Recovery

Commit Means Durable

A committed transaction is a winner whose writes survive recovery.

highlighted = computed this step

Commit means durable

A transaction with a commit record before the crash is a winner. Winner writes are redone and become the authoritative recovered state. Note: committed writes survive even when other writes touched the same item earlier.

commit durability\text{commit durability}

Winner classification

The compiled log has 1 winner. Note: the winner label is rendered from the commit record.

winners=1\text{winners}=1

a simplified redo/undo recovery over a tiny log; ARIES detail (fuzzy checkpoints, CLRs, media recovery) and real durability/fsync are beyond these traces - no product claims.

Commit means durable lsn txn type item before->after 1 T1 begin 2 T1 write A 0->5 3 T1 commit CRASH winners: T1 losers: - recovered state item value A 5 redo LSNs: 2 undo LSNs: -

Redo durable write

The redo list has 1 winner write, and the recovered state has 1 item. Note: the recovered value is shown only in the render.

redo=1,recovered items=1\text{redo}=1,\quad \text{recovered items}=1

a simplified redo/undo recovery over a tiny log; ARIES detail (fuzzy checkpoints, CLRs, media recovery) and real durability/fsync are beyond these traces - no product claims.

Commit means durable lsn txn type item before->after 1 T1 begin 2 T1 write A 0->5 3 T1 commit CRASH winners: T1 losers: - recovered state item value A 5 redo LSNs: 2 undo LSNs: -

Summary

Commit is the boundary that makes a write durable in this model. Note: this is simplified redo and undo; ARIES detail, fuzzy checkpoints, CLRs, media recovery, and real fsync durability are deferred.

winner writes survive\text{winner writes survive}