Component based models and technology

Component Based Models
and Technology

Introduction
In contrast to design, which usually denotes a well-defined phase in the
software life cycle between requirements engineering and implementation,
modeling covers all activities ranging from analytical studies of an application
domain to testing of system parts.
Our experience has shown that these activities require a lot of knowledge and
creativity. Thus it is not purposeful to structure modeling activities by a strict
method. The process of modeling is not predictable and cannot be controlled
in a generic way.
Important properties of design or modeling techniques are the various visual
representation forms (diagrams and descriptions) of several system aspects.
We are not surprised by this focusing on design description and
representation, because our view on modeling agrees that it is simply
impossible to find adequate solutions just by following a strict method.

Style of Representation
The main purpose of the representation of the architecture is the
documentation of (intermediate) results of the modeling process to
communicate the concepts among developers, managers, programmers and
anyone else. On the one hand, it should be relatively (programming) language-
independent, but, on the other hand, the implementation should be rather
straightforward. Due to the importance of reusability, prefabricated components
should be easily embedable.
Most representation techniques use primarily diagrams and charts to capture
the essential model concepts. The reason is the more direct transformation of
mental models into graphic images and their better intuitive comprehensibility.
But graphics have their drawbacks: They tend to require enormous drawing
space which is usually just not available or useful.
Furthermore, transforming a graphic diagram into an executable program is
often a nontrivial, error-prone task. In this respect, textual representations are
superior. Unfortunately, they are less intuitive and prove inferior as thinking
tools or vehicles. So our solution encompasses both, a graphical and a textual
representation.

Composition Representation
The first kind of diagram or description depicts the decomposition of (part of)
the system into components and how these components are coupled. Each
building block has a unique name and a number of essential attributes. The
attributes serve to describe the main characteristics of their components.
Direct coupling expresses that one component uses another directly for some
reason and that the former knows the actions of the latter. Hence all actions
defined by the used part can be invoked.
Indirect coupling means that the first component can only use a (typically
small) set of standardized actions which are forwarded by the mediating
component. Mediating components which do not offer additional functionality
are represented in a collapsed form by a small circle.

Figure depicts a component structure model consisting of three coupled
components.
The second attribute of component2 holds a string value, which can be written
directly behind the attributes name whenever no other component refers to the
string.
The following textual description represents similar structure in a textual form.

The textual description gives a better chance to annotate attributes and couplings. Components
of the same kind, but with different couplings, can be described collectively in a list as shown
for comp4 , comp5, comp6 . Indirect coupling is indicated by a symbolized double arrow (-
>->).
The purpose of the component representation is to describe the static structure of the system. To
complement this picture, we need a description of the dynamic behavior, which is introduced in
the next section.

Collaboration Representation
It is not possible to give one aspect absolute precedence over the other; both the
composition and the collaboration model have to be developed in parallel.
For equal rights reasons, we now start with a textual representation. It first describes
the components by relevant actions and events which can and should be used by
other components. Each action is introduced by a leading ”>” symbol; the events are
marked by an appended ”>” symbol. Next, we describe all collaborations with
causality relationships. A causality relationship associates an event of an originator
component with an action of a receiver component. The meaning is obvious:
Whenever the originator component triggers the event (because of a state change
during an action execution), the associated action is invoked at the specified receiver
component. Optionally, a collaboration relationship may specify a list of arguments
which serve as parameters for the action. The collaboration relations of a component
implemented by a generic event dictionary where the event names serve as keys and
the action commands can be registered appropriately. When an event is triggered in a
specific component at run time, a dispatching mechanism looks up in the event
dictionary and activates one by one the action commands that are registered for the
event.

As in the composition model, equal
components can be described
collectively by arranging them in a list.
Figure 2 depicts the dynamic
collaboration model in a graphic form.

Layered Application Architecture
Composing applications out of components is not as simple as building toy
models out of Lego bricks; therefore we suggest a layered architecture for
highly interactive applications, with well defined functionality at each layer. For
typical information system applications, our architecture model consists of four
layers, as shown in Figure.

