Chapter 3 Idioms

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Chapter 3 - Idioms
CIS 476/566 Software Architecture and Design
Patterns

• Dr. Brahim Medjahed
• brahim_at_umich.edu

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Outline

• Major Types of Patterns
• Idioms

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Three Major Types of Patterns

• Architectural Patterns
• Design Patterns
• Idioms

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Architectural Pattern

• Expresses a fundamental structural organization
  or schema for software systems.
• Provides a set of predefined subsystems,
  specifies their responsibilities, and includes
  rules and guidelines for organizing the
  relationships between them.

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Design Pattern

• Provides a scheme for refining the subsystems or
  components of a software system, or the
  relationships between them.
• Describes commonly recurring structure of
  communicating components that solves a general
  design problem within a particular context.

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Idiom

• Low-level pattern specific to a programming
  language.
• It describes how to solve implementation-specific
  problems in a programming language
• Example memory management in C

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Describing Idioms

• Design Patterns are described
• By means of software design constructs, for
  example objects, classes, inheritance,
  aggregation and use-relationship.
• Idioms are described
• By means of programming language constructs.

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Example Single Thread Execution

• Reason
• To prevent concurrent access to data or other
  resources (aka Critical Section).
• Synopsis
• Synchronize on a same object the portions of code
  that should not be concurrently executed.
• Example
• Several threads set and get a datum held by an
  object. The time of an access is
  non-deterministic.

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Idiom

• class HoldDatum
• private Datum datum
• public synchronized void set (Datum d)
• datum d
• public synchronized Datum get ()
• return datum

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Guarded Suspension

• Reason
• A method should wait to execute until a
  precondition holds.
• Synopsis
• Use an object's methods wait and notify.
• Example
• A queue is used by two threads to exchange
  objects. A thread can take an object from the
  queue only if the queue contains elements.

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Idiom

• // Code requiring pre-condition
• synchronized void guardedMethod ()
• while (! precondition)
• wait ()
• // code requiring precondition
• // Code that changes pre-condition
• synchronized void stateChangingMethod ()
• // code possibly changing precondition
• notify ()
• // Both methods are synchronized on same object

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Balking

• Reason
• Disregard the call to a method of an object, if
  the state of the object is not appropriate to
  execute the call.
• Synopsis
• Test the object's state, in a synchronized block
  for thread safety, and return if the state is
  inappropriate.
• Example
• A toilet with an automatic flusher should not
  begin a flush cycle when another cycle is in
  progress.

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Idiom

• void balkingMethod ()
• synchronized (this)
• if (! someCondition)
• return
• else
• ... // code possibly changing some condition
• ... // do whatever

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Read/Write Lock

• Reason
• To allow concurrent read access to an object, but
  exclusive write access.
• Synopsis
• Use a specialized Scheduler whose method enter is
  replaced by two methods readLock and writeLock.
• Example
• An on-line auction system where remote users get
  and set bids.

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Idiom

• // Example ReadLock
• void synchronized readLock ()
• while (pendingWriters activeWriters gt 0)
• wait ()
• activeReaders
• // being reading

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Idiom (contd)

• // Example WriteLock
• void synchronized writeLock ()
• pendingWriters
• // block readers
• while (activeReaders activeWriters gt 0)
• wait ()
• pendingWriters--
• activeWriters
• // being writing

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Idiom (Contd)

• // Example of done()
• void synchronized done ()
• if (activeWriters gt 0)
• activeWriters--
• else if (activeReaders gt 0)
• activeReaders--
• else
• error ()
• notifyAll ()