Software Architecture - 2

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Software Architecture - 2

• April 14, 2020

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Architectural Patterns - 2

• Implicit Asynchronous Communication Architecture
• Non-buffered event-based Implicit Invocation
• Buffered messaged-based
• Interaction-Oriented Architecture
• Model-View-Controller (MVC)
• Presentation-Abstraction-Control (PAC)
• Distributed Architecture
• Client-server
• Broker
• Service-oriented architecture (SOA)
• Component-based Architecture

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Implicit Asynchronous Communication Architecture

• Overview
• Asynchronous implicit invocation communication
• Non-buffered
• Buffered.
• Publisher-subscriber (producer-consumer)
• Subscribers are interested in some
  events/messages issued by a publisher
• Subscribers register themselves with the event
  source.
• Once an event is fired off by an event source,
  all corresponding subscribers are notified.
• It is up to the subscribers to decide on the
  actions to execute.

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Implicit Asynchronous Communication Architecture

• Publisher-subscriber (producer-consumer)
• The message queue/topic are typical buffered
  asynchronous architectures
• subscribers also need to register their interests
  with the event/message is fired off when
  available on the buffered message queue or topic.
  
• Message queue
• one-to-one or point-to-point architecture between
  message senders and message receivers
• Message topic
• one-to-many architecture between publishers and
  subscribers.

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Non-Buffered Event-Based Implicit Invocations

• The architecture breaks the system into two
  partitions
• Event sources
• event listeners.
• The event registration process connects these two
  partitions.
• There is no buffer available between these two
  parties.

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Non-Buffered Event-Based Implicit Invocations
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Non-Buffered Event-Based Implicit Invocations
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Non-Buffered Event-Based Implicit Invocations
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Non-Buffered Event-Based Implicit Invocations

• The event-based implicit invocation is a good
  solution for user interaction in a user interface
  application. Following are simple Java fragments
  that demonstrate how event sources and event
  targets (listeners) work together in an
  event-driven architecture for a user interface
  application.

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Non-Buffered Event-Based Implicit Invocations

• Applications
• Suitable for interactive GUI component
  communication.
• Suitable for applications that require loose
  coupling between components that need to notify
  or trigger other components to take actions upon
  asynchronous notifications.
• Suitable for the implementation of state
  machines.
• Suitable when event handlings in the application
  are not predictable.

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Non-Buffered Event-Based Implicit Invocations

• Benefit
• Many vendor APIs such as Java AWT and Swing
  components available.
• Easy to plug-in new event handlers without
  affecting the rest of the system.
• Easy to update both of event sources and targets.
  
• Dynamic registration and deregistration can be
  done dynamically at run-time.
• Possibility of parallel execution of event
  handlings.

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Non-Buffered Event-Based Implicit Invocations

• Limitations
• Difficult to test and debug the system since it
  is hard to predict and verify responses and the
  order of responses from the listeners.
• The event trigger cannot determine when a
  response has finished or the sequence of all
  responses.
• There is tighter coupling between event sources
  and their listeners than in message queue based
  or message topic based implicit invocation.

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Buffered Message-Based Software Architecture

• The architecture breaks into three partitions
• Message producers,
• message consumers, and
• message service providers.
• They are connected asynchronously by either
• a message queue
• a message topic.
• This architecture is also considered data-centric.

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Buffered Message-Based Software Architecture

• What is a message?
• A message is a structured data with an id,
  message header, property, and body, e.g. an XML
  document.
• What is messaging?
• A mechanism or technology that handles
  asynchronous or synchronous message delivery
  effectively and reliably.
• A messaging client can send messages to other
  clients and receive messages from other clients.
• A client must register with a messaging
  destination in a connection session provided by
  a message service provider for creating,
  sending, receiving, reading, validating, and
  processing messages.

