Problem 2 - Olympiad

A distributed OLTP system uses two-phase commit (2PC). The transaction manager logs , receives unanimous votes , then crashes before logging . After restart the recovery procedure reads the log. Which log record combination allows the TM to decide the fate of T without remote communication and what is the resulting terminal decision?

A. Only <prepare T> exists → abortB. Only <prepare T> and <yes> from all cohorts exist → commitC. Only <prepare T> and <yes> from at least one cohort exist → commitD. Only <prepare T> and <abort> from any cohort exist → abort

Correct: B

In 2PC if the TM has logged and collected from every cohort it is allowed to commit (it has reached the commit point). Because the crash happened after this but before the log record, the TM can infer it must have decided to commit and can therefore redetermine that decision without contacting cohorts.