Recovery

Checkpoint

A checkpoint records active transactions as recovery metadata.

highlighted = computed this step

Checkpoint

A checkpoint records which transactions were active at that point in the log. This tiny engine keeps checkpoint handling simple and still recomputes the whole recovery result. Note: the active set appears in the log table.

checkpoint record\text{checkpoint record}

Checkpoint record

The compiled log has 1 checkpoint and 1 loser at crash. Note: checkpoint metadata is rendered, not trusted as output.

checkpoints=1,losers=1\text{checkpoints}=1,\quad \text{losers}=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.

Checkpoint lsn txn type item before->after 1 T1 begin 2 T1 write A 0->5 3 T1 commit 4 T2 begin 5 SYS checkpoint active:T2 6 T2 write B 0->9 CRASH winners: T1 losers: T2 recovered state item value A 5 B 0 redo LSNs: 2 undo LSNs: 6

Summary

Checkpoints are scan metadata, not a substitute for recomputing winners, losers, and recovery. Note: this is simplified redo and undo; ARIES detail, fuzzy checkpoints, CLRs, media recovery, and real fsync durability are deferred.

checkpoint metadata\text{checkpoint metadata}