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Transaction Log

A record of all changes made to the database, used for recovery and rollback operations.
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The statement of the theorem

Define the Transaction Log LL as a sequence of log records ⟨L1,L2,…,Lm⟩\langle L_1, L_2, \dots, L_m \rangle. Each record LkL_k is a tuple ⟨TID,Type,Data,OldValue,NewValue⟩\langle \text{TID}, \text{Type}, \text{Data}, \text{OldValue}, \text{NewValue} \rangle, where TID\text{TID} is the transaction ID, Type∈{<BEGIN>,<COMMIT>,<ABORT>,<UPDATE>}\text{Type} \in \{\text{<BEGIN>}, \text{<COMMIT>}, \text{<ABORT>}, \text{<UPDATE>}\}, Data\text{Data} is the identifier of the modified data item, and OldValue\text{OldValue} and NewValue\text{NewValue} are the values before and after the modification, respectively. The log must maintain the strict temporal ordering of all state changes.
Source: Wikipedia