Strong strict two phase locking .28SS2PL.29 Commitment ordering

strong strict 2 phase locking (ss2pl; referred rigorousness or rigorous scheduling) means both read , write locks of transaction released after transaction has ended (either committed or aborted). set of ss2pl schedules proper subset of set of co schedules. property utilized in database systems, , since implies co, databases use , participate in global transactions generate serializable global schedule (when using atomic commitment protocol, needed atomicity in multi-database environment). no database modification or addition needed in case participate in co distributed solution: set of undecided transactions aborted before committing in local generic co algorithm above empty because of locks, , hence such algorithm unnecessary in case. transaction can voted on database system after entering ready state, i.e., completing running task locally. locks released database system after decided atomic commitment protocol, , condition in global co enforcing theorem above kept automatically. interestingly, if local timeout mechanism used database system resolve (local) ss2pl deadlocks, aborting blocked transactions breaks not potential local cycles in global conflict graph (real cycles in augmented conflict graph), database system s potential global cycles side effect, if atomic commitment protocol s abort mechanism relatively slow. such independent aborts several entities typically may result in unnecessary aborts more 1 transaction per global cycle. situation different local wait-for graph based mechanisms: such cannot identify global cycles, , atomic commitment protocol break global cycle, if resulting voting deadlock not resolved earlier in database.

local ss2pl atomic commitment implying global serializability can deduced directly: transactions, including distributed, obey 2pl (ss2pl) rules. atomic commitment protocol mechanism not needed here consensus on commit, rather end of phase-two synchronization point. reason, without considering atomic commitment voting mechanism, automatic global deadlock resolution has not been noticed before co.