Week 07

More Locks and Transactions

Two Phased Locking

Shared and Exclusive Locks

Transactions in SQLite

Aborting Transactions

Rollback

Deadlock Resolution

Additional Notes on Deadlock Prevention

The following is a quote from Database Management Systems by Raghu Ramakrishnan and Johannes Gehrke (19.2.2)

We can prevent deadlocks by giving each transaction a priority and ensuring that lower priority transactions are not allowed to wait for higher priority transactions (or vice versa). One way to assign priorities is to give each transaction a timestamp when it starts up. The lower the timestamp, the higher the transaction's priority, that is, the oldest transaction has the highest priority.

If a transaction Ti requests a lock and transaction Tj holds a conflicting lock, the lock manager can use one of the following two policies:

  • Wait-die: If Ti has higher priority, it is allowed to wait; otherwise it is aborted.
  • Wound-wait: If Ti has higher priority, abort Tj; otherwise Ti waits.

In the wait-die scheme, lower priority transactions can never wait for higher priority transactions. In the wound-wait scheme, higher priority transactions never wait for lower priority transactions. In either case no deadlock cycle can develop.

A subtle point is that we must also ensure that no transaction is perennially aborted because it never has a sufficiently high priority. (Note that in both schemes, the higher priority transaction is never aborted.) When a transaction is aborted and restarted, it should be given the same timestamp that it had originally. Reissuing timestamps in this way ensures that each transaction will eventually become the oldest transaction, and thus the one with the highest priority, and will get all the locks that it requires.

The wait-die scheme is nonpreemptive; only a transaction requesting a lock can be aborted. As a transaction grows older (and its priority increases), it tends to wait for more and more younger transactions. A younger transaction that conflicts with an older transaction may be repeatedly aborted (a disadvantage with respect to wound-wait), but on the other hand, a transaction that has all the locks it needs will never be aborted for deadlock reasons (an advantage with respect to wound-wait, which is preemptive).

Hand-Written Demonstration