1. What is the use of standby redolog:

The Advantage of having Standby Redo Logs is that every records written into the Online RedoLogs of the Primary Database
is transferred to the Standby database and written into the Standby Redo Logs at the same time.
therefore, you it minimizes the probability of Data Loss on the Standby Database.

For real time apply, it is mandatory to have redolog.

Without Standby Redo Logs, an Archived Redo Log is created by the RFS process and when it has completed,
this Archived Redo Log is applied to the Standby Database by the MRP process . An incomplete ArchiveLog file cannot be applied on the Standby Database and will not be used in a Failover situation. This causes a certain data loss.

If you have Standby Redo Logs, the RFS process will write into the Standby Redo Log ,when there record entered in online redolog and when a log switch occurs, the Archiver Process of the Standby Database will archive this Standby Redo Log to an Archived Redo Log, while the MRP process applies the information to the Standby Database. In a Failover situation, you will also have access to the information already written in the Standby Redo Logs, so the information will not be lost.

2. What are different protection modes in dataguard

There are three modes .

1. MAXIMUM PROTECTION
This mode provides maximum protection. It guarantees zero data loss. In this mode the redo/transaction data must be written to both primary redo log and standby redo log. For any reason(mostly N/W issue) if it is unable to write to standby, Then primarya will get shutdown.

2. MAXIMUM AVAILABILITY:
It provides the highest level of data protection that is possible without affecting the availability of the primary database.Transactions are not allowed to commit until all redo data are written to the online redo logs and propagated to at least one synchronized secondary database. If for any reason, the primary database cannot propagate its redo stream to one secondary database, the primary will NOT shutdown and operates as it it were in maximum performance mode until issues are fixed.

3. MAXIMUM PERFORMANCE:(default one)
This is the default protection mode. With this protection mode, a transaction is committed as soon as the redo data needed to recover the transaction is written to the local (online) redo log.

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