Durability
When a transaction commits, its results will survive failures (e.g. of the application, OS, DB system … even of the disk).
Makes it possible for a transaction to be a legal contract.
Implementation is usually via a log
- DB system writes all transaction updates to its log
- to commit, it adds a record “commit(Ti)” to the log
- when the commit record is on disk, the transaction is committed.
- system waits for disk ack before acking to user
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