Comparison among General and nested protocols:
The nested protocol is suitable for a system in which
• The message send-receive cost is high as well as broadcast not available.• The require for concurrency within phase 1 and concurrency within phase 2 is low,• The participant as well as cohort structure of the transaction is static or universally known.
Most data management systems have chosen for the nested commit protocol for these reasons. Alternatively the general two phase commit protocol is appropriate if:
a) Broadcast is the normal mode of interposes communication (in that case the coordinator sends two messages as well as each process sends two messages for a total of 2N messages.)Aloha net, Ethernet, ring-nets as well as space-satellite nets have this property.
b) Parallelism between the cohorts of a transaction is desirable (the nested protocol has merely one process active at a time during commit processing.)
Summary of Recovery protocols:
• The consistency lock protocol segregate transactions from inconsistencies due to concurrency.
• The DO-REDO-UNDO log record protocol permits for and uncommitted actions.
• The write-ahead-log protocol assures that the log is ahead of non-volatile storage so that undo as well as redo can always be performed.
• The two-phase commit protocol coordinates the commitment of independent participants (or cohorts) within a transaction.
The following table describes the virtues of the write-ahead-log as well as two-phase-commit protocols. It observes the possible situations after a crash. The applicable issues are whether an update to the object survived (was written to non-volatile storage), as well as whether the log record corresponding to the update survived. One mill never have to redo an update whose log record isn’t written because: Only committed transactions are redone as well as COMMIT writes out the transaction's log records prior to the commit completes. Therefore the (no, no, redo) case is precluded by two-phase commit. Likewise write-ahead-log (WAL) precludes the (no, yes,*) cases because an update is never written before its log record. The other cases must be obvious.
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