Interview Questions - Oracle Data Guard - 2

1) What is a Redo Transport Service in Oracle Data Guard?

The Redo Transport Service is responsible for transmitting redo data from the primary database to the standby database. It ensures that changes made to the primary database are replicated to the standby database, keeping them synchronized. The two modes of redo transport are synchronous and asynchronous.

2) How is log transport initiated from the Primary to Standby database?

   Log transport from the Primary to Standby database in Oracle Data Guard is initiated through a series of steps and mechanisms that ensure the redo data (changes made to the database) is reliably transmitted and applied to the standby database. Here’s how the process works:

A) Redo Generation:

   As transactions are committed on the primary database, redo entries are generated and written to the online redo log files.

B) Log Writer Process (LGWR):

   The Log Writer process is responsible for writing redo entries from the redo log  buffer in memory to the online redo log files on disk.

C) Archiver Process (ARCn):

   When a log switch occurs (i.e., the current redo log file is full and a new log file is  started), the Archiver process copies the contents of the filled online redo log file to an archive log file. This is necessary for data recovery purposes.

D) Redo Transport Services:

Redo Transport Services manage the transmission of redo data to the standby database. These services can operate in synchronous or asynchronous mode, depending on the chosen Data Guard protection mode.

Synchronous Transport:

   In synchronous transport mode, the redo data is sent to the standby database as it is generated.

   The LGWR process writes the redo data to the standby redo log files on the standby database and waits for an acknowledgment that the write is complete before confirming the transaction commit on the primary database. This ensures no data loss but can introduce latency.

Asynchronous Transport:

   In asynchronous transport mode, the redo data is sent to the standby database fter the transaction is committed on the primary database.

   This mode reduces latency and has less impact on the primary database’s  performance but carries a risk of data loss if the primary database fails before the redo data is transmitted and applied to the standby database.

E) Network Transport:

The redo data is transmitted over the network from the primary database to the  standby database. Data Guard uses Oracle Net Services (SQL*Net) for this  communication, ensuring secure and reliable data transfer.

F) Standby Redo Log Files:

On the standby database, the redo data is written to standby redo log files. These  files are equivalent to the online redo log files on the primary database and are used to apply the redo data to the standby database.

G) Log Apply Services:

   The Log Apply Services on the standby database are responsible for applying the redo data to the standby database, keeping it synchronized with the primary database.

   Depending on the standby database configuration, the redo data can be applied in real-time or at intervals, allowing for flexibility in how up-to-date the standby database is kept.

H) Acknowledgments:

   In synchronous mode, the standby database sends an acknowledgment back to the primary database once the redo data is successfully written to the standby redo log files. The primary database then confirms the transaction commit.

   In asynchronous mode, the acknowledgment is not required for the primary  database to confirm the transaction commit, allowing for reduced latency but with potential data loss risks.

3. Explain the concept of the Far Sync instance in Oracle Data Guard.

A Far Sync instance in Oracle Data Guard serves as a remote redo transport destination that acts as a relay between the Primary and Standby databases. It allows for zero data loss configurations without the need for a physical standby at the remote site.

4. How does Oracle Data Guard handle network failures between the Primary and Standby databases?

Oracle Data Guard handles network failures by utilizing features like Fast-Start Failover and automatic detection of network issues. Fast-Start Failover can automatically fail over to a standby database in case of a communication failure, ensuring minimal downtime.

5. Enumerate the advantages associated with the maximum protection mode.

The advantages of Maximum Protection mode in Oracle Data Guard include:

Zero data loss in case of a primary database failure.

Synchronous redo transport to ensure that transactions are committed on the    standby before being acknowledged on the primary.

Highest level of data protection at the cost of potential performance impact.