CSSE374 Iteration 3: More Object Design with GoF Patterns

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Below Pattern-making is an old art. This is
Batik, which is a national art form in Indonesia.
From http//en.wikipedia.org/wiki/Batik.
CSSE374 Iteration 3 More Object Design with
GoF Patterns
Steve Chenoweth Day 33, Feb 6, 2009
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Today coming up

• Today
• More Object Des with GoF patterns (Ch 36) ?
• Revisit what you think you can deliver in P6?
• Monday, Feb 9
• HW 8 applying these ideas to your project
• Persistence frameworks (Ch 37)
• Tuesday, Feb 10
• UML deployment components (Ch 38)
• Thursday, Feb 12
• P6 due!
• Documenting archs (Ch 39)
• Friday, Feb 13
• P6A Turn in doc describing lessons learned from
  project
• More on iterative agile (Ch 40)

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More Object Des with GoF patterns

• This is not all the GoF patterns. ?
• Big application focus in this Ch is error
  handling.
• In NextGen POS
• Failover to a local service when a remote service
  fails
• Local caching
• Support for third-party POS devices, such as
  different scanners
• Handling credit, debit, and check payments
• Note the overall idea that designing these things
  has been deferred to Iteration 3!

Ch 36
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Heres the whole GoF list

• Creational Patterns
• Abstract Factory
• Builder
• Factory Method
• Prototype
• Singleton
• Structural Patterns
• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

• Behavioral Patterns
• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template Method
• Visitor

Ones we did / will do are in red.
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NextGen POS Error Handling

• Typical tough situation remote service failure
  how to avoid closing the store!
• E.g., were using a remote product database, and
  the connection is interrupted.
• Similarly for tax calculation, inventory, credit
• What would you do to prevent these situations
  from being catastrophic? (Quiz question 1)

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Lets analyze the books solution

• The ServicesFactory will always return an adapter
  to a local product information service.
• The local products adapter is not really an
  adapter to another component. It will itself
  implement the responsibilities of the local
  service.
• The local service is initiated to a reference to
  a second adapter to the true remote product
  service.
• If the local service finds the data in its cache,
  it returns it otherwise, it forwards the request
  to the adapter for the external service.
• The local cache is actually 2 levels in-memory
  and on-disk.

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Adapters for product information
How would you handle the stale cache problem?
(Quiz question 2)
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Advantages / disadv of local cache

• Faster performance
• Backup for remote failure
• May not agree (stale cache problem)
• Local is likely to be incomplete
• Note that these are quality attribute decisions!

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This is a GoF Proxy

• Pattern Proxy
• Context/Problem Direct access to a real object
  is not desired or possible.
• Solution Add a level of indirection with a
  surrogate proxy object that implements the same
  interface as the subject object, and has
  responsibility for controlling or enhancing
  access to it.

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Error guidelines / patterns

• Exceptions are especially appropriate when
  dealing with resource failures (disk, memory,
  network, database, etc.)
• Pattern Convert Exceptions (Exception
  Abstraction)
• Within a subsystem, avoid emitting lower level
  exceptions. Convert into ones meaningful at the
  subsystem level. It wraps the lower-level one
  and adds higher-level info.

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Error guidelines / patterns, cntd

• Pattern Name the problem, not the thrower
• Assign a name to an exception that describes why
  the exception is being thrown, not the exception.
  This makes it easier for the programmer to
  understand what to do about it, and lets you
  group similar exceptions by rationale.
• Pattern Centralized error logging
• Use a Singleton-accessed central error logging
  object and report all exceptions to it. In a
  distributed system, each local singleton will
  collaborate with a central error logger.

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Error guidelines / patterns, cntd

• Pattern Error Dialog
• Use a standard Singleton-accessed
  application-independent non-UI object to notify
  users of errors.
• It wraps one or more UI dialog objects (suchas
  a GUI modal dialog, text console, sound beeper,
  or speech generator) and delegates the
  notification of the error to the UI objects.
• Thus, output could go to both a GUI dialog and to
  a speech generator.
• It will also report the exception to the
  centralized error logger.

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Error Handling in UML
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Abstract Factory

• Weve already discussed factories which
  create objects for us, when the main user of
  those would be too complex by including this.
• Abstract factory adds one more level of
  indirection you might do the object creations
  in different ways, depending upon some variables.
• Java creates different GUI objects depending on
  what O/S it is operating under.
• In NextGen POS, how to use the software for
  different hardware, for IBM versus NCR POS
  terminals.

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Abstract Factory, cntd

• Context / Problem How to create families of
  related classes that implement a common
  interface? E.g.,
• cashDrawer JavaPOSDevicesFactory.getInstance().g
  etNewCashDrawer()
• returns a cashDrawer for the right class (IBM vs
  NCR), depending on how you want it done.
• Solution Define a factory interface (the
  abstract factory). Define a concrete factory
  class for each family of things to create.
  Optionally, define a true abstract class that
  implements the factory interface and provides
  common services to the concrete factories that
  extend it.

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Abstract Factory, cntd