Component-Based Software Engineering

Report
ASU Open Course System
Component-Based Software Engineering
Module 1 Part 4
Component Composition
Dr. Eman M. Saleh Al-Maghary
Email: [email protected] – Ext. 1269
SE Department
Faculty of Information Technology
http://fit.asu.edu.jo – http://OpenCourse.asu.edu.jo
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Component-Based Software
Engineering
1302481
• Prerequisite: 1302386
• Text Books:
• “Software Engineering, Ian Sommerville, 9th Ed.,
2012.
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Lecture Contents
• Different types of component composition
• Adapter components
• Composition adapters and emergent
properties
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Component composition
• The process of assembling components to
create a system.
• Composition involves integrating
components with each other and with the
component infrastructure.
• Normally you have to write ‘glue code’ to
integrate components.
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Chapter 17
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Types of composition
• Sequential composition where the composed
components are executed in sequence. This involves
composing the provides interfaces of each component.
• Hierarchical composition where one component calls on
the services of another. The provides interface of one
component is composed with the requires interface of
another.
• Additive composition where the interfaces of two
components are put together to create a new
component. Provides and requires interfaces
of integrated component is a combination
of interfaces of constituent components.
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Chapter 17
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5
Types of component
composition
Chapter 17
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Examples
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Sequential composition
the components are executed in •
sequence to provide some
required effect
• The outputs from the ‘provides’
interface from the first
component executed become
the inputs for the provides
interface for the 2nd unit called.
Example: Simple Connect
Component and the Chat
Component
Executed
First
Simple
Connect
Application/Server
Address +User
name
Chat
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Hierarchical composition
• In this case, one
component (defined in
the requires interface) is
called directly from within
the body of the other
component.
• The calling component
must know the name and
the signature of the called
component.
Calling
Chat
myButton.label = “Send”
Called
Button
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Additive composition
• In this case, we put two
components together
so that the provides
interface includes
operations that come
from both of the
composed components.
Chatting Room
Video
Audio
session
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life streaming
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Interface incompatibility
• Parameter incompatibility where
operations have the same name but are of
different types.
• Operation incompatibility where the names
of operations in the composed interfaces
are different.
• Operation incompleteness where the
provides interface of one component is a
subset of the requires interface of another.
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Interface incompatibility
Possible solutions:
Write wrapping code which bridges the gaps
OR
Use an adaptor component to bridge the
gap
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Adaptor components
• Address the problem of component
incompatibility by reconciling the interfaces of
the components that are composed.
• Different types of adaptor are required
depending on the type of composition.
• An addressFinder and a mapper component
may be composed through an adaptor that strips
the postal code from an address and passes this
to the mapper component.
Chapter 17
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Component adaptation-wrapping
• white-box wrapping is applied :
When a software team has full access to the
internal design and code for a component, whitebox wrapping is applied. This wrapping examines
the internal processing details and makes codelevel modifications to remove any conflicts.
• Grey-box wrapping is applied when the
component library provides a component
extension language or API that enables conflicts
in interfaces to be removed or masked.
• Black-box wrapping: Write “glue code” which
bridges the gaps
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Components with incompatible
interfaces
• The component
addressFinder
through its location
method produces a
string which is the
address of the
property, including
street number and
name and town
• The component
mapper through its
displayMap method,
expects a string
which is a postcode
only (not a complete
address)
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Composition through an adaptor
• The component postCodeStripper is the
adaptor that facilitates the sequential
composition of addressFinder and mapper
components.
Chapter 17
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• The component postCodeStripper is the adaptor that
facilitates the sequential composition of addressFinder
and mapper components.
Adapter component
- Wrapping Code
address = addressFinder.location (phonenumber) ;
postCode = postCodeStripper.getPostCode (address) ;
mapper.displayMap(postCode, 10000)
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An adaptor linking a data
collector and a sensor
An adaptor component may be used is in hierarchical composition
Chapter 17
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Photo library composition
Chapter 17
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Interface semantics
• You have to rely on component
documentation to decide if interfaces that
are syntactically compatible are actually
compatible.
