Oracle Database Architecture

The Oracle instance = SGA + oracle processes.

Oracle database = Data files + Control Files + Redo log files

PMON (Process Monitor)	- when a session is broken: · the rollback is performed (PROCESS RECOVERY) · the memory used became available - restart the Dispatcher and server processes if they are stopped - registers information about the instance and dispatcher processes with the network listener
SMON (System Monitor)	- when an instance is broken or restarted: a rollback is performed (INSTANCE RECOVERY). In an Oracle Real Application Clusters database, the SMON process of one instance can perform instance recovery for other instances that have failed. - cleans up temporary segments that are no longer in use - coalesces contiguous free extents in dictionary managed tablespaces that have PCTINCREASE set to a non-zero value.
DBWn (Database Writer)	- writes modified blocks from the database buffer cache to the data files. The database selects an appropriate default setting for this initialization parameter or adjusts a user-specified setting based on the number of CPUs and the number of processor groups. However, the DB_WRITER_PROCESSES initialization parameter could be used to specifies the number of DBWn processes. Oracle Database allows a maximum of 20 database writer processes (DBW0-DBW9 and DBWa-DBWj).
LGWR (Log Writer)		- write to disk the content of redo log buffer
CKPT (Checkpoint)		- tell to the DBWn to write the buffer cache information to the disk files
CJQ0 (Coordinator Job Queue)		- check for scheduled jobs create processes Jnnn processes which will run the particular jobs.
Jnnn (Job slave process)		- run different scheduled jobs. For more information about the Oracle jobs follow the link.
Dnnn (Dispatcher process)		- receive requests from the listener and acts like a requests coordinator for the Oracle Shared (MTS) Server processes. For more information about the Shared (MTS) Server architecture follow the link.
ARCn (Archiver Process)		- archive the log files (if database is in ARCHIVELOG mode).