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Point-to-Point Messaging (P2P)

• A P2P messaging architecture is composed of
  message queues, senders, and receivers.
• Each message is sent to a specific queue
  maintained by the consumer
• The queue retains all messages until either the
  messages are consumed or the messages expire.
• Each message has only one consumer, i.e., the
  message will be gone once it is delivered.
• This approach allows multiple receivers but only
  one of them will get the message.
• P2P messaging requires every message in the queue
  be processed by a consumer.

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Point-to-Point Messaging (P2P)
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Publish-Subscribe Messaging (PS)

• The PS messaging architecture is a hub-like
  architecture, where publisher clients send
  messages to a message topic that acts like a
  bulletin board.
• Message topic publishers and subscribers are not
  aware of each other.
• One difference between PS from P2P is that each
  topic message can have multiple consumers.
• The system delivers the messages to all its
  multiple subscribers instead of single receiver
  as in the message queue system.
• Normally a message topic consumer must subscribe
  the topic before it is published

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Publish-Subscribe Messaging (PS)
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Buffered Message-Based Software Architecture

• Applications
• Suitable for systems where the communication
  needs buffered message-based asynchronous
  implicit invocation for performance and
  distribution purposes.
• The provider wants the components not to depend
  on information about other components'
  interfaces, so that components can be easily
  replaced.
• The provider wants the application to run whether
  or not all other components are up and running
  simultaneously.
• A component can send information to another and
  continue to operate on its own without waiting
  for an immediate response.

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Buffered Message-Based Software Architecture

• Benefits
• Providing high degree of anonymity between
  message producer and consumer.
• The message consumer does not know who produced
  the message (user independence), where the
  producer lives on the network (location
  independence), or when the message was produced
  (time independence).
• Supporting for concurrency among consumers and
  between producer and consumers.

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Buffered Message-Based Software Architecture

• Limitations
• Capacity limit of message queue. This is not an
  inherent limitation but an implementation issue
  that can be eased if the queue is implemented as
  a dynamic data structure (e.g., linked lists).
• However, there is an absolute limit based on
  available memory. It is also difficult to
  determine the numbers of agents needed to satisfy
  the loose couplings between agents.
• Complete separation of presentation and
  abstraction by control in each agent generate
  development complexity since communications
  between agents only take place between the
  control of agents.
• Increased complexity of the system design and
  implementation

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Chapter 9INTERACTION ORIENTED SOFTWARE
ARCHITECTURES
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INTERACTION ORIENTED SOFTWARE ARCHITECTURES

• The key point - the separation of user
  interactions from data abstraction and business
  data processing.
• The interaction oriented software architecture
  decomposes the system into three major
  partitions
• Data module
• Control module
• View presentation module
• The data module provides the data abstraction
  all business logic.
• The view presentation module is responsible for
  data output presentation and it may provide an
  input interface for user input.
• The control module determines the flow of control
  involving view selections, communications between
  modules, job dispatching, and certain data
  initialization and system configuration actions.
• Multiple view presentations in different formats
  are allowed

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INTERACTION ORIENTED SOFTWARE ARCHITECTURES

• Two major style
• Model-View-Controller (MVC)
• Presentation-Abstraction-Control (PAC)
• Both of MVC and PAC are used for interactive
  applications multiple talks and user
  interactions.
• They are different in their flow of control and
  organization.
• The PAC is an agent based hierarchical
  architecture
• The MVC does not have a clear hierarchical
  structure and all three modules are connected
  together.

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Model-View-Controller (MVC)

• Objects of different classes take over the
  operations related to the application domain (the
  model), the display of the application's state
  (the view), and the user interaction with the
  model and the view (The controller).
• Models
• The model of an application is the
  domain-specific software simulation or
  implementation of the application's central
  structure.
• Views
• In this metaphor, views deal with everything
  graphical they request data from their model and
  display the data.
• Controllers
• Controllers contain the interface between their
  associated models and views and the input devices
  (e.g., keyboard, pointing device, time)

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MVC-I

• The MVC-I is a simple version of MVC architecture
  where the system is simply decomposed into two
  sub-systems The Controller-View and the Model.
• Basically, the Controller-View takes care of
  input and output processing and their interfaces
  the Model module copes with all core
  functionality and the data.
• The Controller-View module registers with
  (attaches to) the data module.

