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ch11-2 - Chapter 11 Distributed Concurrency Control 11.32...

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Chapter 11 Distributed Concurrency Control 11.32 Primary Copy 2PL Implement lock management at a number of sites and make each lock manager responsible for a set of lock units. A dictionary is needed to keep the allocation of lock managers information. Changes from centralized 2PL to primary copy 2PL are minimal. 11.33 Distributed 2PL There is a lock manager at every site Concurrency control is accomplished by the cooperation of lock manager at the sites where data are involved in the set of transactions. If database is not replicated, distributed 2PL becomes primary copy 2PL. For replicated database, a Read One Write All protocol is needed. Communication structure 15
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TM DP LM Chapter 11 Distributed Concurrency Control same site Messages of communication Lock request Lock granted End of database operation Release lock Differences between distributed 2PL and centralized 2PL can be observed by looking at the communication structures. 114 TIEMSTAMP-BASED CONCURRENCY ALGGORITHM Lock method maintains serializability by mutual exclusion. Timestamp method maintains serializability by assigning a unique timestamp to every transaction 16
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Chapter 11 Distributed Concurrency Control and executing transactions accordingly. What is a timestamp? W an identifier for transaction a used to permit ordering u monotonicity – timestamps generated by the same TM are monotonically increased in values. How to assign a timestamp value H use a monotinically increasing counter, but this is difficult in distributed environment t use a two-tuple form <local-counter-value, site-identifier> s use <local-system-clock, site-identifier> Note site ID is put in the least significant position to avoid the situation that timestamps have the same local counters. ts(T i ) – the timestamp value of transaction T i .
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