CORBA-Based Enterprise System Development

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CORBA-Based Enterprise System Development
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Problems and Challenges

• Multiple platforms, languages and systems
• Mixture of client-server and mainframe-based
  applications built as stand-alone systems
• Proprietary, legacy systems
• Conflicting data formats and semantic definitions
• Integration not planned in original designs

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Possible Solutions

• System development as composition rather than
  programming
• Middleware, design patterns and frameworks
• Component-based software engineering
• Standard-based open system development
• Building a standardized information technology
  infrastructure
• Planning and developing a long term architectural
  vision

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Component Software

• Uniform access to services
• Uniform discovery of resources and object names
• Uniform error handling methods
• Uniform security policies

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Where to Begin?

• Uniform communication infrastructure
• Platform (hardware, operating systems,
  communication protocols) independence
• Uniform interaction protocol
• Transparent programming in heterogeneous
  environment
• Language independence
• Location transparency
• Separation of interface and implementation
• Common building blocks
• Domain independent
• Domain specific

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Object Management Architecture
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Problem I. Common Communication Infrastructure

• Key hide difference
• Issues
• Difference in hardware communication
• Difference in network protocols and operating
  systems
• Difference in languages
• Difference in invocation methods and exception
  handling

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Solution Common Object Request Broker
Architecture (CORBA)
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Role of Object Request Broker
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How Does ORB Work?
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Problem II. System Integration and Reuse

• Key standard component model
• Issues
• How to describe an object/server/function/applicat
  ion
• How to find an object
• How to compose applications
• How to reuse object/components

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Goal Ideal Integration Model
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Toward A Scalable and Manageable Solution
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Solution Common Integration Infrastructure

• Separation of interface from implementation ?
  Interface Definition Language (CORBA IDL)
• Services not objects ? Independent interface
  specification
• Unified naming and invocation model
• Find services through interface repository
• Common services and facilities

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Mapping Solution to CORBA
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How Does CORBA Work?
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How Does CORBA Work? - continued

• Using IDL interface to separate client and object
  implementation from ORB
• Client only sees object interface not
  implementation
• Plug-and-play composition
• Client does not pass request directly to object
• Request is always passed through ORB
• Result location/language/OS/platform transparency

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How to Use CORBA - Role of OMG Language Mapping
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How to Use CORBA - Producing IDL, Client/Object
Implementation
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How to Use CORBA - Integrating Imported Object
with Client Implementation
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Building CORBA Object Summary

• Define object interface using OMG IDL (p. 24/25)
• Making choices of
• Implementation language
• runtime platform and OS
• the ORB it will connect to
• whether it will run local to its client or
  remotely
• the network hardware or protocol it will use, etc
• Write code for the object
• Compile IDL interface, which generates Stub and
  Skeleton code
• Linking implementation code with Skeleton code
  connects the object to the ORB
• Integrating purchased object/component (p.26)

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CORBA IDL

• IDL is pure specification, not implementation
• IDL file creates multiple language bindings
• Platform independence
• An IDL interface not necessarily correspond to
  single object implementation
• Interface inheritance supported
• Supports dynamic binding
• Supports multiple implementation

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IDL Example
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IDL Specification of Course Registration Model

• Module CourseRegistration
• // Forward Declarations
• Interface Course
• Interface FacultyMember
• Interface Student
• attribute string name
• attribute string address
• attribute unsigned long studentId
• attribute string major
• attribute float gradePointAverage
• exception ClassFull
• void enroll (in Course course)
• raises (ClassFull)
• exception HasNotCompleteReqts
• void graduate ()
• raises (HasNotCompleteReqts)

• Typedef sequenceltCoursegt CourseList
• CourseList class_list()
• void notify_cancellation (in Course course)

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IDL Specification - continued

• interface Course
• attribute string courseSubject
• attribute unsigned short maxSize
• enum SchoolSemesters
• FALL, SPRING, SUMMER
• attribute SchoolSemesters semester
• attribute unsigned long time
• attribute string days
• attribute unsigned short year
• void register_student (in Student student)
• exception RoomSpaceUnavailable
• void request_scheduling (in Time time,
• in string days,
• in SchoolSemesters semester,
• in unsigned short year,
• raises (RoomSpaceUnavailable)
• void cancel_class()