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MVC-I

• The Model module notifies the Controller-View
  module of any data changes so that any graphics
  data display will be changed accordingly the
  controller also takes appropriate action upon the
  changes.
• The connection between the Controller-View and
  the Model can be designed in a pattern of
  subscribe-notify whereby the Controller-View
  subscribes to the Model and the Model notifies
  the Controller-View of any changes.
• In other words, the Controller-View is an
  observer of the data in the Model module.

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MVC-I
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MVC-I

• Simple GUI example designed in MVC-I.
• The View has a GUI interface with two text
  fields, for the user to enter a new number in one
  of the text fields and the accumulated summation
  is displayed in the other text field.
• The summation is held in the Model module.
• Model provides all business logics including all
  getter and setter methods.

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MVC-I

• Whenever the data in the Model is updated it will
  notify the registered GUI components of changes,
  and then the GUI components will update their
  displays.
• This is why we say that the data in the Model of
  MVC architecture is active rather than passive.
• Actually, for this specific example there is no
  need to have separated Model to notify the change
  because actionPerformed() can take care all
  necessary changes.
• We just want to use this example to see how MVC-I
  architecture works.

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MVC-II
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MVC-II

• The MVC in figure 9.2 is the same as MVC-I in
  figure 9.1 except that the controller and the
  view are separated.

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MVC-II
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MVC-II

• Fig 9.4 depicts a sequence diagram for a generic
  MVC architecture.

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MVC-II

• After clients start up the MVC application, the
  controller initializes the Model and View, and
  attaches the View and itself to the Model (this
  is called registration with the Model.
• Later, the Controller intercepts a user request
  either directly from command line or through the
  View interface, and forwards the request to the
  Model to update the data in the Model.
• The changes in the Model will trigger the Model
  to notify all attached or registered listeners of
  all changes, and the interface in the View will
  also be updated right way.

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MVC-II
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MVC-II

• The myServlet Servlet sets an item value and
  stores this item in a JavaBean named myBean, then
  transfers the control to a JSP page named
  fromServlet.jsp which retrieves the item from the
  myBean and displays it on a Web page.
• Whenever the data is changed the display is also
  changed.
• The following Java classes show a MVC-II template
  to provide more detailed explanations on the
  MVC-II architecture.

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MVC

• Applications
• Suitable for interactive applications where
  multiple views are needed for a single data model
  and the interfaces are prone to data changes
  frequently.
• Suitable for applications where there are clear
  divisions between controller, view, and data
  modules so that different professionals can be
  assigned to work on different aspects of such
  applications concurrently.

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MVC

• Benefits
• There are many MVC vendor framework toolkits
  available.
• Multiple views synchronized with same data model
• Easy to plug-in new or change interface views,
• Very effective for developments if Graphics
  expertise professionals, programming
  professionals, and data base development
  professionals are working in a team in a designed
  project.

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MVC

• Does not fit well agent-oriented applications
  such as interactive mobile and robotics
  applications.
• Multiple pairs of controllers and views based on
  the same data model make any data model change
  expensive.
• The division between the View and the Controller
  is not clear in some cases.

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Presentation-Abstraction-Control (PAC)

• The PAC architecture is similar to MVC but with
  some important differences.
• The PAC was developed from MVC to support the
  application requirement of multiple agents in
  addition to interactive requirements.
• In PAC, the system is decomposed into a hierarchy
  of many cooperating agents.

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Presentation-Abstraction-Control (PAC)

• Each agent has three components (Presentation,
  Abstraction, and Control).
• The Control component in each agent is in charge
  of communications with other agents.
• The top-level agent provides core data and
  business logics.
• The bottom level agents provide detailed specific
  data and presentations.
• A middle level agent may play a role of
  coordinator of low-level agents.
• There are no direct connections between
  Abstraction component and Presentation component
  in each agent.

