Architecture

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Architectural patterns
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Patterns

• Architectural patterns
• Fundamental structural organization for software
  systems.
• High-level subdivision of the system.
• Highest level of pattern
• Design patterns
• Organization of classes
• Intermediate level of pattern
• Idioms
• Organization within a single class or a few
  classes
• Lowest level of pattern
• Some architectural patterns
• Layers
• Pipes and filters
• Broker
• Model-View-Controller (MVC)
• Master-slave

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Categories of architectural patterns

• From mud to structure
• Patterns Layers, Pipes and filters
• Distributed systems
• Patterns Broker, Master-Slave
• Interactive systems
• Patterns Model-View-Controller (MVC)

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Architectural Pattern - Layers

• Decompose overall system task into cooperating
  subtasks Examples
• TCP/IP, and other protocol stacks
• Java application, JVM, OS, physical machine
• Information systems Presentation, Controller,
  Model, Data
• Upper layer asks lower layer for service
• Interfaces between layers must be stable
• Standardization Public or proprietary
• Java interface is a keyword of the language
• Error handling
• Exceptions
• Special values for signaling errors, like null or
  -1

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Layers consequences

• Benefits
• Reuse of layers
• Support for standardization
• A network protocol specifies a layer, but does
  not implement it!
• Dependencies are kept local
• Exchangeability
• An implementation of a layer can be exchanged
  with another implementation
• With the same interface
• Liabilities
• Lower efficiency
• Difficult to establish correct granularity of
  layers
• Layers inside layers might help

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Architectural Pattern - Pipes and filters

• System process a stream of data.
• Each processing step is encapsulated in a filter
  component.
• Examples
• A shell in an operating system, like Linux/UNIX
  or MS-DOS
• find a data.txt sort more
• Find all lines with a in data.txt sort the
  lines show the lines, one screen at a time
• Compilation of a Java program
• Lexical analysis syntax analysis semantic
  analysis code generation
• Generally
• Data source filter filter data sink
• The vertical bar is called a pipe.
• Implementation
• Cooperating processes
• No shared memory Uses standard input (System.in)
  and standard output (System.out)
• Cooperating threads
• Shared memory Bounded buffers
• Example The bounded buffer exercise

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Pipes and filters, consequences

• Benefits
• Flexibility by filter exchange
• Plug-in a new filter, and you have another
  application
• Flexibility by recombination
• Reuse of filter components
• Useful for prototyping
• Efficient in parallel processing
• Liabilities
• Sharing information is hard
• Data transformation overhead
• Data must be adapted to the next filter
• Error handling

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Distributed systems

• A layered system can be distributed
• Each layer running on a separate computer
• Called multi-tiered system
• A pipes and filters system can be distributed
• Each filter running on a separate computer
• Pipes are network connections

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Architectural Pattern - Broker

• Coordinates communication between distributed
  components
• Decouples clients and servers
• Broker features
• Register / deregister servers
• Locate servers
• Forward messages
• Examples
• CORBA
• Common Object Request Broker Architecture
• Some chat / messenger systems

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Broker, consequences

• Benefits
• Location transparency
• Clients do not need to know where servers are.
• Servers can be moved to other computers.
• Changeability and extensibility
• Servers can be changed
• Keep the same interface
• Reusability
• Components can be reused in other application
• Liabilities
• Efficiency
• Fault tolerance
• Broker is not working gt nothing is working

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Architectural Pattern Master-Slave

• Divide and conquer
• Master use same subservice from slaves
• Master functions
• Split work
• Call slaves
• Combine results
• Examples
• Parallel processing
• Fault tolerance
• Computational accuracy

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Master-Slave, consequences

• Benefits
• Faster computation
• Split the problem over threads and machines.
• Easy scalability.
• Robustness
• Slaves can be duplicated
• Correctness
• Slave can be implemented differently to minimize
  sementic errors
• Liabilities
• Communication overhead
• Not all problems can be divided

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Interactive systems

• Interaction with the user
• Through graphical user interfaces
• System responds to events (user inputs)
• Functional core of the system must be kept
  independent of the user interface
• You must be able to add a new user interface to
  the system.
• A single system can have many user interfaces
• PC interface
• Web interface
• PDA interface
• Mobil phone interface

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Architectural Pattern Model-View-Controller (MVC)

• Divides an application into 3 parts
• Model
• Core functionality and data
• View
• Displays information to the user
• Same model component can have many views
• Controller
• Handles user input
• User interface View controller
• Changes in the model are automatically propagated
  to the view
• Use observer-observable design pattern

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Model-View-Controller in Java

• The Java GUI frameworks Swing (and AWT) uses a
  modified version of MVC
• Each Swing component has
• Model
• Holding the state of the component
• View
• The visual part
• Controllers
• Are attached by programmers
• Example JButton
• Model ButtonModel getModel()
• View ButtonUI getUI()
• Controllers addActionListener(),
  addItemListener(), attChangeListener()
• Example, JavaBeans AccountConstrained (Model)
  AccountFrame (View controller)

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Model-View-Controller, consequences

• Benefits
• Multiple views for the same model
• Synchronized views
• Pluggable views and controllers
• Exchangeability of look and feel
• Port to a new windowing platform does not change
  the model.
• Liabilities
• Increased complexity
• Excessive number of updates
• Intimate connection between view and controller
• Close coupling of (view, controller) and model
• Both view and controller make calls to model

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Distributed systems

• Advantages
• Resource sharing
• Printers, files (HTTP), etc.
• Openness
• Using standard protocols
• Scalability
• Fault tolerance
• Replication
• Disadvantages
• Complexity
• Security
• Manageability
• Unpredictability

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Client-server vs. distributed objects

• Client-Server
• Clients and servers are treated differently
• Distributed objects
• Interacting objects
• Location is irrelevant (taken care of by
  middleware)
• Build on top of client/server
• Like distributed library
• Java RMI
• CORBA

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Service-oriented architecture

• SOA
• Web services
• HTTP for transportation
• SOAP structured data exchange
• UDDI for discovery
• WSDL for description
• XML used everywhere
• Service registry
• Service provider
• Service requestor

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References

• Buschmann et al. Pattern-Oriented Software
  Architecture, Volume1, Wiley 1996
• Martin Fowler Patterns of Enterprise Application
  Architecture, Addison Wesley 2003
• Ian Sommerville Software Engineering, 7th
  edition, Addison Wesley 2004
• 11. Architectural Design
• 12. Distributed System Architectures
• Lethbridge Langaniere Object-Oriented Software
  Engineering, 2nd edition, McGraw-Hill 2005
• 9. Architecting and designing software