• Interface FacultyMember
• attribute String name
• struct OfficeHours
• string time, duration, days
• attribute OfficeHours office_hours
• attribute string office_address
• attribute string department
• exception TeachingLoadExceeded
• void assign_class (in Course course)
• raises (TeachingLoadExceeded)
• typedef unsigned short TeachingHours
• TeachingHours current_teaching_load ()

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CORBA 2 Overview
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Role of ORB
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Role of ORB - A Software Bus
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Principal CORBA Interfaces
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Understanding the ORB Client Side

• Client requests may be passed to ORB through
  either static invocation interface (SII) or
  dynamic invocation interface (DII)
• SII decide object type and operation at compile
  time (static typing), DII at runtime (dynamic
  typing)
• Both allow dynamic binding can select target
  object instance at runtime
• DII cannot check argument type correctness at
  compile time
• One IDL stub for each SI, while one DII shared by
  all dynamic invocations
• SII invocations generally synchronous (blocking),
  DII may be invoked synchronous, asynchronous or
  deferred synchronous

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IDL Stubs

• Client stub is automatically generated by IDL
  compiler
• Client specifies target object instance via its
  object reference and through object IDL
  interfaces
• Client-to-stub interface (marshalling) is defined
  by standard OMG language mapping
• Stub-to-ORB interface proprietary
• The role of client is simply to request services.
  Object activation, deactivation, suspension, etc.
  are either performed automatically by the ORB or
  by customized services located outside the client

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Dynamic Invocation Interface

• Trade off compile time type checking for runtime
  flexibility
• Gives a client the capability, at any time, of
  invoking any operation on any object it may
  access over the network
• Useful for accessing objects for which the client
  has no stub or discovered via naming or trading
  services.
• Server cannot distinguish between SI an d DI
• 4 steps to a DI
• Identify the object to be invoked (e.g. via
  Trader Service)
• Retrieve its interface
• Construct the invocation
• Invoke the request and receive the request

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Management of IDL Interfaces - Interface
Repository

• Allows IDL definitions for objects be stored,
  modified, and retrieved
• Can be used by ORB to
• Provide interoperability between different ORB
  implementations
• Provide type-checking of request signatures,
  whether a request was issued through the DII or a
  stub
• To check the correctness of inheritance graphs

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Interface Repository - continued

• For client objects and users
• To manage installation and distribution of
  interface definitions around your network
• To browse or modify interface definitions or
  other info stored in IDL
• Compilers could compile stubs and skeletons
  directly from IR instead of from the IDL files
• Access IR
• Use utilities provided by ORB vendor
• Write code that invokes the standard IR IDL
  interface mandated by OMG

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Understanding the ORB Implementation Side

• CORBA principle simple client, complex server
• Object builders must write code to handshake with
  ORB

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Implementation Side ORB Operation
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Scenario of Object Invocation on Server Side

• A server process runs distinct from the ORB
• ORB receives a request targeting an object in the
  server. ORB checks its repository and determines
  that neither the server nor the object is
  currently active
• ORB activates server, and server is passed the
  info it needs to communicate with the BOA
• Server calls impl_is_ready on the BOA, indicating
  that the server is ready to activate objects

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Scenario - continued

• BOA calls the servers object activation routine
  for the target object, passing it the object
  reference. Server activates the object
• BOA passes the invocation to the object through
  the skeleton and receives the response, which it
  routes back to the client
• BOA may receive and pass additional request to
  the object
• Server may shut down an object using
  deactivate_obj
• Server may shut down entirely

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Server-Side Structure
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Server-Side Components

• Object adapter provides interfaces between ORB
  and object, implementation depends on specific
  object implementation model
• ORB uses Dynamic Skeleton Interface (DSI) to
  create a proxy skeleton for objects, typically
  remote, whose static skeletons are not bound to
  it
• ORB interfaces provides operations on object
  references, access to interface and
  implementation repository

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Object Implementation Structure

• Most objects are not running and active all the
  time
• ORB, CORBAServices work together to activate the
  objects when necessary
• Context switch hidden from user
• Responsibility of handling object state changes
  shifted to object implementor
• When writing an OMA-compliant object, you have to
  provide a way to save the object state at shut
  down and to checkpoint the object state
• No standard enforcement on this implementor
  responsibility

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Object Adaptors

• Responsible for
• registering implementations
• generating and interpreting object references
• mapping object references to their corresponding
  implementations
• activating and deactivating object
  implementations
• invoking methods, via skeleton or DSI
• coordinating interaction security, in cooperation
  with the Security Object service

