Mandatory Background Processes in Oracle Database
In every Oracle Database environment, certain mandatory background processes are essential for the database’s operation and maintenance. These processes run by default in a read/write database instance that is started with a minimally configured initialization parameter file. However, a read-only database instance disables some of these processes.
This blog discusses the following mandatory background processes:
1. Process Monitor (PMON)
Role: Monitors database processes and cleans up resources.
Function: If a user or background process terminates unexpectedly, PMON ensures that resources (like locks and memory) are released so that other processes can use them.
2. System Monitor (SMON)
Role: Manages instance recovery and cleanup.
Function: On startup, SMON recovers any uncommitted transactions from the last shutdown and cleans up temporary segments that are no longer in use.
3. Database Writer (DBWn)
Role: Writes data to disk.
Function: DBW moves modified data from the buffer cache to data files. This ensures that changes are saved and helps prevent data loss.
4. Log Writer (LGWR)
Role: Manages redo log entries.
Function: LGWR writes the contents of the redo log buffer to disk, protecting the database against data loss. It ensures that all changes made to the database are logged before they are committed.
5. Checkpoint Process (CKPT)
Role: Updates control file and data file headers.
Function: CKPT marks the point at which all data has been written to disk, helping to optimize recovery time by reducing the amount of data that must be read during recovery.
6. Listener Registration Process (LREG)
Role: Registers database instances with the listener.
Function: LREG informs the Oracle Net Listener about the database instance’s status, enabling client connections and ensuring that users can access the database.
7. Manageability Monitor Processes (MMON and MMNL)
Role: Oversees performance monitoring.
Function: MMON collects and manages data for the Automatic Workload Repository (AWR), while MMNL writes Active Session History (ASH) data to disk, helping to track database performance.
8. Recoverer Process (RECO)
Role: Resolves issues in distributed transactions.
Function: In environments with distributed databases, RECO ensures that any in-doubt transactions are resolved, maintaining consistency across different database nodes.
