Oracle Database Instances Startup/Shutdown Sequences
In Oracle Database, the distinction between instances and databases is essential for efficient data management. Let’s explore these concepts and compare the various startup and shutdown sequences.
What is an Oracle Database Instance?
A database instance consists of memory structures that manage physical database files. It is responsible for handling data operations and user requests. Conversely, a database is a collection of physical files created by the CREATE DATABASE statement. While every Oracle database is linked to at least one instance, an instance can exist independently of a database, and vice versa.
Startup Sequence of Oracle Database
Startup Sequence of Oracle Database
A database goes through the following phases when it proceeds from a shutdown state to an open database state :-
| Phase | Mount State | Description |
| 1 | Instance started without mounting database | The instance is started, but is not yet associated with a database |
| 2 | Database mounted | The instance is started and is associated with a database by reading its controlfile. The database is closed to users. |
| 3 | Database open | The instance is started and is associated with an open database. The data contained in the data files is accessible to authorized users. |
Instance and Database Shutdown Sequence
Instance and Database Shutdown Sequence
| Phase | Mount State | Description |
| 1 | Database closed | The database is mounted, but online data files and redo log files are closed. |
| 2 | Database mounted | The instance is started and is associated with a database by reading its control file. The database is closed to users. |
| 3 | Database instance shut down | The database instance is no longer started. |
Database Shutdown Modes
- Shutdown Normal:
- This is the default shutdown mode. The database waits for all users to disconnect and ensures that all ongoing transactions are completed. Once no active sessions are left, the database shuts down gracefully.
- Shutdown Transactional:
- In this mode, the database waits for all active transactions to complete. However, unlike Normal mode, it does not wait for users to disconnect. The shutdown will proceed after all transactions are finished.
- Shutdown Immediate:
- The database stops immediately without waiting for users to disconnect or active transactions to complete. It forces all sessions to be terminated and uncommitted transactions are rolled back. This mode is faster than Normal mode and Transactional mode.
- Shutdown Abort:
- This is the most abrupt shutdown method. The database stops immediately without performing any clean-up activities like rolling back transactions or waiting for users to disconnect. It is used when a quick shutdown is required, but it may require recovery when the database is started again.
Each shutdown mode serves different needs based on how quickly the shutdown is required and whether you need to ensure data integrity or allow for a faster shutdown.
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