Layer 1: User Interface Model
The user interface layer consists entirely of the user interface building blocks, i.e.,
components which have a run-time representation on the screen. Obviously, the
task of these components is visualizing of application data and control devices as
well as capturing user input and triggering events for invoking the associated
commands. Very often, user interface components
can be changed either by direct manipulation by the user (e.g., selecting a list
item) or by changing program states (e.g., updating a displayed list). Since
interaction components cannot assume any functionality of the functional
components, the collaboration can only be configured dynamically in one
direction
Layer 2: Application Model
The application model consists of components which form the functional core of the
application. It has to build up and open the user interface at program start. During
program execution the application components have to maintain the user interface
according to the program state. Actuating a control element at the user interface
usually results in invoking a method of the application model. Since the application
model knows the user interface model as well as the lower-level model, it establishes
the dynamic collaboration relationships among all components. It is purposeful to
decompose a complex application model into several independent components
which either work together via dynamic collaboration or even better are just
synchronized by the underlying layer.

Layer 3: Access Model
The access layer serves as mediator between the user interface or the application
model on the one side and the domain model on the other side. It has two
important tasks: first, synchronizing accesses to and updates of the data (i.e.,
broadcasting changed events to all dependent view or application components)
and second, supplying view components with their data when required. Ideally,
the access layer should completely hide and encapsulate the underlying data, but
provide controlled access. Therefore, only values and copies of data objects
should be passed through to the higher layers. Application-specific access model
components can be viewed as the respective handling instances (authorities) for
the domain components.
Layer 4: Domain Model
The primary purpose of the domain model is to store the application data during
program execution. We prefer to use a structural object model, which is an object-
oriented data model of the domain. The domain model provides all necessary
operations for querying and manipulating the data in a basic, application-
independent form. Furthermore, the domain model is responsible for checking all
integrity conditions and ensuring other semantic characteristics of the data model.
If separated domain models might influence each other, all manipulations should
be done via the access layer. In such cases, only dynamic collaboration can be
used.

Modeling Process Activities
Domain Investigation
Having a sound knowledge on the problem domain is an absolute requirement for being able to
develop systems that solve real problems. For acquiring problem domain knowledge, a number
of techniques have been discovered and proven purposeful, including interviews,
questionnaires and accumulation of artifacts (forms, reports). Additionally, use cases and
scenarios can be used for describing business process activities. These utilities are documented
extensively in the requirements engineering literature.
Conceptual Modeling
"Conceptual modeling denotes an understanding and construction process with the purpose of
forming a model of some part of the world expressed by concepts and phenomena, i.e. a concept
formation process. The purpose of the model may either be the understanding of some existing
system or the design of some new model system."
The resulting model describes or represents concepts and phenomena of the problem domain and
their relationships in an informal way, so that the concepts and phenomena can be communicated
among all involved persons.

User Interface Modeling (Prototyping)
A crucial step in modeling is the development of a user view of the system.
This can be done by involving the experts (i.e., end users) as important participants in the process.
Data Modeling
Especially (but not exclusively) in information system development, data structures form a
decisive factor in the development process. The data engineering community has created a rich
repertoire for many aspects of data modeling.
Behavior Modeling
What we term behavior modeling is a consolidation and simplification of dynamic and state
modeling as described by Rumbaugh et al. [Rumbaugh91]. Here we view the system model as
a state automaton where each system state or mode is expressed in domain subject terms. The
behavior model is formed by a list of transactions. Each transaction is described by a user
command (input), a current state, the resulting visual response (output) and the resulting new
or old state. Additionally, each transaction may specify its implications on the data model. It is
important to understand that our behavior model views the system as a black box with a
known internal state. The behavior model should closely correspond to the conceptual model
of the end user of the system.

