Task $6$: Transactions

a$)$ Give two reasons why we would want transactions in our DBMS in the first place.

b$)$ Explain briefly the ACID properties for transactions.

c$)$ Determine the recovery property $($strict$,$ ACA, recoverable, unrecoverable$)$ for the

following histories:

$H_1$ : $w1(Y)$; $r2(X)$; $w1(X)$; $w3(Y)$; $r2(Y)$; $c1$; $c3$; $c2$

$H_2$ : $r2(X)$; $w2(Y)$; $r1(X)$; $w3(X)$; $c2$; $r1(Y)$; $r3(Y)$; $w1(X)$; $c3$; $c1$

$H_3$ : $r2(X)$; $w2(X)$; $r3(X)$; $w1(Y)$; $c1$; $w3(Y)$; $c3$; $c2$

d$)$ When are two operations in a schedule in conflict?

e$)$ Determine if the following schedule, $H_4,$ conflict serializable by drawing the

precedence graph.

$H_4$ : $r1(Y)$; $w3(X)$; $r3(Y)$; $w2(X)$; $r1(X)$; $c1$; $c2$; $c3$

f$)$ What does it mean to have a deadlock between two or more transactions?

g$)$ We wish to rewrite $H_4$ such that it utilizes locks. Show how the schedule will look after

applying rigorous two$-$phase locking. The new operations needed are rl$1($A$),$ wl$1($A$)$

og ul$1($A$)-$ abbreviations for read$\_$lock$1($A$),$ write$\_$lock$1($A$)$ and unlock$1($A$).$

Remember that read locks can be shared, whereas write locks are exclusive.