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Shared Lock 共享锁/S锁
If a transaction T has obtained a S-lock on item Q, then T can read, but cannot write Q. S-lock is requested using lock-S instruction
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Exclusive Lock 排他锁/X锁
If a transaction T has obtained an X-lock on item Q, then T can both read and write Q. X-lock is requested using lock-X instruction
Each transaction issue lock and unlock requests in two phases:
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Growing Phase 扩张阶段
A transaction may obtain locks, but may not release locks
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Shrinking Phase 收缩阶段
A transaction may release locks, but may not obtain locks
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Each transaction gets a timestamp when it enters the system
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If a transaction X enters the system before Y does,
then TS(X) < TS(Y)
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即：按照事务进入系统的顺序来分配时间戳，
先进的时间戳较小，后进的时间戳较大
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Use the value of the system clock as the timestamp
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Use a logical counter
If TS(X) < TS(Y) , then the produced schedule is equivalent to a serial schedule in which X appears before Y
并发执行的结果，就好像事务是按照时间戳从小到大的顺序串行执行的一样
