Free Transactions and concurrency lessonsTransactions and concurrency · Stage 0 · All or nothing · Step 2

The write-ahead log

See how a sequential log of changes lets the database undo unfinished work and redo committed work after a crash. About 14 minutes.

Quick check

When a transaction commits, what must the database make sure has reached disk before reporting success?

AThe transaction's log records, including its commit record
BEvery data page the transaction modified
CNothing: commit is just a flag in memory

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The transaction's log records, including its commit record. With the log on disk, recovery can redo the changes even if the data pages were never written.

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