Design Patterns for DistributedRealTime Systems ::Active Object:: ::Monitor Object::

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Design Patterns forDistributed/Real-TimeSystems
Active ObjectMonitor Object

• Group 5
• Eric Au
• Sandeep Mistry
• Amirsam Khataei
• Ravisangar Sathiyaseelan

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Introduction

• A Design Pattern is a general reusable solution
  to a commonly occurring problem in software
  design.1
• A Distributed Computing System is a method of
  computer processing in which different parts of a
  program run simultaneously on two or more
  computers that are communicating with each other
  over a network. 2
• A Real-Time Computing System is the study of
  hardware and software systems which are subject
  to a "real-time constraint.3

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Distributed/ Real-Time SystemDesign Pattern

• Its a reusable solution that can be applied
  either
• A Distributed System
• or
• A Real-Time System

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Active Object

• Intent
• Decouples method execution from method
  invocation to enhance concurrency and simplify
  synchronized access to an object that resides in
  its own thread of control. 4
• This design pattern may also be referred to as
  Concurrent Object and Actor

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Structure of an Active Object Design Pattern
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Active Object Sequence Diagram
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Active Object Design Pattern Example - Restaurant

• Customer -gt Client
• Waiter -gt Proxy

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Active Object Design Pattern Example - Restaurant

• Manager -gt Scheduler
• Order Queue -gt Activation Queue

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Active Object Design Pattern Example - Restaurant

• Order -gt Method Request
• Servant -gt Chef

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Active Object Design Pattern Consequences

• Pros
• Enhances concurrency
• Simplifies synchronization
• Method execution order can differ from invocation
  order
• Cons
• Performance overhead
• Hard to debug

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Active Object Design Pattern Known Uses

• CORBA ORBs
• ACE Framework
• Siemens MedCom
• Siemens Call Center management system
• Actors

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Monitor Object

• Intent
• The Monitor Object pattern synchronizes method
  execution to ensure only one method runs within
  an object at a time. It also allows an objects
  methods to cooperatively schedule their execution
  sequences. 5
• This design pattern may also be referred to as
  Thread-safe Passive Object.

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Structure of a Monitor Object Design Pattern

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Monitor Object Design Pattern Sequence Diagram
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Monitor Object Design Pattern Example

• Free toilet

A monitor is an approach to synchronize two or
more computer tasks that use a shared resource.
Separation of toilet and lock
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Monitor Object Design Pattern Example

• Next people waiting in order of arrival

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Monitor Object Design Pattern Pitfalls

• Calling wait() in unstable state
• Forget to release lock when an exception occurs
• Not making the method synchronized when needed

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Monitor Object Design Pattern Another Example
monitor account int balance 0
function withdraw(int amount) if amount lt 0
then error "Amount may not be negative" else if
balance lt amount then error "Insufficient funds"
else balance balance - amount
function deposit(int amount) if amount lt 0
then error "Amount may not be negative" else
balance balance amount
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Monitor Object Design Pattern Consequences

• Pros
• Simplification of concurrency control
  implementation
• Simplify synchronization of methods invoked
  concurrently on an object
• Synchronized methods can cooperatively schedule
  their order of execution
• Cons
• Concurrency remains complicated
• Tightly coupling between object functionality and
  synchronization mechanisms
• Nested monitor lockout
• New problems, deadlocks and starvation

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Monitor Object Design Pattern Known Uses

• Java Objects
• ACE Gateway

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Conclusion

• Active Object
• Separates method invocation from method
  execution.
• Monitor Object
• Synchronizes method execution to ensure only one
  method runs within an object at a time.

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References

• 1 Wikipedia Design Patterns
• http//en.wikipedia.org/wiki/Design_pattern_(compu
  ter_science)
• 2 Wikipedia Distributed Computing
• http//en.wikipedia.org/wiki/Distributed_computing
  
• 3 Wikipedia Real-Time Computing
• http//en.wikipedia.org/wiki/Real-time_system
• 4 Active Object
• http//www.cs.wustl.edu/7Eschmidt/PDF/Act-Obj.pdf
  
• 5 Active Object Pattern
• http//www.cs.uu.nl/docs/vakken/no/active_object_p
  attern.pdf
• 6 Monitor Object
• http//www.cs.wustl.edu/schmidt/PDF/monitor.pdf
• 7 Monitor Object Concurrency Pattern
• http//www.cs.uu.nl/docs/vakken/no/monitor20objec
  t.pdf

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Thank You

• Questions?