Synchronization of Distributed Objects

1
Synchronization of Distributed Objects

• ICS280 Distributed System Middleware
• Xia Zhao
• xzhao_at_ics.uci.edu

2
Overview

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

3
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

4
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

5
Synchronization

• 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

6
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

7
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

8
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

9
Synchronization constraints structure
Object
state
Synchronization constraints
Methods
Message Dispatch
Message Delivery
Input queue
10
Synchronization Constraints syntax / example

• Syntax
• constraint disable pattern1 patternk
• pattern method(x1, , xn) if exp
• Example
• class BoundedBuffer
• size 0
• disable
• put if size MAX
• get if size 0
• method put(item) end put
• method get(client) end get
• end BoundedBuffer

11
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 sizelt1
• method get2(client) end get2
• end Get2Buffer
• Others are more complex cancel, weaker
• Ours simple, practical(semantic inheritance)

12
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/modificat
  ion/reuse
• Composition and evolution

13
Synchronizer structure

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

14
Synchronizer syntax

• Synchronizer name(par1,,parn)
• vari expi
• relationj
• end name
• relation trigger constraint
• constraint disable pattern
  atomic(pattern1,, patternn)
• trigger trigger pattern-gt actions
• pattern object.method(name1,,namen) if exp

15
Synchronizer example1

• Distributed Mutual Exclusion RadioButton
• class Button synchronizer RB(buttons)
• isOn false activated false
• disable for b in buttons
• on if isOn trigger
• off if not isOn b.on -gt activatedtrue
• method on() end on b.off -gtactivatedfalse
  
• method off() end off for b in buttons
• end Button disable
• b.on if activated
• end RB

16
Synchronizer Example2

• Dinning Philosophers
• class Chopstick synchronizer
  indiv(c1,c2,phi)
• isPicked false atomic
• disable pickup if isPicked (c1.pickup(p1) if
  p1phi,
• method pickup(phil) c2.pickup(p2) if
  p2phi)
• isPicked true end indiv
• end pickup
• method drop()
• isPicked false atomic one message,
  one pattern
• end drop atomic different object, no
  undo
• end Chopstick

17
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)

18
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

19
(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

20
Composition-Filters Object Model

• A more elaborate and elegant model than
  traditional OOPL

21
Message Filters
Using Message filters and internal objects to
implement lots of functionality, including
inheritance and delegation
22
Message Filter Types

• Dispatch
• inputfilters
• 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

23
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" internals
clerk Clerk // inherit from clerk
externals bigBrother LogACT // an
external ACT object inputfilters
reifyIn Meta .bigBrother.logMessage
// reify and send message to the
ACT inherit Dispatch clerk.
outputfilters reifyOut Meta
.bigBrother.logMessage //
reify and send message to the ACT end // class
LoggedClerk interface
24
Meta Filter Class Example 2

• class LogACT interface
• methods
• logMessage(Message) returns Nil
• inputfilters
• disp Dispatch inner.
• end // class LogACT interface
• class LogACT implementation
• instvars
• log OrderedCollection
• methods
• logMessage(mess Message) returns Nil
• begin
• log.addLast(mess)
• mess.fire // Message Operations
• end
• end // class LogACT implementation

25
Comparison

• 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
• Frolunds solutions are more abstract, more
  modular. Composition-filter seems dealing with
  complex low-level features