1 chapter 3. synchronization. stempusan national university stem-pnu 2 synchronization in...

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Chapter 3. Synchronization

STEMPusan National University

STEM-PNU

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Synchronization in Distributed Systems

Synchronization in a single machine Same clock Shared data for synchronization

Semaphore for mutual exclusion

Synchronization in distributed system Difficult to handle it No same physical clock No shared data for synchronization

Machine 1 Machine 2

Semaphore

Remote Access : Delay and Error


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Clock

In distributed systems Multiple Physical Clocks : No Centralized Clock

Logical Clock rather than a Global Physical Clock


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Global Clock and Logical Clock

Global Clock TAI (Temps Atomique International) at Paris Time Server Broadcasting from Satellite Granularity

Logical Clock Not absolute time but for ordering Lamport Algorithm

Correction of Clock T(A, tx) < T(B, rx)


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Logical Clock

Logical Clock Not absolute time but for ordering Lamport Algorithm

Correction of Clock C(A, tx) < C(B, rx)

12345

56789

1011121314

678

151617

101112

910

1314

1819

12345

56789

1011121314

678

151617

101617

1819

1819

1819


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Global Clock and Time Server

Global Clock TAI (Temps Atomique International) at Paris Time Server Broadcasting from Satellite Granularity

Global Clock Server Adjust local clock

Cristian’s Algorithm Berkeley Algorithm

Time never runsbackward


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Global State

Global State States of each local system Messages in transit Very important to determine the status of distributed system

Example : Deadlock Detection

Distributed Snapshot : State of distributed systems in a given time


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Mutual Exclusion : Monitor (Coordinator)

In a single system : Easy to implement by semaphore In distributed systems : No shared data for semaphore A Centralized Algorithm

Simple But single point of failure


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Mutual Exclusion : Distributed Algorithm

No central coordinator Rule : When a system receive a request

Not in CS or Not to enter : OK to requester In CS : No reply To enter : compare Timestamp and one with lower TS wins


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Mutual Exclusion : Token Ring

One with token can enter CS If a system wants to enter CS :

wait Token and process it Otherwise, pass token to the next

Token

Wait until

it gets the token


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Mutual Exclusion : Comparison

# of messagesper request

delay

per request

Problem

Monitor 3 2 Crash of

monitor

DistributedAlgorithm

2(n-1) 2(n-1) n points of

Crash

Token Ring 0 to n-1 0 to n-1 Token lost


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Election : Bully Algorithm

When it is found that a coordinator is required


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Election : Ring Algorithm

When it is found that the coordinator has crashed, Circulate message with priority


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Distributed Transaction

Transaction

Example : Flight Reservation

Consistent

State

Consistent

State

Set of operations

BEGIN_TRANSACTION reserve WP -> JFK; reserve JFK -> Nairobi; reserve Nairobi -> Malindi;END_TRANSACTION

(a)

BEGIN_TRANSACTION reserve WP -> JFK; reserve JFK -> Nairobi; reserve Nairobi -> Malindi full =>ABORT_TRANSACTION (b)


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ACID Properties : Atomicity and Consistency

Atomicity. All or Nothing Not Partially Done Example : Failure in Flight Reservation

Consistency. Execution of a transaction preserves the consistency of the database.

State 1 State 2

All

Nothing

State 2’PartiallyDone

Consistent

Consistent


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Transaction States

Active

PartiallyCommitted

Failed Aborted

Committedthe initial state; the transaction stays in this state while it is

executing

after the discovery that normal execution can no longer proceed.

after the transaction has been rolled back and the database restored to its state prior to the start of the transaction.

- restart the transaction or - kill the transaction

ALL

NOTHING


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ACID Properties : Isolation and Durability

Isolation. Although multiple transactions may execute concurrently, each

transaction must be unaware of other concurrently executing transactions.

Intermediate transaction results must be hidden from other concurrently executed transactions.

Durability. After a transaction completes successfully, the changes it has made to

the database persist, even if there are system failures.

DB

Transaction 1

Transaction 2

No Effect


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Example

Transaction : Transfer $50 from account A to account B:1. read(A)

2. A := A – 50

3. write(A)

4. read(B)

5. B := B + 50

6. write(B)

Consistency requirement the sum of A and B is unchanged after the transaction.

Atomicity requirement Durability Isolation


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Example : Concurrent Execution

Two Transactions T1 : transfer $50 from A to B,

T2 transfer 10% of the balance from A to B

Serial Schedule Concurrent Schedule


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Serializability

What happens after these transactions ? Serial Schedule :

Always Correct T1 T2 and T2 T1

Concurrent Schedule Serializable if

Result (T1||T2) = Result(T1 T2) or

Result(T2 T1)


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Transaction : Atomicity

Failure during transaction Abort the partial execution How to discard incomplete transaction

Deferred updates Immediate updates

Safe transaction Decrease failure probability

Rollback Recovery

Return to initial state Without minimum cost


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Safe Transaction

Decrease the possibility of failure

Two Phase Commit Wedding Ceremony Protocol 1st Phase : send ready message 2nd Phase : execute only when OK messages are received from

all


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Log

Log History of operations Used for rollback

(Transaction#, Pre-state, Post-state) (Trans#, DataItem, OldValue, NewValue)

Transaction

operation1

operation3

operation2 SystemSystem

log log log


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Deferred Updates

Reflect Logs after Commit Shadowing

If the transaction fails, then discard logs

Transaction

operation1

operationn

. . .

System

Begin

Commit

Write or update operations

log log log


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(T1, begin)(T1, A, 20, 30)(T2, begin)(T1, B, 30, 40)(T2, B, 40, 50)(T1, commit)(T3, begin)(T2, C, 100, 50)(T2, commit)(T3, D, 10, 20)

Delete them

OK

Deferred Updates : Recovery

Log File

Fail Here

1. Transaction with commit: OK2. Transaction with begin but without commit :

Delete log records


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Transaction

Begin

Commit

Immediate Updates

Execute operation immediately

If the transaction fails, undo operations

operation1

operation3

operation2 System

log log log


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Which one is better ?


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Transaction : Serializability

Serialization Exchange non-conflict operation Conflict operation :

Result(T1.Op,T2.Op) ≠ Result(T2.Op,T1.Op)

Time-stamping Two Phase Locking

r(a) w(a) r(b) w(b)

r(a) w(a)r(b) w(b)

Trans 1

Trans 2

Time


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Time Stamping

If conflict occurs, then abort For read(T,x), ts(T) < tsWR(x), then abort T For write(T,x), ts(T) < tsWR(x) or ts(T) < tsRD(x), then abort T


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Two Phase Locking

2PL

Strict 2PL


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Implementation of Concurrency Control in Distributed Systems

Three Managers TM (Transaction Manager) : Ensure the Atomicity Scheduler : Main Responsibility for Concurrency Control DM (Data Manager) : Simple Read/Write

In a single machine

In distributed systems

1 chapter 3. synchronization. stempusan national university stem-pnu 2 synchronization in...

Documents

token slide

global physical clock

synchronization slide

rx slide

clock shared data

token lost slide

error slide

priority slide