Oracle Processes

There are server and background processes.

Here's a list of background processes:

• SMON
  The System Monitor carries out a crash recovery when a crashed insance is started up again. It also cleans temporary segments.
• PMON
  The Process Monitor checks if a user process fails and if so, does all cleaning up of resources that the user process has aquired.
• DBWR
  The Database Writer writes dirty blocks from the database buffer to the datafiles. How many DBWn Processes are started is determined by the initialization parameter DB_WRITER_PROCESSES. DBWR also writes the actual SCN with the Block.
• LGWR
  The Log Writer writes the redo log buffer from the SGA to the online redo log file.
• MMAN
  The memory manager
• MRP
  Managed recovery process: the process that applies archived redo log to the standby database.
• RFS
  The remote file server process on the standby database receives archived redo logs from the primary database.
• RECO
  The Distributed Transaction Recovery Process finds pending (distributed) transaction and resolves them.
• CKPT
  The Checkpoint Process reqularly initiates a checkpoint which uses DBWR to write all dirty blocks back to the datafiles, thus synchronizing the database. Since a Checkpoint records the current SCN, in a recovery only redo records with a SCN higher than that of the last checkpoint need to be applied.
• ARCH
  The Archiver Process archives redo log files if ARCHIVELOG is enabled.
• Dnnn
  The Dispatcher Process is used in a shared server environment.
• Snnn
  The Shared Server Process is used in a shared server environment.
• LMON
  The lock manager.
• LMD0
  
• QMNn
  AQ Time Manager
• TRWR
  Trace writer
• WMON
  The wakeup monitor process.
• LCKnnn
  Inter-instance locking process.
• SNPnnn
  The snapshot process.
• MMON
  New background process in Oracle 10g.
• DMON
  The Data Guard Broker process.
• SNP
  The snapshot process.

Background processes have v$session.type = 'BACKGROUND'

On Unix, the Oracle Processes can be listed with something like

ps -eaf | grep ora

While background processes do the work for Oracle itself, foreground processes do the work for clients (such as SQL statements).

Foreground processes are created by the listener.

Foreground processes have v$session.type = 'USER'

Oracle uses semaphores to coordinate the work for the processes.