In the text we state that if, in a distributed database system, each site uses a strict two-phase locking concurrency control (all locks are held until commit time) and the system uses a two-phase commit protocol, transactions will be globally serializable. Does the result also hold if the concurrency controls are not strict—read locks are released early—but are two phase?
What will be an ideal response?
No. T1A might release a read lock on x at site A and T2A might subsequently write x.
A similar situation might exist at site B on item y except that the subtransaction of
T1 and T2 might be ordered in the opposite way. Hence the serialization graph for T1
and T2 has a cycle, but do not delay each other. Both might complete, although their
subtransactions are ordered differently at the two sites.
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