concurrency control iii

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Concurrency Control III

Dead LockTime Stamp Ordering

Validation Scheme

Database Implementation – Concurrency Control Yan

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Learning Objectives

Dealing with Deadlock and Starvation Time Stamp Ordering Technique Validation


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Deadlocks Detection

Wait-for graph Prevention

Resource ordering Timeout Wait-die Wound-wait


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Deadlock Detection Build Wait-For graph Use lock table structures Build incrementally or periodically When cycle found, rollback victim

T1

T3

T2

T6

T5

T4T7


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Resource Ordering Order all elements A1, A2, …, An

A transaction T can lock Ai after Aj only if i > j

Problem : Ordered lock requests not realistic in most cases


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Timeout

If transaction waits more than L sec., roll it back!

Simple scheme Hard to select L


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Wait-die Transactions are given a timestamp when they

arrive …. ts(Ti) Ti can only wait for Tj if ts(Ti)< ts(Tj)

...else die


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T1(ts =10)

T2(ts =20)

T3 (ts =25)

wait

wait

Example:

wait?

Very high level: only older ones have the privilege to wait, younger ones die if they attempt to wait for older ones


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Wound-wait Transactions are given a timestamp when they

arrive … ts(Ti) Ti wounds Tj if ts(Ti)< ts(Tj) else Ti waits

“Wound”: Tj rolls back and gives lock to Ti


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T1(ts =25)

T2(ts =20)

T3 (ts =10)

wait

wait

Example:

wait

Very high level: younger ones wait; older ones kill (wound) younger ones who hold needed locks


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Who die? Looks like it is always the younger ones

either die automatically or killed

What is the reason? Will the younger ones starve?

Suggestions?


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Timestamp Ordering Key idea:

Transactions access variables according to an order decided by their time stamps when they enter the system

No cycles are possible in the precedence graph


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Timestamp System time when transactions starts An increasing unique number given to each stransaction

Denoted by ts(Ti)


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The way it works Two time stamps associated with each variable x

RS(x): the largest time stamp of the transactions read it WS(x): the largest time stamp of the transactions write it

Protocol: ri(x) is allowed if ts(Ti) >= WS(x) wi(x) is allowed if ts(Ti) >=WS(x) and ts(Ti) >=RS(x) Disallowed ri(x) or wi(x) will kill Ti, Ti will restart


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ExampleAssuming: ts(T1) = 100, ts(T2) = 200, ts(T3) = 300T1 T2 T3R(x); W(y);

R (y); W(z); R(x);

W(z); R(y); W(x);

x y zRS=-1 RS=-1 RS=-1WS=-1 WS=-1 WS=-1


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ExampleAssuming: ts(T1) = 100, ts(T2) = 200, ts(T3) = 300T1 T2 T3R(x); W(y);W(y);

R (y);R (y); W(z);W(z); R(x); R(x);

W(z);W(z); R(y); R(y); W(x);W(x);

x y zRS=100 RS=-1 RS=-1WS=-1 WS=-1 WS=-1


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R (y);R (y); W(z);W(z); R(x); R(x);

W(z);W(z); R(y); R(y); W(x);W(x);

x y zRS=100 RS=-1 RS=-1WS=-1 WS=100 WS=-1


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R (y); W(z);W(z); R(x); R(x);

W(z);W(z); R(y); R(y); W(x);W(x);

x y zRS=100 RS=200 RS=-1WS=-1 WS=100 WS=-1


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R (y); W(z); R(x); R(x);

W(z);W(z); R(y); R(y); W(x);W(x);

x y zRS=100 RS=200 RS=-1WS=-1 WS=100 WS=300


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R (y); W(z);

R(x);W(z);W(z); R(y); R(y); W(x);W(x);



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R (y); W(z); R(x);

W(z); R(y); R(y); W(x);W(x);


T1 is rolled back


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Net result of TO scheduling Conflict pairs of actions are taken in the order of their

home transactions But the basic TO does not guarantee recoverability


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Validation

An optimistic schemeTransactions have 3 phases:(1) Read

all DB values read writes to temporary storage no locking

(2) Validate check if schedule so far is serializable

(3) Write if validate ok, write to DB


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Time stamps of a transaction Ti Start(Ti) Validation(Ti) Finish(Ti)


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Key idea Make validation atomic If T1, T2, T3, … is validation order, then resulting

schedule will be conflict equivalent to Ss = T1 T2

T3...


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Schedule

T1 T2Read(A)A A+100;

Read(A) A Ax2;

Read(B);B B+100validateWrite(A)Write(B); Read(B) B Bx2; validate Write(A) Write(B);


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Example of what validation must prevent:

RS(T2)={B} RS(T3)={A,B}WS(T2)={B,D} WS(T3)={C}

time

T2start

T2validate

T3validateT3

start

=

T2finishes

T3finishes


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T2finish

phase 3

Example of what validation must prevent:

RS(T2)={B} RS(T3)={A,B}WS(T2)={B,D} WS(T3)={C}

time

T2start

T2validated

T3validated

T3start

=

allow

T3start


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Another thing validation must prevent:RS(T2)={A} RS(T3)={A,B}WS(T2)={D,E} WS(T3)={C,D}

time

T2validated

T3validated

finishT2BAD: w3(D) w2(D)


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finishT2

Another thing validation must prevent:RS(T2)={A} RS(T3)={A,B}WS(T2)={D,E} WS(T3)={C,D}

time

T2validated

T3validated

allow

finishT2


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Validation Rule When start validating T

Check RS(T) WS(U) is empty for any U that started but (did not finish validation before T started)

Check WS(T) WS(U) is empty for any U that started but (did not finish validation before T started validation)


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Exercise:

T: RS(T)={A,B} WS(T)={A,C}

V: RS(V)={B} WS(V)={D,E}

U: RS(U)={B} WS(U)={D}

W: RS(W)={A,D} WS(W)={A,C}

startvalidatefinish


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Exercise:





startvalidatefinish


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Exercise:





startvalidatefinish


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Exercise:





startvalidatefinish

W is rolled bak

concurrency control iii

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