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Presentation-Abstraction-Control (PAC)

• The PAC three components concepts are applied to
  all concrete sub-system architectures.
• It is very suitable for any distributed system
  where all the agents are distantly distributed
  and each of them has its own functionalities with
  data and interactive interface.
• In such a system, all agents need to communicate
  with other agents in a well-structured manner.
• The PAC is also used in applications with rich
  GUI components where each of them keeps its own
  current data and interactive interface and needs
  to communicate with other components.

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Presentation-Abstraction-Control (PAC)

• Of course, some concrete agent needs all three
  components and some other agents do not.
• For some middle level agents the interactive
  presentations are not required, so they do not
  have a Presentation component.
• The control component is required for all agents
  because this is the only way for an agent to talk
  to another agent.
• Figure 9.6 shows a block diagram for a single
  agent in PAC design.

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Presentation-Abstraction-Control (PAC)

• The Control component is a mediator between the
  Presentation component and the Abstraction
  component within the agent, and also a bridge
  between the agent itself and other agents as
  well.
• The Presentation component and the Abstraction
  component are loosely coupled.
• The Presentation component is responsible for
  both data input and data output in GUI interfaces
  where the data come from the Abstraction
  component.
• The Abstraction component is responsible for
  providing logical data concepts and services and
  encapsulating all detailed data manipulation.

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Presentation-Abstraction-Control (PAC)
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Presentation-Abstraction-Control (PAC)

• Assume that the current page is the second to
  last page in the document at this time.
• If the user clicks on the next button, the
  control agent C4 informs agent P4 to update its
  presentation, in this case, it also hides the
  next button since there is no next page after
  last page.
• Agent C4 also informs agent A4 to update the data
  on next button.

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Presentation-Abstraction-Control (PAC)

• After C4 handles all local processing, it
  contacts its parent agent, C1, to let it take
  over.
• After C1 gets the message from C4, it tells A1 to
  move the next page, which is the last page in the
  document, and then asks P1 to display that page.
• C1 also informs C5 to hide the last button since
  the current page is the last page (or let the
  last button stay based on the requirement
  specification).

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Presentation-Abstraction-Control (PAC)

• We can see that all the agents communicate via
  the controls.
• The data structure shown on the upper-right
  corner of Figure 9.7 indicates the pointer and
  data.
• Since PAC2, PAC3, PAC4, and PAC5 are all buttons,
  they have very similar data and presentation
  functionality such as hide, move-over, gray-out
  features their controls, however, are different.
  

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Presentation-Abstraction-Control (PAC)
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Presentation-Abstraction-Control (PAC)

• The sequence diagram in Figure 9.9 shows the
  interaction sequence in the example we discussed
  above.
• When the next button is pressed to display the
  last page in the document PAC4 and PAC1 react as
  follows
• P4 informs C4 that the next button was
  pressed
• C4 sends update to A4
• C4 informs P4 to update its presentation or
  shape
• C4 contacts C1 (a top level agent).
• C1 sends update to A1 to move the pointer to
  next (last page)
• C1 instructs P1 to display the last page.

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Presentation-Abstraction-Control (PAC)
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Presentation-Abstraction-Control (PAC)
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Presentation-Abstraction-Control (PAC)

• Applications
• Suitable for an interactive system where the
  system can be divided into many cooperating
  agents in a hierarchical manner.
• Each agent has its own specific assigned job.
• Suitable when the coupling among the agents is
  expected to be loose so that changes on an agent
  does not affect others.

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Presentation-Abstraction-Control (PAC)

• Benefit
• Support of multi-tasking and multi-viewing.
• Support agent reusability and extensibility.
• Easy to plug-in new agent or replace an existing
  one.
• Support for concurrency where multiple agents are
  running in parallel in different threads or
  different devices or computers.