• Consider an interface for a PhotoLibrary
component:
Chapter 17
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Photo Library documentation
“This method adds a photograph to the library and associates the
photograph identifier and catalogue descriptor with the photograph.”
“what happens if the photograph identifier is already associated
with a photograph in the library?”
“is the photograph descriptor associated with the catalogue entry as
well as the photograph i.e. if I delete the photograph, do I also
delete the catalogue information?”
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The Object Constraint
Language
• The Object Constraint Language (OCL)
has been designed to define constraints
that are associated with UML models.
• It is based around the notion of pre and
post condition specification – common to
many formal methods.
Chapter 17
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The OCL description of the Photo
Library interface
-- The context keyword names the component to which the conditions apply
context addItem
-- The preconditions specify what must be true before execution of addItem
pre: PhotoLibrary.libSize() > 0
PhotoLibrary.retrieve(pid) = null
-- The postconditions specify what is true after execution
post:libSize () = libSize()@pre + 1
PhotoLibrary.retrieve(pid) = p
PhotoLibrary.catEntry(pid) = photodesc
context delete
pre: PhotoLibrary.retrieve(pid) <> null ;
post: PhotoLibrary.retrieve(pid) = null
PhotoLibrary.catEntry(pid) = PhotoLibrary.catEntry(pid)@pre
PhotoLibrary.libSize() = libSize()@pre—1
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Photo library conditions
• As specified, the OCL associated with the Photo Library
component states that:
– There must not be a photograph in the library with the same
identifier as the photograph to be entered;
– The library must exist - assume that creating a library adds a
single item to it;
– Each new entry increases the size of the library by 1;
– If you retrieve using the same identifier then you get back the
photo that you added;
– If you look up the catalogue using that identifier, then you get
back the catalogue entry that you made.
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Chapter 17
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Composition trade-offs
• When composing components, you may find conflicts
between functional and non-functional requirements, and
conflicts between the need for rapid delivery and system
evolution.
• You need to make decisions such as:
– What composition of components is effective for delivering the
functional requirements?
– What composition of components allows for future change?
– What will be the emergent properties of the composed system?
Chapter 17
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Data collection and report
generation components
Chapter 17
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Composition trade-offs
• There are many situations where the solutions to the
composition problems are mutually conflicting.
• In a): many reports may be needed and cannot be
generated both data management and report generator
can be replaced
• In b): - Faster coz no component communication
overheads.
- Data integrity that apply to the database also apply
to reports (no errors in reports)
Chapter 17
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Customization
• We define component customization as the ability of a
consumer to adapt a component prior to its installation or
use.
• Since components are generally treated in black-box
fashion, revealing as little as possible of their
implementation, components can only be customized
using clearly defined customization interfaces.
• A customization interface enables customization and
deployment tools to modify simple properties by providing
instances of other components as parameters to
customization functions.
• Other option is through the component provider (cost) or
open source (free)
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Packaging and Deployment
• Fast Internet connections will allow component consumers to
conveniently download packaged components with
documentation to develop comprehensive software systems.
• A component model must describe how components are
packaged so they can be independently deployed.
• The component must be packaged with anything that the
component producer expects will not exist in the component
infrastructure.
• This may include the program source code, configuration data,
help files, other depending components, and additional
resources.
• A deployment description provides information about the
contents of a package and other information that is necessary
for the deployment process.
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Evolution Support
• Component-based systems require support for system
evolution.
• Components acting as a server for other components might
have to be replaced by newer versions providing new or
improved functionality.
• A new version may not only have a different implementation but
may provide modified or new interfaces.
• Existing clients of such components should, ideally, not be
affected or should be affected as little as possible.
• As a general rule of thumb for engineers substituting
components, component B can immediately replace
component A, if component B provides at least what
component A provided,.
• In addition, old and new versions of a component might need to
co-exist in the same system.
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Key points
• During the CBSE process, the processes of
requirements engineering and system design
are interleaved.
• Component composition is the process of
‘wiring’ components together to create a system.
• When composing reusable components, you
normally have to write adaptors to reconcile
different component interfaces.
• When choosing compositions, you have to
consider required functionality, non-functional
requirements and system evolution.
Chapter 17
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