Transcript 3. Detail (Component) Design --

Component Design
Pedro Mejia Alvarez
CINVESTAV-PN
Main Contents
1. What is component design
2. Basic design principles
3. Modularity and Information hiding
4. Component design process
1. What is component design
——What is a Component?
• OMG Unified Modeling Language
Specification [OMG01] defines a
component as
– “… a modular, deployable, and replaceable part
of a system that encapsulates implementation
and exposes a set of interfaces.”
• OO view: a component contains a set of
collaborating classes
• Conventional view: logic, the internal data
structures that are required to implement
the processing logic, and an interface that
enables the component to be invoked and
data to be passed to it.
1. What is component design
—— OO Component
a n a l y si s c l a ss
P ri n t Jo b
n u m b e rO f P a g e s
n u m b e rO f S i d e s
p a p e rT y p e
m a g n if ic a t io n
p ro d u c t i o n F e a t u re s
d e si g n c o m p o n e n t
c o m p u t e Jo b Co st ( )
c o m p u t e Jo b
p a ssJo b t o P ri n t e r( )
P ri n t Jo b
i n i t i a t e Jo b
< < in t er f ace> >
co m p u t eJo b
e la b o r a t e d d e s ig n c la s s
Pr in t J o b
comput ePageC ost ( )
comput ePaper C ost ( )
comput ePr odC ost ( )
comput eTot alJobC ost ( )
number O f Pages
number O f Sides
paper Type
paper W eight
paper Size
paper C olor
magnif icat ion
color Requir ement s
pr oduct ionFeat ur es
< < in t er f ace> >
in it iat eJo b
buildW or kO r der ( )
checkPr ior it y ( )
passJobt o Pr oduct ion( )
collat ionO pt ions
bindingO pt ions
cover St ock
bleed
pr ior it y
t ot alJobC ost
W O number
comput ePageC ost ( )
comput ePaper C ost ( )
comput ePr odC ost ( )
comput eTot alJobC ost ( )
buildW or kO r der ( )
checkPr ior it y ( )
passJobt o Pr oduct ion( )
1. What is component design
—— Conventional Component
d esig n co m p o nent
g et Jo b Dat a
Co m p ut ePag eCo st
accessCo st sDB
elab o r at ed m o d ule
Pag eCo st
in: num berPages
in: num berDocs
in: sides= 1 , 2
in: color= 1 , 2 , 3 , 4
in: page size = A , B , C, B
out : page cost
in: j ob size
in: color= 1 , 2 , 3 , 4
in: pageSize = A , B , C, B
out : B PC
out : SF
g e t Jo b Dat a ( n u m b e rP ag e s, n u m b e rDo cs,
sid e s, co lo r, p ag e S ize , p ag e Co st )
acce ssCo st sDB (j o b S ize , co lo r, p ag e S ize ,
B P C, S F )
co m p u t e P ag e Co st( )
j o b size ( JS ) =
n u m b e rP ag e s * n u m b e rDo cs;
lo o ku p b ase p ag e co st ( B P C) -->
acce ssCo st sDB ( JS , co lo r) ;
lo o ku p size f act o r ( S F) -->
acce ssCo st DB ( JS , co lo r, size )
j o b co m p le xit y f act o r ( JCF) =
1 + [ ( sid e s-1 ) * sid e Co st + S F]
p ag e co st = B P C * JCF
2. Basic Design Principles
• Class Design Principles
• Package Design Principles
• Package Coupling Principles
2. Basic Design Principles
——Class Design Principles
• Single Responsibility Principle (SRP)
• Open/Closed Principle (OCP)
• Liskov Substitution Principle (LSP)
– a.k.a. Design by Contract
• Dependency Inversion Principle (DIP)
• Interface Segregation Principle (ISP)
2. Basic Design Principles
—— Single Responsibility Principle (SRP)
A class should have only one reason to change
Robert Martin
Related to and derived from cohesion, i.e. that elements
in a module should be closely related in their function
Responsibility of a class to perform a certain function
is also a reason for the class to change
2. Basic Design Principles
—— SRP Example
All-in-one wonder
Always changes to 4vector
Separated responsibilities
Changes to rotations or boosts
don't impact on 4vector
2. Basic Design Principles
—— SRP Summary
• Class should have only one reason to
change
– Cohesion of its functions/responsibilities
• Several responsibilities
– mean several reasons for changes →
