Which file records all changes made to the database and is used only when recovering an instance?
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Archive log file
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Redo log file
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Control file
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Alert log file
Online redo log files record all database changes in real-time. They are essential for instance/crash recovery when an instance fails abnormally, allowing database rollback and rollforward operations.
The (online) redo log file records every change made to an Oracle database and is used specifically for instance recovery — replaying committed and uncommitted changes since the last checkpoint after a crash. Archive log files are copies of filled redo logs kept for media recovery (restoring from backup across a longer time span), not instance recovery. The control file tracks physical database structure/metadata (datafile locations, SCNs), and the alert log is a diagnostic text log of database events/errors — neither is used to redo changes during recovery.