Transaction durability

Transaction durability means that an acknowledged committed transaction survives failures covered by the database's durability model. A successful statement that has not committed is not yet such a promise. The failure model and storage configuration matter: surviving a process crash does not automatically mean surviving destruction of every storage replica.

Write-ahead logging provides one mechanism. Before reporting a durable commit, persist the required log records. Modified data pages can be written later because recovery can replay their committed changes from the log. The log-before-data rule also prevents a page from reaching persistent storage without the recovery information needed to interpret it.^[ref] Durability therefore concerns persistence of the committed result, whereas transaction atomicity concerns which parts of a transaction may take effect. Faster asynchronous commit settings can deliberately weaken the interval covered by the acknowledgment.