more frequent changes
• Sounds simple enough
– Not so easy in real life
– Tradeoffs with complexity, repetition,
opacity
2. Basic Design Principles
—— Open/Closed Principle (OCP)
Modules should be open for extension,
but closed for modification
Bertrand Meyer
Object Oriented Software Construction
Module: Class, Package, Function
New functionality → new code, existing code remains unchanged
"Abstraction is the key" → cast algorithms in abstract interfaces
develop concrete implementations as
needed
2. Basic Design Principles
—— Abstraction and OCP
Client is closed to changes
in implementation of Server
Client is open for extension
through new Server
implementations
Without AbsServer the Client
is open to changes in Server
2. Basic Design Principles
—— Liskov Substitution Principle
(LSP)
All derived classes must be substituteable
for their base class
Barbara Liskov, 1988
The "Design-by-Contract" formulation:
All derived classes must honor the contracts
of their base classes
Bertrand Meyer
2. Basic Design Principles
—— LSP: The Square-Rectangle Problem
Clients (users) of Rectangle expect
that setting height leaves width
unchanged (and vice versa)
Square does not fulfill this expectation
Client algorithms can get confused
2. Basic Design Principles
—— Dependency Inversion Principle (DIP)
Details should depend on abstractions.
Abstractions should not depend on details.
Robert Martin
Why dependency inversion? In OO we have ways to
invert the direction of dependencies, i.e. class inheritance
and object polymorphism
2. Basic Design Principles
—— DIP Example
Dependency changed from
concrete to abstract ...
The abstract class
is unlikey to change
... at the price of a dependency
here, but it is on an abstraction.
Somewhere a dependency on
concrete Server must exist,
but we get to choose where.
2. Basic Design Principles
—— DIP and Procedural Design
Procedural:
Call more concrete routines
Dependence on (reuseable)
concrete modules
In reality the dependencies are
cyclic → need multipass link and
a "dummy library"
The BaBar Framework classes
depend on interfaces
Can e.g. change data store
technology without disturbing
the Framework classes
2. Basic Design Principles
—— ISP Explained
• Multipurpose classes
– Methods fall in different groups
– Not all users use all methods
• Can lead to unwanted dependencies
– Clients using one aspect of a class also
depend indirectly on the dependencies of
the other aspects
• ISP helps to solve the problem
– Use several client-specific interfaces
2. Basic Design Principles
—— ISP Example: UIs
The Server "collects" interfaces
New UI → Server interface changes
All other UIs recompile
UIs are isolated from each other
Can add a UI with changes in
Server → other UIs not affected
2. Basic Design Principles
—— Three Package Design Principles
• Reuse-Release Equivalency Principle
• Common Closure Principle
• Common Reuse Principle
2. Basic Design Principles
—— Reuse-Release Equivalency
Principle (REP)
The unit of reuse is the unit of release
Bob Martin
It is about reusing software
Reuseable software is external software,
you use it but somebody else maintains it.
There is no difference between commercial
and non-commercial external software for reuse.
2. Basic Design Principles
—— REP Summary
• Group components (classes) for reusers
• Single classes are usually not reuseable
– Several collaborating classes make up a package
• Classes in a package should form a reuseable and
releaseable module
– Module provides coherent functionality
– Dependencies on other packages controlled
– Requirements on other packages specified
• Reduces work for the reuser
2. Basic Design Principles
—— Common Closure Principle
(CCP)
Classes which change together belong together
Bob Martin
Minimise the impact of change for the programmer.
