Component Models - PowerPoint PPT Presentation

Presentation Transcript

Component Models

Agenda

Introduction to Components
Object/Component Services
Component Examples
Component Models

DCOM & MTS
CORBA
Enterprise Javabeans

Introduction to Components

Component Software, Clemens Szyperski, Addison Wesley, 1998
Raising Components, J. Williams, Application Development Trends, Sept. 200
Component Based Development Using Componentised Software, www.cbdiforum.com, Nov. 1999
Problem

failure to deliver quality software at right time inhibits business
prepackaged applications not satisfactory due to:

integration problems
inflexibility
difficulty in upgrading
cost

Characteristics of Desirable Solution

must be adaptable to business and technology changes
must be easily and rapidly upgradeable
must be piecewise replaceable
must be customizable to business requirements
must support integration of other packages

Components Are A (The) Solution

components collaborate at semantic level to do task
Issues:

What is a component?
Why aren’t objects a solution?
How do components that have no previous knowledge of each other collaborate?
How can components help in improving quality?
How can components help in improving delivery times?

Analogies for Components

hardware components,ICs

doesn't reflect true nature of SW

deliver blueprint/plan of product, not final product
computers instantiate software; measures must be taken to prevent copies

must distinguish between software and its instances, plans and a building, etc.

Analogies for Components:Confusing Abstraction & Instances

plans can be:

parameterized
applied recursively
scaled
instantiated any number of times
delivered; instances created from its parameterization

instances can't

$analogies for components math$

Analogies for Components:Math

math characterization

isolation of aspects, their orthogonal treatment, their static capturing

fails to capture engineering and marketing aspects

functional & non-functional, interacting whole
reuse, time to market, quality, viability

Components

parts meant to be assembled into a larger part

implications

components come in various sizes
components meet at an interface
components are units of sale

But…

components provide services rather than a physical manifestation -- more like a catalog entry describing characteristics than a particular piece of hardware
components are not the same as software objects

Objects vs. Components

Businesses and Components

business services as components

makes them reusable, replaceable and upgradeable
services approach matches business operation
supports drive toward web
black box components

lessen dependencies and can be certified

component reuse more productive than code reuse
larger chunk size is better fit for work assignment
well-defined and stable interfaces ease integration
can buy or outsource at higher level than code

Business Objects

Instant CORBA, R. Orfali, D. Harkey, J. Edwards, Wiley & Sons, 1997

"self-contained deliverable that has a user interface, state, and knows how to cooperate with other separately developed business objects"
application-independent concepts that represent real-world concerns, recognizable by an end user
application provides an environment to execute business objects
must have dynamic binding and well-defined interfaces to be implemented independently
must recognize events in its environment, change attributes, and interact with other business objects
variation of MVC pattern

Business Objects/Components

business objects
business process objects
presentation objects
business object components

Business Objects/Components Described

business objects

encapsulate storage, metadata, concurrency, business rules for an entity, and how object reacts to changes in view or model

business process objects

business logic at enterprise level
for long-lived processes
defines how object reacts to environmental changes, such as from a business transaction or a message from another business object

Business Objects/Components Described

presentation objects

visual representation of the object

business object component

business object, one or more presentation objects, and a process object

Characteristics of Components

identifiable for recognition and reference
traceable throughout development to allow replacement
replaceable by components with same services
accessible only through interfaces to avoid implementation dependencies
interfaces are immutable to allow replacement
interfaces are accurately documented

Optional Characteristics of Components

hidden physical implementation
independent of implementation of other components
encapsulation of implementation
independent of language, tool or platform
runtime assembly and upgrading
services are generic
well-defined means of extending
object-oriented behind interface

Why Objects Failed to Solve Application Development Problems

Objects have internal dependencies (inheritance)

what happens to object instance’s inherited methods when superclass changes?

Name space collisions were common
No standards were available to interface objects
Don’t really want object instances, want classes
"A map is not the territory it represents, but if correct, it has a similar structure to the territory, which accounts for its usefulness” - Alfred Korzybski, father of general semantics
No notion of independence or late composition
Object technologies ignore aspects of economies, markets, technical consequences

Why?

problem: change, adaptation, unforeseen contexts on server side
solution: flexibility by replaceable modules, enabled by separation of concerns

Basic issues

remove communication dependencies: use proxies
locate objects by name: use brokers
hide details, enable reusability and reconfiguration
but: slower performance, more points of failure

More basic issues

create objects: use factories
find factories: use directories
not enough room for all objects: use passivation/activation
services not always available: use asynchronous messaging
reduce dependencies on infrastructure: use containers
maintain long-term state: use stable storage to persist
coordinate among multiple entities: use transactions
etc.

From: CORBA Fundamentals and Programming, J. Siegel, Wiley & Sons, 1996
CORBAservices

life cycle, naming, persistence, event notification*
transaction*, concurrency control, relationship, externalization
security, time
query, licensing, property
trader, collections, startup

Object Life Cycle Service

create
find

persisted object
find by diverse criteria

passivate/activate
remove
move, copy

CORBA interface exists; responsibility of programmer to implement

Naming Service

Primary services

bind name to object
lookup object bound to name and return reference

allows hierarchy of names
can’t associate anything else with name

see Trader Service

Persistent Object Service

Transient objects

common
allow for garbage collection

from client’s view, object instance always there
datastore: e.g., flat files, relational or OO DBMS
ways to control persistence

connection: state is maintained by infrastructure
store/restore: state is maintained by programmer

Concurrency Control Service

Designed for use with Transaction Service
General lock management model
Five levels of locking granularity to control overhead

Relationship Service

how objects are related to each other
roles

e.g., employment relationship: employer and employee
multiple roles per object allowed

nodes

collection point for all roles an object has
traversal of all nodes exactly once
useful for operations like move and copy when

role and node objects serve as proxies

allows objects that don’t include relationships to have them
allows tracing and traversing independent of objects

Relationship Characterizations

Type

e.g., ownership type relates person and car

Role
Degree

number of required roles
e.g., 3 for book checkout: library, book, patron

Cardinality

maximum number of relationships involving role
e.g., employer has many employee relationships

Semantics: attributes and operations of relationship

e.g., due date attribute for book checkout

Externalization Service

Recording and playing back an object’s state as a stream

like what in Java?
like what other service?

But not dynamically stored

externalize/internalize:

like life cycle move/copy, but:

no way to specify destination
internalization possible without network connection to existing object
internalization may be delayed for long periods of time, if at all

Security Service

identify and authenticate user
authorize: provide/prevent access
audit
secure communicated data
administer

Query Service

provides single source for querying all objects
standard query language
standard interfaces to query evaluators (which abstract access to object/DBMS/etc.)
standard collections of objects
query evaluators to implement standard interface