Concurrency Control of Transactions - 2PL
Consider two transactions T and U as defined in the following.
T: y = read(k); x = read(i); write(j,22);
U: write(i,33); write(j,44);
- Give two interleavings of transactions T and U that are serially equivalent but not with two-phase locking.
- Give two interleavings of transactions T and U that are serially equivalent and with two-phase locking.
Concurrency Control of Transactions - OCC
Consider optimistic concurrency control as applied to two transactions T and U as defined in the following.
T: x = read(i); write(j,66);
U: write(i,77); write(j,88);
Suppose the transactions T and U are active at the same time as one another. Describe the outcome in each of the following cases. Note that in each case your description must include whether transactions T and U commit or abort and why. Remember that writes are not carried out until after validation.
- T’s request to commit comes first and backward validation is used;
- U’s request to commit comes first and backward validation is used;
- T’s request to commit comes first and forward validation is used;
- U’s request to commit comes first and forward validation is used.
Concurrency Control of Transactions
- How does time-stamp ordering (TSO) ensure serializability?
- What are advantages and disadvantages of the optimistic concurrency control method for controlling concurrent access to shared data?
- Compare two-phase locking, optimistic concurrency control, and timestamp ordering methods for controlling concurrent access to shared data.
Deadlock detection and prevention in distributed systems
Wait-die and wound-wait are two practical algorithms for distributed deadlock prevention.
- Describe the common idea shared by these two algorithms.
- State one advantage and one disadvantage of wound-wait over wait-die.
- A process with transaction timestamp 64 needs a resource held by a process with transaction timestamp 82. Explain what happens (1) when wait-die algorithm is used; and (2) when wound-wait algorithm is used.
- Why the methods wait-die and wound-wait cannot apply in the deadlock prevention if processes are allowed to hold more than one resource? Give one example to explain your reason.
- What is the main difference between deadlock prevention and deadlock avoidance in handling deadlocks in distributed systems?
Suggested answers are available here.
