of the 3rd?
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Consistent

• Example specification fragment for a
  word-processor

The whole text should be kept in lines of equal
length. The length is specified by the user.
Unless the user gives an explicit hyphenation
command, a carriage return should occur only at
the end of a word.
What if the length of a word exceeds the length
of the line?
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Complete

• Internal completeness
• the specification must define any new concept or
  terminology that it uses
• glossary helpful for this purpose
• the specification must document all the needed
  requirements
• difficulty when should one stop?

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Incremental

• Referring to the specification process
• start from a sketchy document and progressively
  add details
• Referring to the specification document
• document is structured and can be understood in
  increments

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Classification of specification styles

• Informal, semi-formal, formal
• Operational
• Behavior specification in terms of some abstract
  machine
• Descriptive
• Behavior described in terms of properties and
  input/output relation.

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Example 1

• Specification of a geometric figure E

E can be drawn as follows 1. Select two points
P1 and P2 on a plane 2. Get a string of a certain
length and fix its ends to P1 and P2 3. Position
a pencil as shown in next figure 4. Move the pen
clockwise, keeping the string tightly stretched,
until you reach the point where you started
drawing
this is an operational specification
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A descriptive specification

• Ellipse E is described by the following equation
• ax2 by2 c 0
• where a, b, and c are suitable constants

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Another exampleSorting an array of elements

• Let a be an array of n elements. The result of
  its sorting is an array b of n elements such that
  the first element of b is the minimum of a (if
  several elements of a have the same value, any
  one of them is acceptable) the second element of
  b is the minimum of the array of n-1 elements
  obtained from a by removing its minimum element
  and so on until all n elements of a have been
  removed.

OP

• The result of sorting array a is an array b
  which is a permutation of a and is sorted.

DES
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How to verify a specification?

• Observe dynamic behavior of specified system
  (simulation, prototyping, testing specs)
• Analyze properties of the specified system
• Analogy with traditional engineering
• physical model of a bridge
• mathematical model of a bridge

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Data Flow Diagrams (DFDs)

• A semi-formal operational specification
• System viewed as collection of data manipulated
  by functions
• Data can be persistent
• they are stored in data repositories
• Data can flow
• they are represented by data flows
• DFDs have a graphical notation

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Graphical notation

• bubbles represent functions
• arcs represent data flows
• open boxes represent persistent store
• closed boxes represent I/O interaction

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Example
specifies evaluation of (a b) (c a d)
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A construction method (1)
1. Start from the context diagram
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A construction method (2)
2. Proceed by refinements until you reach
elementary functions (preserve balancing)
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A library example
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Refinement ofGet a book
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Patient monitoring systems
The purpose is to monitor the patients vital
factors--blood, pressure, temperature, --reading
them at specified frequencies from analog devices
and storing readings in a DB. If readings fall
outside the range specified for patient or
device fails an alarm must be sent to a nurse.
The system also provides reports.
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A refinement
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More refinement
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An evaluation of DFDs (1)

• Easy to read, but
• Informal semantics
• How to define leaf functions?
• Inherent ambiguities

• Outputs from A, B, C are
• all needed?
• Outputs for E and F are
• produced at the same time?

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An evaluation of DFDs (2)

• Control information is absent

Possible interpretations (a) A produces datum,
waits until B consumes it (b) B can read the
datum many times without consuming it (c) a pipe
is inserted between A and B
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Formalization/extensions

• There have been attempts to formalize DFDs
• There have been attempts to extend DFDs (e.g.,
  for real-time systems)

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UML diagrams

• UML (Unified Modeling Language) is a collection
  of languages that provide specific notations to
  specify, analyze, visualize, construct, and
  document the artifacts of a software system
• In this chapter, we cover use-case diagram,
  sequence diagram, and collaborative diagrams from
  UML.

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UML use-case diagrams

• Defines a global view of the actors involved in a
  system and the actions that the system performs,
  which in turn provides an observable result that
  is of value to the actors.
• Partitions the overall functionality of the
  system into transactions with respect to the
  actor and illustrates how actors interact with
  them.
• Actors define different roles such as people,
  computer systems, environment

borrow
book

return
book

librarian

customer

library
update

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Use case diagram

• Association the communication path between an
  actor and a use case that it participates in
• Extend The insertion of additional behavior into
  a base use case that does not know about it
• Generalization relation between a general use
  case and a more specific use case that inherits
  from it.
• Include the insertion of additional behavior
  into a base use case that explicitly describes
  the insertion

Base use case
Request catalog
Association
Actor
Place order
ltltextentgtgt
Extension use case
ltltincludegtgt
ltltincludegtgt
ltltincludegtgt
Supply Customer data
Arrange payment
Parent use case
Order produce
Inclusion use case
Child use case
Pay cash
Arrange credit
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UML sequence diagrams

• Describe how objects interact by exchanging
  messages
• Provide a dynamic view

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UML sequence diagram example Ticket selling box
office
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UML collaboration diagrams

• Give object interactions and their order
• Equivalent to sequence diagrams

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UML collaborative diagram example Ticket selling
box office
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Finite state machines (FSMs)

• Can specify control flow aspects
• Defined as

a finite set of states, Q a finite set of
inputs, I a transition function d Q x I ? Q (d
can be a partial function)
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Example a lamp
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Another examplea plant control system
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A refinement
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Classes of FSMs

• Deterministic/nondeterministic
• FSMs as recognizers
• introduce final states
• FSMs as transducers
• introduce set of outputs
• . . .

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FSMs as recognizers
qf is a final state
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FSMs as recognizers
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Limitations

• Finite memory
• State explosion
• Given a number of FSMs with k1, k2, kn states,
  their composition is a FSM with k1 k2 kn.
  This growth is exponential with the number of
  FSMs, not linear (we would like it to be k1 k2
  kn )

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State explosion an example
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The resulting FSM
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FSM simulator