Consider a distributed transaction processing system that uses a serial validation optimistic concurrency control at each site and a two-phase commit protocol. Show that deadlocks are possible under these conditions.
What will be an ideal response?
A subtransaction enters its validation phase when the prepare message arrives. It
votes ready or abort based on the result of the validation test, but cannot start the
write phase until the commit or abort message arrives in Phase 2 of the commit
protocol, preventing other (sub)transactions at that site from entering validation. If
T1 and T2 both have cohorts at sites A and B and both start the commit protocol at
approximately the same time, it is possible that T1A enters validation at site A and
T2B enter validation at site B. A deadlock results if
1. T1 cannot complete, since T1A cannot exit from its validation phase until T1B
completes validation and T1B is waiting for T2B.
2. T2 cannot complete, since T2B cannot exit from its validation phase until T2A
completes validation and T2A is waiting for T1A.
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