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Until now we have talked about schedules having only committed transactions. We must also take into consideration schedules containing aborted transactions, which brings the recoverability notion. 

Example 4. Consider two transactions

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Here w1[x] will be undone by a2, causing lost update for T1.

If in a schedule,

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• a transaction performs a dirty read operation from an uncommitted transaction

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• and its commit operation is delayed till the uncommitted transaction either commits or

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• aborts

then such a schedule is called as a  Recoverable Schedule. recoverable schedule.

If a schedule only reads data written by already committed transactions, it called cascadeless schedule. It avoids Recoverable schedules also avoid cascading aborts.

TBD strict schedules

CC protocol WIP

A strict schedule only reads or writes data written by already committed transactions. Strict schedules are recoverable, do not require cascading aborts, and actions of aborted transactions can be undone by restoring the original values of modifed objects.

TBD schedules diagram tree.

CC protocol WIP

A CC protocol ensures that only schedules A CC protocol ensures that only schedules with desirable properties are generated.

MV2PL (S2PL vs non-S2PL)

MVOCC

MVTO

WRITE SNAPSHOT https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/tr-95-51.pdf

READ SNAPSHOT https://dl.acm.org/doi/10.1145/2168836.2168853

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