Logging

Read/append only data structure on stable media
→ Recovery manager appends transaction log records
→ Aborting and failure recovery reads transaction log records

UNDO: Old version of objects that have been modified by a transaction
- Can be used to undo database changes on abort
REDO: New versions of objects that have been modified by a transaction
- Can be used to redo the work done by a transaction that commits
BEGIN/COMMIT/ABORT: Record when transactions begin, commit, or abort

Example

Five transactions: ${T_{0}, T_{1}, T_{2}, T_{3}, T_{4}}$
Transaction status

${T_{1}, T_{3}}$ committed
${T_{2}}$ aborted
${T_{0}, T_{4}}$ active

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Write-Ahead Logging

Allowing indices on stable store to be updated prior to a LOG can lead to inconsistency

Undo Rule

Log record for an update is appended to the LOG file before the corresponding data page is written to stable store
→ Guarantees atomicity

Redo Rule

All log records for a transaction are appended to the LOG file before acknowledging a commit
→ Guarantees durability