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Presentation-Abstraction-Control (PAC)

• Limitations
• Overhead due to the control bridge between
  presentation and abstraction and the
  communication of controls among agents.
• Difficult to determine the right number of the
  agents due to the loose coupling and high
  independence between agents
• Complete separation of presentation and
  abstraction by control in each agent generate
  development complexity since communications
  between agents only take place between the
  controls of agents.

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Presentation-Abstraction-Control

• Context
• Development of an interactive system with the
  help of agents
• Problem
• A system consists of a set cooperating agents.
• Each agent is responsible for one particular task
• Solution
• Structure the application as a tree-like
  hierarchy of PAC agent.
• Each agent is responsible for one aspect of the
  systems functionality

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Presentation-Abstraction-Control

• Each agent consists of three components
  presentation, abstraction, and control
• The presentation component provides the visible
  behavior of the agent
• The abstraction component maintains the data
  model and operations on the data model
• Control component connects the presentation and
  abstraction and communication with other agents.
• The top level agent provides the functional core
  of the system
• Bottom level agents represent self-contained
  semantic concepts on which the user can act, such
  as a spreadsheet
• Intermediate level agents represent either
  combinations of, or relationships between, low
  level agents.
• Known uses
• Network Traffic Management
• Mobile Robot

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PAC Pattern - Structure
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PAC Pattern - Structure
ViewMediator Agent
BarChart Agent
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Chapter 10DISTRIBUTED ARCHITECTURE
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Overview

• A distributed system can be modeled by the
  client-server architecture, and this forms the
  basis for multi-tier architectures alternatives
  are the broker architecture such as CORB, and the
  Service-Oriented Architecture (SOA).
• The important features of a distributed
  architecture are its service location
  transparency, and its services reliability and
  availability.
• Additionally, there are several technology
  frameworks to support distributed architectures,
  including .NET, J2EE, CORBA, .NET Web services,
  AXIS Java Web services, and GloBus Grid services.

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Client-Server
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Client-Server
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Client-Server

• Advantages
• Separation of responsibilities such as user
  interface presentation and business logic
  processing.
• Reusability of server components.
• Disadvantages
• Lack of heterogeneous infrastructure to deal with
  the requirement changes.
• Security complications.
• Server availability and reliability.
• Testability and scalability.
• Fat clients with presentation and business logic
  together.

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Multi-tiers
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Multi-tiers
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Broker Architectural Style

• Overview
• The broker architecture is a middleware
  architecture used in distributed computing to
  coordinate and facilitate communication between
  registered servers and clients.
• It can be used to structure distributed software
  systems with decoupled components that interact
  by remote service invocations.
• A broker component is responsible for
  coordinating communication, such as forwarding
  requests, as well as for transmitting results and
  exceptions.
• A broker can be either an invocation-oriented
  service, to which clients send invocation
  requests for brokering, or a document or message-
  oriented broker to which clients send a message
  (such as an XML document).
• Client and the server never interact with each
  other directly.
• A broker system is also called the proxy-based
  system.

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Broker Architectural Style

• Servers make their services available to their
  clients by registering and publishing their
  interfaces with the broker.
• Clients can request the services of servers from
  the broker statically or dynamically by look-up.
• A broker component is responsible for
  coordinating communications brokering the
  service requests, locating a proper server,
  forwarding and dispatching requests, and sending
  responses or exceptions back to clients.
• CORBA (Common Object Request Broker Architecture)
  is a good implementation example of the broker
  architecture.

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Broker Architectural Style

• In addition, the clients can dynamically invoke
  the remote methods even if the interfaces of the
  remote objects are not available at the
  compilation time.
• Client has a direct connection to its
  client-proxy and server has direct connection to
  its server-proxy.
• The proxy talks to the mediator-broker.

