<P> SS2PL mentioned above is a variant (special case) of CO and thus also effective to achieve distributed and global serializability . It also provides automatic distributed deadlock resolution (a fact overlooked in the research literature even after CO's publication), as well as Strictness and thus Recoverability . Possessing these desired properties together with known efficient locking based implementations explains SS2PL's popularity . SS2PL has been utilized to efficiently achieve Distributed and Global serializability since the 1980, and has become the de facto standard for it . However, SS2PL is blocking and constraining (pessimistic), and with the proliferation of distribution and utilization of systems different from traditional database systems (e.g., as in Cloud computing), less constraining types of CO (e.g., Optimistic CO) may be needed for better performance . </P> <Ol> <Li> The Distributed conflict serializability property in its general form is difficult to achieve efficiently, but it is achieved efficiently via its special case Distributed CO: Each local component (e.g., a local DBMS) needs both to provide some form of CO, and enforce a special vote ordering strategy for the Two - phase commit protocol (2PC: utilized to commit distributed transactions). Differently from the general Distributed CO, Distributed SS2PL exists automatically when all local components are SS2PL based (in each component CO exists, implied, and the vote ordering strategy is now met automatically). This fact has been known and utilized since the 1980s (i.e., that SS2PL exists globally, without knowing about CO) for efficient Distributed SS2PL, which implies Distributed serializability and strictness (e.g., see Raz 1992, page 293; it is also implied in Bernstein et al. 1987, page 78). Less constrained Distributed serializability and strictness can be efficiently achieved by Distributed Strict CO (SCO), or by a mix of SS2PL based and SCO based local components . </Li> <Li> About the references and Commitment ordering: (Bernstein et al. 1987) was published before the discovery of CO in 1990 . The CO schedule property is called Dynamic atomicity in (Lynch et al. 1993, page 201). CO is described in (Weikum and Vossen 2001, pages 102, 700), but the description is partial and misses CO's essence . (Raz 1992) was the first refereed and accepted for publication article about CO algorithms (however, publications about an equivalent Dynamic atomicity property can be traced to 1988). Other CO articles followed . (Bernstein and Newcomer 2009) note CO as one of the four major concurrency control methods, and CO's ability to provide interoperability among other methods . </Li> </Ol> <Li> The Distributed conflict serializability property in its general form is difficult to achieve efficiently, but it is achieved efficiently via its special case Distributed CO: Each local component (e.g., a local DBMS) needs both to provide some form of CO, and enforce a special vote ordering strategy for the Two - phase commit protocol (2PC: utilized to commit distributed transactions). Differently from the general Distributed CO, Distributed SS2PL exists automatically when all local components are SS2PL based (in each component CO exists, implied, and the vote ordering strategy is now met automatically). This fact has been known and utilized since the 1980s (i.e., that SS2PL exists globally, without knowing about CO) for efficient Distributed SS2PL, which implies Distributed serializability and strictness (e.g., see Raz 1992, page 293; it is also implied in Bernstein et al. 1987, page 78). Less constrained Distributed serializability and strictness can be efficiently achieved by Distributed Strict CO (SCO), or by a mix of SS2PL based and SCO based local components . </Li> <Li> About the references and Commitment ordering: (Bernstein et al. 1987) was published before the discovery of CO in 1990 . The CO schedule property is called Dynamic atomicity in (Lynch et al. 1993, page 201). CO is described in (Weikum and Vossen 2001, pages 102, 700), but the description is partial and misses CO's essence . (Raz 1992) was the first refereed and accepted for publication article about CO algorithms (however, publications about an equivalent Dynamic atomicity property can be traced to 1988). Other CO articles followed . (Bernstein and Newcomer 2009) note CO as one of the four major concurrency control methods, and CO's ability to provide interoperability among other methods . </Li>

Explain concurrency control mechanisms. discuss the need with example