GATE CS Transactions and Concurrency Control questions

Consider concurrent execution of two transactions $T 1$ and $T 2$ in a DBMS, both of which access a data object $A$. For these two transactions to not conflict on $A$, which one of the following statements must be true?

A schedule of three database transactions $T_{1}, T_{2}$, and $T_{3}$ is shown. $R_{i}(A)$ and $W_{i}(A)$ denote read and write of data item $A$ by transaction $T_{i}, i=1,2,3$. The transaction $T_{1}$ aborts at the end. Which other transaction(s) will be required to be rolled back? …

Consider the database transactions T1 and T2 , and data items X and Y . Which of the schedule(s) is/are conflict serializable? …

​​​​An audit of a banking transactions system has found that on an earlier occasion, two joint holders of account $A$ attempted simultaneous transfers of Rs. $10000$ each from account $A$ to account $B$. Both transactions read the same value, Rs.$11000$, as the initial balance in $A$ and were allowed to go through. $B$ was credited Rs.$10000$ …

​​​​Consider the following read-write schedule $\text{S}$ over three transactions $T_{1}, T_{2}$, and $T_{3}$, where the subscripts in the schedule indicate transaction IDs: $S: r_{1}(z) ; w_{1}(z) ; r_{2}(x) ; r_{3}(y) ; w_{3}(y) ; r_{2}(y) ; w_{2}(x) ; w_{2}(y) ;$ Which of the following transaction schedules is/are conflict equivalent to …

​​​​​Once the $\text{DBMS}$ informs the user that a transaction has been successfully completed, its effect should persist even if the system crashes before all its changes are reflected on disk. This property is called

​​​​Which of the following statements about the Two Phase Locking ($2 \mathrm{PL}$) protocol is/are TRUE?

Let $\textit{R}_{i}(z)$ and $\textit{W}_{i}(z)$ denote read and write operations on a data element $z$ by a transaction $\textit{T}_{i},$ respectively. Consider the schedule $\textit{S}$ with four transactions. $S: \; R_{4}(x) R_{2}(x) R_{3}(x)R_{1}(y) W_{1}(y) W_{2}(x) W_{3}(y) R_{4}(y)$ Which one of the following serial schedules is conflict …

Suppose a database system crashes again while recovering from a previous crash. Assume checkpointing is not done by the database either during the transactions or during recovery. Which of the following statements is/are correct?

Let $r_i(z)$ and $w_i(z)$ denote read and write operations respectively on a data item $z$ by a transaction $T_i$. Consider the following two schedules. $S_1: r_1(x)r_1(y)r_2(x)r_2(y)w_2(y)w_1(x)$ $S_2: r_1(x)r_2(x)r_2(y)w_2(y)r_1(y)w_1(x)$ Which one of the following options is correct?

​​​​​Let $S$ be the following schedule of operations of three transactions $T_1$, $T_2$ and $T_3$ in a relational database system: $$R_2(Y), R_1(X), R_3(Z), R_1(Y)W_1(X), R_2(Z), W_2(Y), R_3(X), W_3(Z)$$ Consider the statements $P$ and $Q$ below: $P$: $S$ is conflict-serializable. $Q$: If $T_3$ commits before $T_1$ finishes, then $S$ …

Consider the following two statements about database transaction schedules: Strict two-phase locking protocol generates conflict serializable schedules that are also recoverable. Timestamp-ordering concurrency control protocol with Thomas’ Write Rule can generate view serializable schedules that are not conflict serializable Which of the above statements …

In a database system, unique timestamps are assigned to each transaction using Lamport's logical clock. Let $TS(T_{1})$ and $TS(T_{2})$ be the timestamps of transactions $T_{1}$ and $T_{2}$ respectively. Besides, $T_{1}$ holds a lock on the resource $R,$ and $T_{2}$ has requested a conflicting lock on the same resource $R.$ The following …

Two transactions $T_1$ and $T_2$ are given as $T_1:r_1(X)w_1(X)r_1(Y)w_1(Y)$ $T_2:r_2(Y)w_2(Y)r_2(Z)w_2(Z)$ where $r_i(V)$ denotes a $\textit{read}$ operation by transaction $T_i$ on a variable $V$ and $w_i(V)$ denotes a $\textit{write}$ operation by transaction $T_i$ on a variable $V$. The total number of conflict serializable …

Consider the following two phase locking protocol. Suppose a transaction $T$ accesses (for read or write operations), a certain set of objects $\{O_1,\ldots,O_k \}$. This is done in the following manner: $\text{Step 1}$. $T$ acquires exclusive locks to $O_1,\ldots,O_k$ in increasing order of their addresses. $\text{Step 2}$. The required …

Which one of the following is NOT a part of the ACID properties of database transactions?

Consider the following database schedule with two transactions $T_{1}$ and $T_{2}$. $S= r_{2}\left(X\right); r_{1}\left(X\right); r_{2} \left(Y\right); w_{1} \left(X\right); r_{1} \left(Y\right); w_{2} \left(X\right); a_{1}; a_{2}$ Where $r_{i} (Z)$ denotes a read operation by transaction $T_{i}$ on a variable $Z$, $w_{i}(Z)$ denotes a write …

Suppose a database schedule $S$ involves transactions $T_1,\ldots,T_n$ . Construct the precedence graph of $S$ with vertices representing the transactions and edges representing the conflicts. If $S$ is serializable, which one of the following orderings of the vertices of the precedence graph is guaranteed to yield a serial schedule?

Consider a simple checkpointing protocol and the following set of operations in the log. (start, T4); (write, T4, y, 2, 3); (start, T1); (commit, T4); (write, T1, z, 5, 7); (checkpoint); (start, T2); (write, T2, x, 1, 9); (commit, T2); (start, T3); (write, T3, z, 7, 2); If a crash happens now and the system tries to recover using both undo and redo …

Consider the following transaction involving two bank accounts $x$ and $y$. read(x); x:=x-50; write (x); read(y); y:=y+50; write(y) The constraint that the sum of the accounts $x$ and $y$ should remain constant is that of