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Broker Architectural Style

• Components
• Broker
• Stub
• Skeleton
• Bridge
• Network

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Broker Component

• Co-ordinates communications passes on requests
  and returns replies.
• The broker keeps all servers registration
  information including their functionality and
  services as well as location information.
• The broker provides APIs for clients to request,
  servers to respond, registering or unregistering
  server components, transferring messages, and
  locating servers.

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Stub Component

• Client-side proxy It mediates between client and
  broker and provides additional transparency
  between them.
• To the client, a remote object appears like a
  local one.
• The proxy hides the inter-process communication
  at protocol level and performs marshalling of
  parameter values and unmarshaling of results from
  the server.
• The stub is generated at the static compilation
  time and deployed to the client side to be used
  as a proxy for the client.

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Skeleton Component

• Server-side proxy It is also statically
  generated by the service interface compilation
  and then deployed to the server side.
• It encapsulates low-level system-specific
  networking functions like what client-proxy does
  and provides high-level APIs to mediate between
  the server and the broker.
• It receives the requests, unpacks the requests,
  unmarshals the method arguments, and calls the
  appropriate service.
• When it receives the result back from the server
  it also marshals the result before sending it
  back to the client.

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Bridges

• These optional components are used to hide
  implementation details when two brokers
  interoperate.
• It encapsulates underlying network detail
  implementation and mediates different brokers
  such as DCOM, .NET Remote and Java CORBA brokers.
  
• They can take requests and parameters in one
  format and translate them to another format.
• A bridge can connect two different networks based
  on different communication protocols.

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Network Component

• Network The connections between the components
  are the network with designated protocol
  standards such as TCP/IP OIIP or SOAP.
• The request carries data in a format of message
  document or method invocation.

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Broker model
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Brokers with client-server proxy
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Brokers with client-server proxy
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Brokers with client-server proxy
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Brokers with client-server proxy

• Advantages
• Server component implementation and location
  transparency.
• Changeability and extensibility.
• Simplicity for clients to access server and
  server portability.
• Interoperability via broker bridges.
• Reusability.
• Feasibility of runtime changes of server
  components (add or remove server components on
  the fly).
• Disadvantages
• Inefficiency due to the overhead of proxies
• Low fault-tolerance
• Difficulty in testing due to the amount of
  proxies

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CORBA
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Service-Oriented Architecture (SOA)

• Overview
• A Service Oriented Architecture (SOA) starts with
  a businesses process.
• A service is a business functionality that is
  well-defined, self-contained, independent from
  other services, and published and available to be
  used via a standard programming interface.
• Software manages business processes through a SOA
  with well-defined and standard interfaces that
  can build, enhance, and expand their existing
  infrastructure more flexible.

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Service-Oriented Architecture (SOA)

• SOA services can be extensively reused within a
  given domain or product line, and even among
  legacy systems.
• Loose coupling of serviceorientation provide
  great flexibility for enterprises to make use of
  all available service recourses regardless of
  platform and technology restrictions.
• The connections between services are conducted by
  common and universal message oriented protocols
  such as the SOAP Web service protocol, which can
  deliver requests and responses between services
  loosely.
• A connection can be established statically or
  dynamically.

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Service-Oriented Architecture (SOA)
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Service-Oriented Architecture (SOA)
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SOA Implementation in Web Services
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Service-Oriented Architecture (SOA)

• Advantages
• Loose-coupling is the key attribute of SOA.
• Each service component is independent from other
  services due to the stateless service feature.
• The implementation of a service will not affect
  the application of the service as long as the
  exposed interface is not changed.
• A client or any service can access other services
  regardless of their platform, technology,
  vendors, or language implementations.
• Any service can be reused by any other service.
  Because clients of a service only need to know
  its public interfaces, service composition and
  integration become much easier.
• SOA based business application development comes
  much more efficient in term of time and cost.
• Loosely coupled services make themselves easy to
  scale.
• The coarse-grained, document-oriented, and
  asynchronous service features enhance the
  scalability attribute.