Design Patterns For Distributed/real-time Systems:::active Object::::monitor Object:

Design Patterns for Distributed/Real-Time Systems ::Active Object:: ::Monitor Object:: Guide Faculty Mrs. Barkha Vijh Presented By; kaushik

Mayank

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]

Distributed/ Real-Time System Design Pattern It’s a reusable solution that can be applied either A Distributed System or A Real-Time System

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

Structure of an Active Object Design Pattern

Active Object Sequence Diagram

Active Object Design Pattern Example - Restaurant • Customer -> Client

• Waiter -> Proxy

Active Object Design Pattern Example - Restaurant • Manager -> Scheduler

• Order Queue -> Activation Queue

Active Object Design Pattern Example - Restaurant • Order -> Method Request

• Servant -> Chef

Active Object Design Pattern Consequences

• Pro’s

– Enhances concurrency – Simplifies synchronization – Method execution order can differ from invocation order

• Con’s – Performance overhead – Hard to debug

Active Object Design Pattern Known Uses • • • • •

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

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 object’s methods to cooperatively schedule their execution sequences. “ [5] This design pattern may also be referred to as Thread-safe Passive Object.

Structure of a Monitor Object Design Pattern Client Monitor +synchronized method_1()() +synchronized method_2()() 1

1

Client2 * MonitorCondition +notify()() +release()()

MonitorLock +acquire()() +release()()

Monitor Object Design Pattern Sequence Diagram

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

Monitor Object Design Pattern Example

• Next people waiting in order of arrival

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

Monitor Object Design Pattern Another Example monitor account { int balance := 0 function withdraw(int amount) { if amount < 0 then error "Amount may not be negative" else if balance < amount then error "Insufficient funds" else balance := balance - amount }

}

function deposit(int amount) { if amount < 0 then error "Amount may not be negative" else balance := balance + amount }

Monitor Object Design Pattern Consequences • Pro’s

– Simplification of concurrency control implementation – Simplify synchronization of methods invoked concurrently on an object – Synchronized methods can cooperatively schedule their order of execution

• Con’s

– Concurrency remains complicated – Tightly coupling between object functionality and synchronization mechanisms – Nested monitor lockout – New problems, deadlocks and starvation

Monitor Object Design Pattern Known Uses • Java Objects • ACE Gateway

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.

References • [1] Wikipedia: Design Patterns – http://en.wikipedia.org/wiki/Design_pattern_(computer_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_pattern.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/monitor%20object.pdf

Thank You

Questions?