Component Modeling
Component modeling encompasses the activities of finding and identifying all components,
defining attributes, defining actions, defining events, and defining couplings and collaborations
of the proposed composition and collaboration model.
Components are usually already indicated by nouns of the glossary. They often correspond
to nouns, but in contrast to object modeling, closely related things which cannot exist
independently are grouped together within one component. Adjectives which can be associated
with component nouns are often candidates for attributes and verbs for actions. Since no
component is an island, some attributes may refer to other components, and/or events can be
identified which may be of interest to the environment
Interface Definition Modeling
We assume that a number of components can be reused from a library of prefabricated parts.
Only components which are not already available have to be implemented by means of a
program. For this purpose we suggest deriving class interface definitions out of the component
model. Developing the class definitions requires some experience to find good compromises
between optimal suitability for the system at hand and enough flexibility for future changes or
reuse engagements for the component.
Implementation and Test
Perhaps surprisingly, these activities belong to the modeling activities, too. But creating an
executable system in parallel with other modeling activities has the advantage that many
hindsights gained from programming can improve the overall process and the product.
Additionally, feasibility or efficiency problems of any kind arise much earlier, and developers
are motivated by an already working subsystem.

Component TechnologiesComponent Technologies

Overview
 Introduction
 ACME Architectural Description Language
 Java Bean Component Model
 COM, DCOM, MTS and COM+
 CORBA Component Model (CCM)
 .NET Component Model
 OSGI Component Model

Introduction
 A Short Historical Perspective
 Component Interface and Connections
 Performing Services Transparently

A Short Historical Perspective
 Programming languages, can be seen from either
 The run-time point of view or,
 The design and reuse perspective

Component Interface and
Connections
 ADLs primarily address the issues related to the
early phases of software engineering:
 Design
 Analysis
 They identify a number of concepts, such as:
 Architecture, configurations, connectors, bindings,
properties, hierarchical models, style, static analysis and
behavior.

Component Interactions
Iteractions with
traditional software entities
Interactions
with
other
components
Interact
with
other
component
Interactions with
component
infrastructure
Components
Traditional
software
entities
Component
Infrastructure

Majors steps in CBD lifecycle
Aspect Phase Actor
Interface Definition Designer
Assembly Assembly Architect
Implementation Implementation Developer
Lifecycle Packaging,
Deployment
Administrator
Framework,
run-time support
Execution End User

ACME Architectural Description Language
 Components and Ports
 Connectors and Roles
 Systems and Attachments
 Representations and Bindings
 Properties, Constraints, Types and Styles

Components and Ports
 Components
 Represent the computational elements and data
stores of a system.
 Ports
 Are the points of interaction between a
component and its environment.
Component
Port

Connectors and Roles
 Connectors
 Represent interactions between components such as
method calls or an SQL connection between a client and
a database server.
 The interface of a connector is defined as a set of
roles
Connector
Role

Systems and Attachments
 The structure of a system is specified by a set of
components, a set of connectors, and a set of
attachments.
 Attachment
 Links a component port to a connector role.
Attachement

Representations and Bindings
Connector
Component
Port
Role
Attachement
Binding

Java Bean Component Model
 Key Features
 Interface of a Component
 Implementation of a Component
 Components Assembly
 Packaging and Deployment

Key Features
 Bean Box
 "A Java Bean is a reusable software component that
can be manipulated visually in a builder tool”.
 The Java Bean was designed for the construction of
graphical user interface (GUI).
 Explicitly tailored to interact in two different contexts:
 At composition time, within the builder tool.
 At execution time, with the runtime environment.

Interface of a Component
 This model defines four types of port:
 methods,
 properties,
 event sources and
 event sinks called listeners.
Read-only property
Write-only property
Property
Method
Event source
Event sink (listener)
Bounded property
v Vetoable property
ro
wo
1 Unicast event source
Ports

Implementation of a Component
 Most bean components are implemented by a simple
Java object, the object being encapsulated in the
component, but there are more sophisticated
implementations possible.
 Wrapping a legacy object.
 Multiple-objects implementation.
 Dependency on traditional entities.

