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Synchronization of Distributed Objects

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Synchronization of Distributed Objects ICS280: Distributed System Middleware Xia Zhao xzhao_at_ics.uci.edu – PowerPoint PPT presentation

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Title: 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
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