Hamish Burke | 2025-06-16

Related to: #databases

Immediate Update Algorithm

• Changes written to database during transaction execution (before it commits)
  • Means changes immediately visible to other transactions (violates isolation in ACID), unless Concurrency Control Mechanism used
• Needs undo and redo
• Undo
  • If Transaction fails before committed
  • If a transaction was uncommitted at the time of the crash
• Redo
  • If Transaction committed but changes lost (eg system crash)
  • If a transaction committed after the checkpoint but before the crash
• Done through logging (think Pencil->StrokeHistory)

<T1, start>
<T1, X, 100, 150> // transaction, item, before, after
<T1, Y, 200, 170>
<T1, commit>