This is a file that contains several important messages relevant to the database administrator. This file is located in the specified path defined in the BACKGROUND_DUMP_DEST variable. This variable is dynamic, meaning we can change it without shutting down the database.

This file contains the following messages:

1. Commands to start and stop the database, including the start times and types of database startups and the times of transitioning from the NOMOUNT state to the MOUNT state and then to the OPEN state.
2. All operations that change the physical structure of the database, such as the ALTER DATABASE command, including creating, deleting, and renaming data files, as well as redo files and resizing data files, and putting data files offline.
3. Operations related to tablespaces, such as creation, deletion, and also putting tablespaces in Hot Backup Mode and offline mode.
4. All log switch operations.
5. Operations that change variables via the ALTER SYSTEM command.
6. Moment of detecting corrupt blocks in the data files.
7. Moment of detecting deadlock issues, where locks occur between two sessions, each waiting for the other.
8. Problems that occur during file opening, specifically during the transition from the NOMOUNT state to the MOUNT state or from the MOUNT state to the OPEN state.
9. Moment of checkpoint failure.

Naturally, we can review the Alert Log file via the operating system through the BACKGROUND_DUMP_DEST path. This file is consistently named alert_(SID).log, where SID is the instance name.

Thus, the Alert Log file is a single file for each instance containing information about the operation of the database and the instance.