Log file consists of a sequence of small records:

• <START T> ... transaction T begins
• <COMMIT T> ... transaction T completes successfully
• <ABORT T> ... transaction T fails (no changes)
• <T,X,v> ... transaction T changed value of X from v

• we refer to <T,X,v> generically as <UPDATE> log records
• update log entry created for each WRITE (not OUTPUT)
• update log entry contains old value (new value is not recorded)