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Synchronization of Distributed Objects. ICS280: Distributed System Middleware Xia Zhao Overview. Review of Object-Orientation & Actor Design goals and principles Single object: synchronization constraints Multiple objects: synchronizer Conclusion

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synchronization of distributed objects

Synchronization of Distributed Objects

ICS280: Distributed System Middleware

Xia Zhao

  • Review of Object-Orientation & Actor
  • Design goals and principles
  • Single object: synchronization constraints
  • Multiple objects: synchronizer
  • Conclusion
  • Related work: Composition-Filters Model

Synchronization of Distributed Objects

object orientation review
Object Orientation Review
  • Object and Class
  • Interface and Encapsulation
  • Method and Message
  • Inheritance
    • Semantic inheritance: When A inherits B, A can be treated as B

Synchronization of Distributed Objects

actor underlying framework
Actor: underlying framework
  • Hewitt 1977, Agha 1986
  • Asynchronous objects execute concurrently
  • Message passing: only inter-object communication
    • asynchronous: non-block
    • reliable: guaranteed to reach destination
    • arriving order may not be sending order
    • dispatch: message causes method execution
    • actor: one thread of control, no intra actor concurrency, one message queue

Synchronization of Distributed Objects

  • Definition: Ordered Message Dispatch
  • Synchronization: correct order
  • Two types of synchronization:
    • Shared Buffer: put/get, single object
      • Synchronization Constraints
    • Multimedia: audio/video, objects group
      • Synchronizer

Synchronization of Distributed Objects

design goals and principles
Design Goals and Principles
  • A general construct, not a specific OOPL
  • Maintain encapsulation, enhance reuse: both objects and constraints
  • Separation of concerns
    • Single object: how vs. when
    • Object groups: entity vs. context
  • Language Support at both levels, in uniform way
    • Otherwise programmer need to invent the other

Synchronization of Distributed Objects

single object synchronization constraints
Single object: Synchronization Constraints
  • Separation of concerns:
    • methods specification: how
    • synchronization constraints: when
  • ease of reasoning
  • ease of modification: independent
  • ease of implementation: add constraints
  • integration with inheritance

Synchronization of Distributed Objects

example bounded buffer
Example: Bounded buffer
  • A shared buffer can hold at most max elements
  • A producer can put one element if buffer not full
  • A consumer can get one element if buffer not empty
  • Coordination can be implemented in producer and consumer
  • But it mixes how(functionality) and when(coordination), and compromises abstraction, modularity, reuse
  • So, buffer should be the center for coordination

Synchronization of Distributed Objects

synchronization constraints structure
Synchronization constraints: structure



Synchronization constraints


Message Dispatch

Message Delivery

Input queue

Synchronization of Distributed Objects

synchronization constraints syntax example
Synchronization Constraints: syntax / example
  • Syntax

constraint ::= disablepattern1; …; patternk

pattern ::= method(x1, …, xn) ifexp

  • Example

class BoundedBuffer

size := 0;


put if size = MAX;

get if size = 0;

method put(item) … end put;

method get(client) … end get;

end BoundedBuffer;

Synchronization of Distributed Objects

synchronization constraints inheritance
Synchronization Constraints: inheritance
  • Constraints inheritance is different from method inheritance
  • Constraints inheritance enables incremental modification
  • Method: More stringent constraints
  • Example

class Get2Buffer inherits BoundedBuffer

disable get2 if size<=1;

method get2(client)… end get2;

end Get2Buffer;

  • Others are more complex: cancel, weaker
  • Ours: simple, practical(semantic inheritance)

Synchronization of Distributed Objects

objects group synchronizer
Objects Group: Synchronizer
  • Separation of concerns
    • Individual objects’ encapsulation
    • Coordination constraints among them
  • Transparent
    • objects are not aware of coordinator
    • no explicit control exchange, just message observation
  • Ease of reasoning
  • Rely on interface, ease of encapsulation/modification/reuse
  • Composition and evolution

Synchronization of Distributed Objects

synchronizer structure










Synchronizer: structure
  • Observes and constrains
  • States
  • Trigger: message, action, state
  • Atomicity Constraints: mutual
  • Disabling Constraints: one way

