DATAGUARD SERVICES AND MODES

DATAGUARD SERVICES:  To transfer the logs between primary and standby database oracle provide the few services which is required to run the data guard smoothly  following section explain how DG managed the transmission of redo data from primary database to standby database.

Redo Transport Services: This service control the automated transfer of redo data from production Database to one or more archival destination (standby Database side). Redo transport services perform following task.

Ø  Transfer the redo data from Primary system to standby system in the DG configuration.

Ø  Manages the process of resolving the gap in the archive redo log files due to network failure.

Ø  This service automatically detect missing or corrupted archived redo log files  on a standby   system& automatically  retrieved replacement archived redo log files from the Primary Database on standby Database.

Ø  Data Guard Redo Transport Services are used to transfer this redo data to the standby site synchronously or asynchronously

       Redo apply services: (MRP & RFS): 

  Ø   One the redo is transferred to the standby site, Data Guard apply service provide two different method for applying the data to a standby Database. Data guard redo apply service (physical standby) and Data guard Sql apply service(logical standby).

      Ø  Data Guard Redo Transport Services are used to transfer this redo data to the standby site synchronously or   asynchronously.

        Role Transitions: We can change the role of Primary Database from Primary to standby and standby system to Primary system using a either switch over or fail over operations. Role transition are invoked manually used sql statements. we can also use the oracle data guard broker to simplify role transition and automate fail over using OEM or the DGMGR command line interface.

Switch over Role: A switch over is a role reversal between the Primary Database and one of its standby Database.This is typically done for planned maintenance at the Primary system. During switch over the Primary Database transition to a standby role and standby Database transition to Primary role.

Fail over Role: Fail over is when the Primary Database is unavailable fail over is performed only in the event of a failure of the Primary Database and the fail over result in a transition of standby Database to a Primary role.

 Data Guard Protection Mode: Oracle data protection concept basically used for safeguard against data loss. Oracle 9i introduced the Maximum protection and Maximum availability concept.

      Maximum performance:

T        Ø   This is the default protection mode. It provides the highest level of data protection that           is possible without  affecting the performance of a Primary Database. 

        Ø  Redo data is written to one or more standby Database but this is done asynchronously with  respect to transaction commitment, so Primary Database performance is unaffected by delays is writing redo data to the standby Database.

         Ø   This allows the transactions to commit as soon as all redo data generated by those transaction has written to online redo log.

         Ø   This protection mode offer slightly less data protection than maximum availability Mode. And has minimal impact on Primary Database performance.

    Maximum protection:

       Ø  This protection mode ensures that no data loss will occur if the Primary Database fails. To provide  this level of protection, the redo data needed to recover a transaction must be written to both the online redo log and to at least one standby Database before the transaction commits.

        Ø   To ensure that data loss cannot occur, the Primary Database will shut down, rather than continue processing transactions.

     Maximum availability:

      Ø    This protection mode provides the highest level of data protection that is possible without effecting the availability of a Primary Database. Transactions do not commit until all redo data neded to recover those transactions has been written to the online redo log and to at least one standby Database.

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