3.1.9 Transactions and Concurrency

1. A server manages data items, and allows read and write operations. Give three serialisable schedules of the following transactions
   • T: R(j),R(i),W(j,44),W(i,33)
   • U: R(k),W(i,55),R(i),W(k,66)
2. For transactions T and U, explain which of the following interleavings can occur with strict two phase locking and with optimistic concurrency control. Compare the possible outcomes in timestamping where TS_T < TS_U and TS_T > TS_U
   1. R_T(i), W_U(i, 55), W_T(j, 44), W_U(j, 66)
   2. R_T(i), W_T(j, 44), W_U(i, 55), W_U(j, 66)
   3. W_U(i, 55), W_U(j, 66), R_T(i), W_T(j, 44)
   4. W_U(i, 55), R_T(i), W_U(j, 66), W_T(j, 44)
   5. W_U(i, 55), R_T(i), W_T(j, 44), W_U(j, 66)
3. Define an inconsistent retrieval. What pattern of operational conflicts may lead to an inconsistent retrieval
4. Why might the start time of a transaction not be the best time to allocate its timestamp? Given the timestamps of two committed transactions, can you always draw their serialisation graphs? Compare the overhead of implementing locking with that of timestamping

The answer.