Implementations of Bean Components
Object
Method
Method call
Binding
A simple implementation A more complex implementation

Components Assembly
 Assembly is one of the key features of Java Bean
though no not specific solution is provided.
 Different ways of assembling components are
supplied.
Component-based assembly Heterogeneous assembly

Packaging and Deployment
 Java Beans define a model for packaging
components into archives.
 Includes the definition of dependency relationships between
the package items.
 The customization code can be more complex than
the component itself!
 Each package item can be marked "Design Only", so
that they can be removed in a final application.

COM, DCOM, MTS and COM+
 Interfaces and Assembly
 Implementation
 Framework
 Lifecycle

Interfaces and Assembly
 A COM interface is seen as a C++ virtual class and
takes the form of a list of data and function
declarations without associated code.
 All interfaces are descendants of the IUnknown
interface.
Component interface
Interface
Component implementation

Implementation
 A COM object is a piece of binary code, written in any
programming language, as long as the compiler
generates code following the binary interoperability
convention.

Framework
 Standard interfaces
 A simple run-time

Lifecycle
 COM, and COM+ are strictly execution time and
binary component models.
 No lifecycle issues are explicitly supported.

CORBA Component Model (CCM)
 Interface and Assembly
 Framework : The Container Approach

Lifecycle

Interface and Assembly
 A component interface is made of ports divided into:
 Facets
 Receptacles
 Event sources
 Event sinks
Component interface
Attribute
Facet
Event source
Event sink
Ports
Receptacle Segment

Framework : The Container Approach
 Services can be made available to components without having
to change that component’s source code.
CCM run-time infrastrucure
container

Lifecycle
 CCM is the best effort to date:
 To gather the advances made in different fields,
 To include a wide spectrum of lifecycle activities,
while still claiming efficiency and heterogeneity
capabilities,
 However, the whole does not provide the feeling
of being as “simple” as claimed.

.NET Component Model
 Interfaces and Assembly
 Implementation
 Framework
 Lifecycle

Interfaces and Assembly
 programming language approach for component
programming.
 The program contains the information related to the
relationships with other “components”, and that the
compiler is responsible for generating the information
needed at execution.
 There is no explicit concept of connection but rather the
traditional list of imported and exported resources.

Implementation
 A component (assembly) is made of modules, which
are traditional executable files (DLL).
 Modules cannot be assemblies, thus the .NET model
is not hierarchical.
Component interface
Attribute
Method
Event source
Ports
Event source
Modules

Framework
 .NET relies on the traditional programming
approach : the framework is seen as the language run-
time support.
 Transaction control relies on MTS.

Lifecycle
 Assemblies (and their modules) are local to an
application, and thus different DLLs with same name can
run simultaneously.
 Each assembly has a versioning information about itself
and about the assemblies it depends on.
 Version control is delegated to the dynamic loader, which selects
the “right” version.
 Significantly improve the application packaging and
deployment.
 Early lifecycles phases do not seem to have received
much attention.

OSGI Component Model
 Components
 Interface of a Bundle Component
 Assembly of Bundle Components

Implementation of a Bundle Component

Components
 A bundle use three kinds of ports to express its
interactions with
 Traditional technology
 Other components
 The run-time environment
 Bundles may listen to events published by the framework
such as the insertion of a new component in a system.

Interface of a Bundle Component
Package export
Package import
Service use
Ports
Service interface
static
dynamic
Interface of a bundle component

Assembly of Bundle Components
 A system is an evolving set of bundle
components.
 A bundle component publishes a service interface
 It can attach to it a set of properties describing its
characteristics.
 A component requires an interface for its use,
 It will select one via a query expression based on these
properties.
 This flexibility also has its counterpart
 There is no guarantee than the service will continue to
be available

Implementation of a Bundle Component
 JAR archive containing:
 Service components
 Java packages
 Other resources files
Package
Resource
Service component
Activator

Thank you!Thank you!
Mayukh Maitra (2K11/SE/038)
Saransh Garg (2K11/SE/063)
Siddharth Malhotra (2K11/SE/074)
Isha Panesar (2K11/SE/034)

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