Transcript Architecture

Architectural patterns
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
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Layers
Pipes and filters
Broker
Model-View-Controller (MVC)
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Categories of architectural patterns
• From mud to structure
– Patterns: Layers, Pipes and filters
• Distributed systems
– Patterns: Broker
• Interactive systems
– Patterns: Model-View-Controller (MVC)
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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: Can be described using interfaces
– 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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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
• Finds 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
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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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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
– (Napster registry + RMI / RPC registry)
• Does not forward messages
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Broker, consequences
• Benefits
– Location transparency
• Clients do no 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 => nothing is working
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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
• Web interface
• PDA interface
• Mobil phone interface
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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:
http://www.geocities.com/pbanta/ModelViewController.html
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Model-View-Controller,
consequences
• Benefits
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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
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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
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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
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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
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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
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Lethbridge & Langaniere ObjectOriented Software Engineering, 2nd
edition, McGraw-Hill 2005
9. Architecting and designing software
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