Synchronization of Distributed Objects

synchronizer syntax
Synchronizer: syntax

Synchronizer name(par1,…,parn)

vari := expi;


end name;

relation ::= trigger | constraint

constraint ::= disable pattern | atomic(pattern1,…, patternn)

trigger ::= trigger pattern-> actions

pattern ::= object.method(name1,…,namen) if exp

Synchronization of Distributed Objects

synchronizer example1
Synchronizer: example1

Distributed Mutual Exclusion: RadioButton

class Button synchronizer RB(buttons)

isOn := false; activated := false;

disablefor b in buttons:

on if isOn; trigger

off if not isOn; b.on -> {activated:=true;};

method on() … end on; ->{activated:=false;};

method off() … end off; for b in buttons:

end Button; disable

b.on if activated

end RB;

Synchronization of Distributed Objects

synchronizer example2
Synchronizer: Example2

Dinning Philosophers

class Chopstick synchronizer indiv(c1,c2,phi)

isPicked := false; atomic

disable pickup if isPicked; (c1.pickup(p1) if p1=phi,

method pickup(phil) c2.pickup(p2) if p2=phi);

isPicked = true; … end indiv;

end pickup;

method drop()

isPicked = false; … atomic: one message, one pattern

end drop; atomic: different object, no undo

end Chopstick

Synchronization of Distributed Objects

synchronizer composition
Synchronizer: composition
  • Composing Disabling and Atomicity Constraints

synchronizer composed(o, p)

disable o.m if exp;

atomic(o.m, p.n);

end composed;

  • Multiple synchronizers constrains one object
    • one chopstick is constrained by two synchronizers
  • Incremental Strengthening of Atomic Constraints

Philosopher needs two chopsticks and one spoon

strengthen indiv with (spoon.pickup(p3) if p3 = phil)

Synchronization of Distributed Objects

synchronizer evaluation order
Synchronizer: Evaluation Order
  • Synchronizer and object: two-way
    • testing constraint, possibly dispatching, triggering state change: atomic
  • Different objects in atomic constraint
    • both m1 and m2 are to o, but m1 prevents m2
  • Synchronization Constraints and Synchronizer
    • evaluating synchronization constraints, evaluating synchronizer constraints, dispatching message: atomic

Synchronization of Distributed Objects

not conclusion
(Not) Conclusion
  • Express message ordering constraints at high-level, object-oriented, and uniform manner
  • Synchronization constraints: part of object
    • when/how, inheritance
  • Synchronizer: separate entity
    • messages in group----atomic multicast: order; transaction: effect
    • synchronizer: user-specified order, supplementary
  • Separation of concern, abstraction, reuse

Synchronization of Distributed Objects

composition filters object model
Composition-Filters Object Model

A more elaborate and elegant model than traditional OOPL

Synchronization of Distributed Objects

message filters
Message Filters

Using Message filters and internal objects to implement lots of functionality, including inheritance and delegation

Synchronization of Distributed Objects

message filter types
Message Filter Types
  • Dispatch


disp : Dispatch = { inner.* };

  • Abstract Communication Types(ACT)
    • Ordinary class, plus manipulating first-class representation of messages
    • Meta Filter Class: Message interactions are intercepted and transformed into first-class representations

Synchronization of Distributed Objects

meta filter class example 1
Meta Filter Class Example: 1

class LoggedClerk interface

comment "This is a subclass of clerk of which all the incoming and outgoing messages are logged by the external ACT object named bigBrother";


clerk : Clerk; // inherit from clerk


bigBrother : LogACT; // an external ACT object


reifyIn : Meta = { [*.*]bigBrother.logMessage };

// reify and send message to the ACT

inherit : Dispatch = { clerk.* };


reifyOut : Meta = { [*.*]bigBrother.logMessage };

// reify and send message to the ACT

end // class LoggedClerk interface

Synchronization of Distributed Objects

meta filter class example 2
Meta Filter Class Example: 2

class LogACT interface


logMessage(Message) returns Nil;


disp : Dispatch = {inner.* };

end; // class LogACT interface

class LogACT implementation


log : OrderedCollection;


logMessage(mess : Message) returns Nil


log.addLast(mess);; // Message Operations


end; // class LogACT implementation

Synchronization of Distributed Objects

  • With MAUD 1993
    • MAUD also has a message interception mechanism, it requires development and replacement of some special classes. Composition-filter provides a general framework and several common solutions
  • With Frolund 1996
    • Frolund’s solutions are more abstract, more modular. Composition-filter seems dealing with complex low-level features

Synchronization of Distributed Objects