# Distributed Databases Small Assignment

I don’t understand this Computer Science question and need help to study.

Assign 4:

Questions

8.1,8.2 and 8.3 from text book

Question 4:

Which of the following histories are conflict equivalent?

H1 ={W2(x);W1(x);R3(x);R1(x);W2(y);R3(y);R3(z);R2(x)}

H2 ={R3(z);R3(y);W2(y);R2(z);W1(x);R3(x);W2(x);R1(x)}

H3 ={R3(z);W2(x);W2(y);R1(x);R3(x);R2(z);R3(y);W1(x)}

H4 ={R2(z);W2(x);W2(y);W1(x);R1(x);R3(x);R3(z);R3(y)}

Question 5:

One says that history H is recoverable if, whenever transaction

Ti reads (some item x) from transaction Tj (i <> j) in H and Ci occurs in H, then

Cj <S Ci. Ti “reads x from” Tj in H if

1. Wj(x) <H Ri(x), and

2. Aj not <H Ri(x), and

3. if there is some Wk(x) such that Wj(x) <H Wk(x) <H Ri(x), then Ak <H Ri(x).

Which of the following histories are recoverable?

H1 ={W2(x);W1(x);R3(x);R1(x);C1;W2(y);R3(y);R3(z);C3;R2(x);C2}

H2 ={R3(z);R3(y);W2(y);R2(z);W1(x);R3(x);W2(x);R1(x);C1;C2;C3}

H3 ={R3(z);W2(x);W2(y);R1(x);R3(x);R2(z);R3(y);C3;W1(x);C2;C1}

H4 ={R2(z);W2(x);W2(y);C2;W1(x);R1(x);A1;R3(x);R3(z);R3(y);C3}

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