Processes Supporting an Instance

A number of processes are associated with an instance and perform specific tasks for the instance.

Listener
The Listener is a process that listens on the network for requests coming in to an Oracle instance. AListener
process can support one or more Oracle instances. The Listener acts as the intermediary between user
requests from remote machines and the instance. The failure of the Listener will mean that the instance it
supports is not accessible by a remote client—such as any application that accesses the instance.

Background Processes
Awhole set of processes run in the background on an Oracle server, supporting the actions of the Oracle
instance. The main processes are listed here, with the standard acronyms of the processes in parentheses:
❑ Database Writer (DBWR). Writes data blocks from the database buffers, as described in the fol-
lowing section.
❑ Log Writer (LGWR). Writes redo log information from the redo log buffer in memory to the
redo log on disk.
❑ System Monitor (SMON). Monitors the health of all components of an Oracle instance, and
helps to recovery the database when it is restarted after a crash.
❑ Process Monitor (PMON). Watches individual user processes that access the Oracle instance
and cleans up any resources left behind by an abnormal termination of a user process.
❑ Archiver (ARC). Writes a copy of a filled redo log file to an archive location. An Oracle instance
can have up to 10 Archiver processes.