When a change is needed, it is good for the programmer
if the change affects as few packages as possible, because
of compile and link time and revalidation
2. Basic Design Principles
—— CCP Summary
• Group classes with similar closure
together
– package closed for anticipated changes
• Confines changes to a few packages
• Reduces package release frequency
• Reduces work for the programmer
2. Basic Design Principles
—— Commom Reuse Principle
(CRP)
Classes in packages should be reused together
Bob Martin
Packages should be focused, users should
use all classes from a package
CRP for packages is analogous to SRP for classes
2. Basic Design Principles
—— CRP Summary
• Group classes according to common
reuse
– avoid unneccessary dependencies for
users
• Following the CRP often leads to
splitting packages
– Get more, smaller and more focused
packages
• Reduces work for the reuser
2. Basic Design Principles
—— Three more package design principles
• Acyclic Dependencies principles
• Stable Dependencies principles
• Stable Abstractions principles
2. Basic Design Principles
—— The Acyclic Dependencies
Principle (ACP)
The dependency structure for packages must be
a Directed Acyclic Graph (DAG)
Stabilise and release a project in pieces
Avoid interfering developers
Morning after syndrome
Organise package dependencies in a top-down hierarchy
2. Basic Design Principles
—— Dependencies are a DAG
It may look complicated,
but it is a DAG (Directed
Acyclic Graph)
Can exchange
ObjyIO and RootIO
2. Basic Design Principles
—— Dependency Cycles
A cycle between Framework
and ObjyIO
Must develop together
May need multipass link
2. Basic Design Principles
——Stable Dependencies Principle
(SDP)
Dependencies should point in the direction of stability
Robert Martin
Stability:
corresponds to effort required to change a package
stable package
hard to change within the project
Stability can be quantified
2. Basic Design Principles
—— SDP Example
Bad
Good
A is responsible
for B, C and D.
It depends on E,
→ irresponsible
A is responsible for
B, C, D and E. It will
be hard to change
E depends on
F, G and H. A
depends on it. E
is responsible and
irresponsible.
E depends on A,
F, G and H. It is
irresponsible and
will be easy to
modify.
2. Basic Design Principles
—— SDP Summary
• Organise package dependencies in the
direction of stability
• Dependence on stable packages
corresponds to DIP for classes
– Classes should depend upon (stable)
abstractions or interfaces
– These can be stable (hard to change)
2. Basic Design Principles
—— Stable Abstractions Principle
(SAP)
Stable packages should be abstract packages.
Unstable packages should be concrete packages.
Robert Martin
Stable packages contain high level design. Making them
abstract opens them for extension but closes them for
modifications (OCP). Some flexibility is left in the stable
hard-to-change packages.
3. Modularity and Information hiding
——Modularity
• Computer systems are not monolithic:
– they are usually composed of multiple,
interacting modules.
• Modularity has long been seen as a
key to cheap, high quality software.
• The goal of system design is to decide:
– – what the modules are;
– – what the modules should be;
– – how the modules interact with oneanother.
3. Modularity and Information hiding
—— What is a module?
• Common view: a piece of code. Too limited.
• Compilation unit, including related declarations
and interface
• David Parnas: a unit of work.
• Collection of programming units (procedures,
classes, etc.)
– with a well-defined interface and purpose within the
entire system,
– that can be independently assigned to a developer
3.Modularity and Information hiding
—— Why modularize a system?
• Management: Partition the overall development effort
– – Divide and conquer
• Evolution: Decouple parts of a system so that changes to
one part are isolated from changes to other parts
– Principle of directness (clear allocation of requirements to
modules, ideally one requirement (or more) maps to one
module)
– – Principle of continuity/locality (small change in requirements
triggers a change to one module only)
• Understanding: Permit system to be understood
– as composition of mind-sized chunks, e.g., the 7±2 Rule
– with one issue at a time, e.g., principles of locality,
encapsulation, separation of concerns
• Key issue: what criteria to use for modularization?
3. Modularity and Information hiding
—— Information hiding
• Hide secrets. OK, what’s a “secret”?
– Representation of data
– Properties of a device, other than required
properties
– Implementation of world models
– Mechanisms that support policies
• Try to localize future change
– Hide system details likely to change
independently
– Separate parts that are likely to have a different
rate of change
– Expose in interfaces assumptions unlikely to
change
3. Modularity and Information hiding
—— Interface vs. Implementation
Users and implementers of a module have different views of it.
Interface: user’s view of a module.
