Log-Based Recovery A log is kept on stable storage. The log is a sequence of log records,and maintains a record of update activities on the database. When transaction 7;starts,it registers itself by writing a <T;start>log record Before T;executes write(X),a log record <Ti X,Vi,V2> is written,where Vi is the value of X before the write (the old value), and V2 is the value to be written to X(the new value) When T;finishes it last statement,the log record <T;commit>is written. Two approaches using logs Deferred database modification Immediate database modification Database System Concepts-6th Edition 16.12 @Silberschatz,Korth and SudarshanDatabase System Concepts - 6 16.12 ©Silberschatz, Korth and Sudarshan th Edition Log-Based Recovery A log is kept on stable storage. The log is a sequence of log records, and maintains a record of update activities on the database. When transaction Ti starts, it registers itself by writing a <Ti start>log record Before Ti executes write(X), a log record <Ti , X, V1 , V2> is written, where V1 is the value of X before the write (the old value), and V2 is the value to be written to X (the new value). When Ti finishes it last statement, the log record <Ti commit> is written. Two approaches using logs Deferred database modification Immediate database modification