Total marks: $10$

Q$\mathrm{.}2$Set. $($B$)$

i$)$ Consider a variant of the tree protocol called the forest protocol. The database is organized as a forest of rooted trees. Each transaction Ti must follow the following rules:

$$ The first lock in each tree may be on any data item.

$$ The second, and all subsequent, locks in a tree may be requested only if the parent of the requested node is currently locked.

$$ Data items may be unlocked at any time.

$$ A data item may not be relocked by Ti after it has been unlocked by Ti $.$

Show that the forest protocol does not ensure serializability. $(6$ marks$)$

ii$)$

Consider the following two transactions:

T$34$: read$($A$)$;

read$($B$)$;

if A $=0$ then B :$=$ B $+1$;

write$($B$).$

T$35$: read$($B$)$;

read$($A$)$;

if B $=0$ then A :$=$ A $+1$;

write$($A$).$

a$.$ Add lock and unlock instructions to transactions T$31$ and T$32,$ so that they observe the two$-$phase locking protocol. $(2$ marks$)$

b$.$ Can the execution of these transactions result in a deadlock? $(2$ marks$)$

Total marks:$10$