Oracle Real Application Clusters (RAC) is a software component we can add to a high-availability solution that enables users on multiple machines to access a single database with increased performance. RAC comprises two or more Oracle database instances running on two or more clustered machines and accessing a shared storage device via cluster technology. To support this architecture, the machines that host the database instances are linked by a high-speed interconnect to form the cluster. Oracle RAC offers features in the following areas:

- Scalability

- Availability

- Load balancing

- Failover

- Online backup is one which is taken at the time the database is running in the ARCHIVELOG mode.

- Following files should be backed up during the Offline Backup: All data file, all control file and all online redo log file. But in the case of the online backup following files is backed up: All data files all archived redo log files and one control file via the alter database command.

- Offline database we cannot rely upon but in case of online database no issues.

- Offline backup no archived mode is required but in the online backup it is must.

Oracle HTTP Server is managed by OPMN which stands for Oracle Process Manager and Notification Server. With the use of opmnctl utility we can start, stop and restart Oracle HTTP Server. If we don’t use this utility the configuration management infrastructure cannot detect or communicate with the Oracle HTTP Server processes, and problems may occur.

The startproc command for the case of Windows is: ORACLE_HOME\opmn\bin> opmnctl [verbose] startproc ias-component=HTTP_Server.

To stop Oracle HTTP Server, use the stopproc command: Windows: ORACLE_HOME\opmn\bin> opmnctl [verbose] stopproc ias-component=HTTP_Server.

Basic Replication : Basic replication is implemented using standard CREATE SNAPSHOT or CREATE MATERIALIZED VIEW statements. It can only replicate data and not procedures, indexes replication is always one-way, and snapshot copies are read only.

Advanced Replication : Advanced replication supports various configurations of updatable snapshot, multi-master and update anywhere replication. It is more difficult to configure but allows data and other database objects like indexes and procedures to be replicated.

Differences between Basic and Advanced replications:

- With basic replication, data replicas provide read-only access to the table data whereas advanced replication features extend the capabilities of basic read-only replication by allowing applications to update table replicas throughout a replicated database system.

- With Basic Replication applications can query data from local data replicas. On the other hand with advanced replication, data replicas anywhere in the system can provide both read and update access to a table's data.

The CACHE option means how many sequences will be stored in memory for access by the application objects. The performance is faster. However in case of the database is down the data is memory is lost for the sequence.

The NO CACHE option means values are not stored in memory. So there might be some performance issue. 

A sequence is a database object that is used to generate sequential number.

CREATE SEQUENCE seqname [increment] [minimum value][maximum value][start][cache][cycle]               Nextval and currval lets us get the next value and current value from the sequence.

- %FOUND

 This is a Boolean variable which evaluates to TRUE if the last row is successfully fetched.

- %NOTFOUND

 This is a Boolean variable which evaluates to TRUE if the last row is not successfully fetched. This means there are no more rows to fetch.

- %ROWCOUNT

 Returns the number of rows fetched by the cursor.

- %ISOPEN

If the cursor is open, it evaluates to TRUE else FALSE.

%type is used to declare a field of a table while %rowtype is used to declare a record with the same type as specified in that table, view or cursor.

Example of %type:

DECLARE

         v_EmployeeName emp.ename%TYPE

Example of %rowtype

DECLARE

          v_empployee emp%ROWTYPE;