# Which of the following histories are recoverable?

*label*Computer Science

*timer*Asked: Nov 21st, 2015

**Question description**

One says that history H is recoverable if, whenever transaction Ti reads (some item x) from transaction Tj (i 6= 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. Ajnot ≺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}