Put the database in ARCHIVELOG mode

Command

SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE ARCHIVELOG;
ALTER DATABASE OPEN;
-- Verify:
ARCHIVE LOG LIST;

What it does

Puts the database in archive log mode; filled redo logs are archived, making hot backups and point-in-time recovery possible.

When to use it

For the backup and recovery strategy of a production database (mandatory before RMAN, Data Guard).

Caution

Requires an instance restart; plan the downtime. Make sure the archive destination (FRA or LOG_ARCHIVE_DEST) has enough space; if it fills up, the DB halts. On RAC: stop all instances with srvctl stop database -d DB, open a single instance with STARTUP MOUNT, switch to ARCHIVELOG, shut it down and start with srvctl start database -d DB (11gR2+ does not require cluster_database=false).