Safe Commits

Commit After the Writer

A reader commit is safe only after the writer it depended on has committed.

highlighted = computed this step

Commit after the writer

A reader can depend on a writer. If the reader later commits, the writer must have committed first for this check to pass.

writer commit before reader commit\text{writer commit before reader commit}

Show the dependency

There is 1 external read dependency in the diagram.

dependencies=1\text{dependencies}=1

Safe commit and rollback schedules are tiny finite histories; logging, undo, ARIES, timing, isolation-level, product, and performance claims are out of scope.

Recoverable scheduleeventtxopitemvalue1T1wX102T2rX3T1c4T2c reads-from factsreadEventreaderitemwriterwriteEventvalue2T2XT1110 external read dependencieswriterreaderitemwriteEventreadEventT1T2X12 classificationsfactvaluerecoverableyescascadelessnostrictno violationsnkindtx/readerwriteritem1cascadelessT2T1X2strictT2T1X

Check the commit order

The writer commits before the reader, so the recoverable label is yes.

recoverable=yes\text{recoverable}=yes

Summary

This check only talks about commit order for reads-from dependencies. It does not claim a lock protocol or recovery algorithm.

recoverable only\text{recoverable only}