How Oracle Database Writes To The Redo Log?

The redo log of a database consists of two or more redo log files. The database requires
a minimum of two files to guarantee that one is always available for writing while the
other is being archived (if the database is in ARCHIVELOG mode).
LGWR writes to redo log files in a circular fashion. When the current redo log file fills,
LGWR begins writing to the next available redo log file. When the last available redo
log file is filled, LGWR returns to the first redo log file and writes to it, starting the
cycle again. The numbers next to each line indicate the sequence in which LGWR writes to each redo log file.
Filled redo log files are available to LGWR for reuse depending on whether archiving
is enabled.
¡ If archiving is disabled (the database is in NOARCHIVELOG mode), a filled redo log
file is available after the changes recorded in it have been written to the datafiles.
¡ If archiving is enabled (the database is in ARCHIVELOG mode), a filled redo log file
is available to LGWR after the changes recorded in it have been written to the
datafiles and the file has been archived.

Oracle Database uses only one redo log files at a time to store redo records written
from the redo log buffer. The redo log file that LGWR is actively writing to is called the
current redo log file.
Redo log files that are required for instance recovery are called active redo log files.
Redo log files that are no longer required for instance recovery are called inactive redo
log files.
If you have enabled archiving (the database is in ARCHIVELOG mode), then the
database cannot reuse or overwrite an active online log file until one of the archiver
background processes (ARCn) has archived its contents. If archiving is disabled (the
database is in NOARCHIVELOG mode), then when the last redo log file is full, LGWR
continues by overwriting the first available active file.

A log switch is the point at which the database stops writing to one redo log file and
begins writing to another. Normally, a log switch occurs when the current redo log file
is completely filled and writing must continue to the next redo log file. However, you
can configure log switches to occur at regular intervals, regardless of whether the
current redo log file is completely filled. You can also force log switches manually.
Oracle Database assigns each redo log file a new log sequence number every time a
log switch occurs and LGWR begins writing to it. When the database archives redo log
files, the archived log retains its log sequence number. A redo log file that is cycled
back for use is given the next available log sequence number.
Each online or archived redo log file is uniquely identified by its log sequence number.
During crash, instance, or media recovery, the database properly applies redo log files
in ascending order by using the log sequence number of the necessary archived and
redo log files.