Online Redo Log File

Each transaction is recorded in the redo logs.Redo logs stroe all changes made to the database. In the oracle database every instance has online redo log to protect the database in any case of an instance failure.Every oracle database instance has its own online redo log group. 
Online redo log files are filled with redo records are also called redo entry. If you change any value in a table then you generate a redo record that describe changes to the data segment block for the table and the transaction table of the rollback segments.Redo entries record all data that you can use to reconstruct all changes made to the database.Online redo log also protects rollback data, when you recover the database using redo data.

 In the above fig you can see a set of identical copies of online redo log files. DBA can set up the oracle database to maintain copies of online redo log files to avoid losing database information. The Log Writer LGWR background process writes the same information to all online redo log files in a group.So, Oracle server needs minimum two online rredo log file groups for the normal operation of database.In each group each online redo log file is known as member and each member in a group has Log Sequence Number.The current log sequence number is stored in the control file and in the header of all data files.
Using the alert log,v$logfile, v$loghist, v$recovery_log, you can monitor redo logs.

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