Question $3$

Some operations are called idempotent operations because they can be repeated multiple times without any side effects. RPCs that implement idempotent operations are a good fit for at$-$least$-$once call semantics. Which of the following functions is NOT idempotent? In the following, x and y are arguments passed by the calling function, while the server is holding a Cassandra key$-$value store K$.(1$ point$)$

$1$ point

void f$($int x$,$ int y$)\{$ Update K with key$=$x to have value$=$y$+1$; $\}$

void f$($int x$,$ int y$)\{$ Update K with key$=$x to have value$=$y; $\}$

void f$($int x$,$ int y$)\{$ Update K with key$=$x to have value$=$value$+$y; $\}$

void f$($int x$,$ int y$)\{$ Unless entry already exists, add an entry in K using key$=$x$,$ value$=$y; $\}$