• describes only what a user needs to know to use the module
• makes it easier to understand and use
• describes what services the module provides, but not how
it’s able to provide them
3. Modularity and Information hiding
—— What Is an Interface?
• Interface as a contract - whatever is published by a
module that
– Provided interface: clients of the module can depend on and
– Required interface: the module can depend on from other
modules
• Syntactic interfaces
– How to call operations
• List of operation signatures
• Sometimes also valid orders of calling operations
• Semantic interfaces
– What the operations do, e.g.,
• Pre- and post-conditions
• Use cases
3. Modularity and Information hiding
—— Further Principles
• Explicit interfaces
– Make all dependencies between modules explicit
(no hidden coupling)
• Low coupling - few interfaces
– Minimize the amount of dependencies between
modules
• Small interfaces
– Keep the interfaces narrow
• Combine many parameters into structs/objects
• Divide large interfaces into several interfaces
• High cohesion
– A module should encapsulate some well-defined,
coherent piece of functionality (more on that later)
3. Modularity and Information hiding
—— Coupling and Cohesion
• Cohesion is a measure of the
coherence of a module amongst the
pieces of that module.
• Coupling is the degree of interaction
between modules.
• You want high cohesion and low
coupling.
3. Modularity and Information hiding
—— Degrees of Cohesion
3. Modularity and Information hiding
—— Coincidental cohesion
• The result of randomly breaking the project
into modules to gain the benefits of having
multiple smaller files/modules to work on
– Inflexible enforcement of rules such as:
“every function/module shall be
between 40 and 80 lines in length” can
result in coincidental coherence
• Usually worse than no modularization
– Confuses the reader that may infer
dependencies that are not there
3. Modularity and Information hiding
—— Logical cohesion
• A “template” implementation of a number of quite
different operations that share some basic course
of action
– variation is achieved through parameters
– “logic” - here: the internal workings of a module
• Problems:
– Results in hard to understand modules with
complicated logic
– Undesirable coupling between operations
• Usually should be refactored to separate the
different operations
3. Modularity and Information hiding
—— Example of Logical Cohesion
3. Modularity and Information hiding
—— Temporal Cohesion
• Temporal cohesion concerns a module organized to
contain all those operations which occur at a similar
point in time.
• Consider a product performing the following major
steps:
– Initialization, get user input, run calculations,
perform appropriate output, cleanup.
• Temporal cohesion would lead to five modules named
initialize, input, calculate, output and cleanup.
• This division will most probably lead to code duplication
across the modules, e.g.,
– Each module may have code that manipulates one
of the major data structures used in the program.
3. Modularity and Information hiding
—— Procedural Cohesion
• A module has procedural cohesion if all the
operations it performs are related to a sequence of
steps performed in the program.
• For example, if one of the sequence of operations
in the program was “read input from the keyboard,
validate it, and store the answers in global
variables”, that would be procedural cohesion.
• Procedural cohesion is essentially temporal
cohesion with the added restriction that all the
parts of the module correspond to a related action
sequence in the program.
• It also leads to code duplication in a similar way.
3. Modularity and Information
hiding
—— Procedural Cohesion
3. Modularity and Information hiding
—— Communicational Cohesion
• Communicational cohesion occurs when
a module performs operations related to
a sequence of steps performed in the
program (see procedural cohesion) AND
all the actions performed by the module
are performed on the same data.
• Communicational cohesion is an
improvement on procedural cohesion
because all the operations are
performed on the same data.
3. Modularity and Information hiding
—— Functional Cohesion
• Module with functional cohesion focuses on
exactly one goal or “function”
– (In the sense of purpose, not a programming
language “function”).
• Module performing a well-defined operation is
more reusable, e.g.,
– Modules such as: read_file, or draw_graph are more
likely to be applicable to another project than one
called initialize_data.
• Another advantage of is fault isolation, e.g.,
– If the data is not being read from the file correctly,
there is a good chance the error lies in the read_file
module/function.
3. Modularity and Information hiding
—— Informational Cohesion
• Informational cohesion describes a module as performing a number
of actions, each with a unique entry point, independent code for
each action, and all operations are performed on the same data.