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Server-Side Summary

• Thin clients, fat servers
• ORB handles transparent communication
• Object adapter provides interfaces between ORB
  and object implementation represents
  implementation of object model
• To client, server always there, always available
  and always in consistent state
• Object implementation must support the
  realization of this client-side simplicity

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CORBAservices

• Representing basic functions needed by most
  application developments
• Reduced effort for application development and
  encourages compatible systems
• Bases for component-based software development
• Real savings for end-user companies adopting OMG
  technology
• Declared in IDL
• Explicit operation sequencing dependencies
• No implementation descriptions

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CORBAservices in OMA
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CORBAservices Architecture
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Event Service

• Defines generic interfaces for passing
  information among multiple sources and consumers
• Sources and consumers dont need to have direct
  knowledge of each other, thus de-coupling
  consumers from event sources with grouping and
  delivery mechanism managed by the Service
• Can be used as multicast mechanism without direct
  connection between sender and receivers

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Event Service Functions

• Supports multiple styles of interaction (between
  application and the Service two principal
  styles push and pull
• PUSH - event source makes out call to consumers
• PULL - event source waits for consumer to make a
  call back in order to receive the next event
  notification
• PULL consists of polled and blocking mode
• Supports different styles of interactions
  simultaneously all interoperating at the same
  time through the same event channel

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Event Service Interfaces
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Event Service Objects

• Event factory object implements the Lifecycle
  service operations specific to the Event service,
  and responsible for creating event channel
  objects
• Event channel object supports several interfaces
  for event notification and other operations

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Event Service Scenario
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Naming Service

• A general directory service to be used by most
  applications
• Provides mapping between object name and
  reference
• Can be used as an interface wrapper over
  existing naming directory services
• Names maybe object names or operation names
• Name bindings are always relative to a scope
  called naming context names are unique to their
  naming context
• Name resolution is mapping from name to object
  within a context

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Naming Context Hierarchy Example
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Naming Services

• Key operations bind and resolve
• Primary objects in the Naming service are naming
  context objects
• Schema that defines the directory trees and the
  naming conventions used in these trees is an
  application design choice
• Names are represented as a sequence of
  structures. Each structure is a (name, kind)
  pair. The intention is that the structure
  sequence would be converted into path names for
  use in platform specific environment

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Naming Service

• Naming service usually requires following
  conventions defined
• Definition of the local naming schema
• structure of naming contexts
• rules for extending the context
• Definition of the local naming conventions
• Well-known names
• conventions for new names
• semantics and values for the kind field

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Overview of CORBAdomains and CORBAfacilities

• CORBAdomains address interoperability within a
  vertical domain, e.g. healthcare, manufacturing,
  telecom, financial services, etc.
• CORBAfacilities address interoperability across
  vertical domains by providing a set of common
  facilities, e.g. compound documents and system
  management facilities, needed by multiple domains
• CORBAservices focus on enabling capabilities,
  CORBAfacilities and CORBAdomains focus on
  interoperability issues

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More Detailed View of OMA
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Horizontal CORBAfacilities

• Distributed Document Component Facility (DDCF)
• for transparent manipulation of compound
  documents in distributed environment
• Based on OpenDoc specifications
• Common Management Facilities
• based on submission from X/Open consortium
• System management automates the handling of
  computer support services across a distributed
  enterprise, e.g. remote update installation,
  monitoring and maintenance of security policies,
  etc

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Horizontal CORBAfacilities

• Internationalization and Time Operations
  Facilities
• Former supports multinational data types
  including output formats and conversions among
  formats
• Latter defines similar capabilities on time
  objects and conversions
• E.g. character classification, date/time formats,
  numeric formatting, monetary formatting, etc.
• Data Interchange Facility
• Interpretation, conversion and exchange among
  different data formats

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Primary Source of Information

• L. Bass, P. Clements and R. Kazman, Software
  Architecture in Practice, Addison-Wesley, 1998
• T.J. Mowbray and W.A. Ruh, Inside CORBA -
  Distributed Object Standards and Applications,
  Addison-Wesley, 1997
• K. Wreder and Y. Deng, Architecture-Centered
  Enterprise System Development and Integration
  Based on Distributed Object Technology,
  Proceedings of COMPSAC99.