– In informational cohesion, each function in a module can
perform exactly one action.
• It corresponds to the definition of an ADT (abstract data type) or
object in an object-oriented language.
• Thus, the object-oriented approach naturally produces designs with
informational cohesion.
• Each object is generally defined in its own source file/module, and
all the data definitions and member functions of that object are
defined inside that source file
3. Modularity and Information hiding
—— Levels of Coupling
3. Modularity and Information hiding
—— Content Coupling
•
One module directly refers to the content of the other
– module 1 modifies a statement of module 2
• Assembly languages typically supported this, but not highlevel languages
• COBOL, at one time, had a verb called alter which could
also create self-modifying code (it could directly change an
instruction of some module).
– module 1 refers to local data of module 2 in terms of some kind
of offset into the start of module 2.
• This is not a case of knowing the offset of an array entry this is a direct offset from the start of module 2's data or
code section.
– module 1 branches to a local label contained in module 2.
• This is not the same as calling a function inside module 2 this is a goto to a label contained somewhere inside
module 2.
3. Modularity and Information hiding
—— Common Coupling
• Common coupling exists when two or more modules
have read and write access to the same global data.
• Common coupling is problematic in several areas of
design/maintenance.
– Code becomes hard to understand - need to know
all places in all modules where a global variable gets
modified
– Hampered reusability because of hidden
dependencies through global variables
– Possible security breaches (an unauthorized access
to a global variable with sensitive information)
• It’s ok if just one module is writing the global data and
all other modules have read-only access to it.
3. Modularity and Information hiding
—— Common Coupling
• Sometimes necessary, if a lot of data has to be
supplied to each module
3. Modularity and Information hiding
—— Control Coupling
• Two modules are control-coupled if module 1 can
directly affect the execution of module 2, e.g.,
– module 1 passes a “control parameter” to
module 2 with logical cohesion, or
– the return code from a module 2 indicates NOT
ONLY success or failure, but also implies some
action to be taken on the part of the calling
module 1 (such as writing an error message in
the case of failure).
• The biggest problem is in the area of code re-use:
the two modules are not independent if they are
control coupled.
3. Modularity and Information hiding
—— Stamp Coupling
• It is a case of passing more than the required data
values into a module, e.g.,
– Passing an entire employee record into a function
that prints a mailing label for that employee. (The
data fields required to print the mailing label are
name and address. There is no need for the salary,
SIN number, etc.)
• Making the module depend on the names of data fields
in the employee record hinders portability.
– If instead, the four or five values needed are passed
in as parameters, this module can probably become
quite reusable for other projects.
• As with common coupling, leaving too much
information exposed can be dangerous.
3. Modularity and Information hiding
—— Data Coupling
• Data coupling exhibits the properties
that all parameters to a module are
either simple data types, or in the
case of a record being passed as a
parameter, all data members of that
record are used/required by the
module. That is, no extra information
is passed to a module at any time
3. Modularity and Information hiding
—— Others Coupling
• Routine call
– increases connectedness of a system
• Type use
– use in ClassA types from ClassB (complex
modifications)
• Inclusion or import
– occurs when CompA incs./imports CompB
• External
– occurs when calling OS system calls, DBMS
services, etc.
4. Component design process
——Component Level Design-I
• Step 1. Identify all design classes that
correspond to the problem domain.
• Step 2. Identify all design classes that
correspond to the infrastructure domain.
• Step 3. Elaborate all design classes that are not
acquired as reusable components.
– Step 3a. Specify message details when classes or
component collaborate.
– Step 3b. Identify appropriate interfaces for each
component.
– Step 3c. Elaborate attributes and define data types and
data structures required to implement them.
– Step 3d. Describe processing flow within each operation in
detail.
4. Component design process
—— Component-Level Design-II
• Step 4. Describe persistent data sources
(databases and files) and identify the classes
required to manage them.
• Step 5. Develop and elaborate behavioral
representations for a class or component.
• Step 6. Elaborate deployment diagrams to
provide additional implementation detail.
• Step 7. Factor every component-level design
representation and always consider
alternatives.