The processes in an Oracle Database system can be categorized into two
major groups:
•
User processes that run the application or Oracle
tool code.
•
Oracle Database processes that run the Oracle
database server code. They include server processes and background processes.
When a user runs an application program or an Oracle tool such as
SQL*Plus, Oracle Database creates a user process to run the user’s application.
Oracle Database also creates a server process to execute the commands issued by
the user process. In addition, the Oracle server also has a set of background
processes for an instance that interact with each other and with the operating
system to manage the memory structures and asynchronously perform I/O to write
data to disk, and perform other required tasks.
The process structure varies for different Oracle Database
configurations, depending on the operating system and the choice of Oracle
Database options. The code for connected users can be configured as a dedicated
server or a shared server.
•
With dedicated server, for each user, the database
application is run by a user process that is served by a dedicated server
process that executes Oracle database server code.
•
A shared server eliminates the need for a dedicated
server process for each connection. A dispatcher directs multiple incoming
network session requests to a pool of shared server processes. A shared server
process serves any client request.
Server
Processes
Oracle
Database creates server processes to handle the requests of user processes
connected to the instance. In some situations when the application and Oracle
Database operate on the same computer, it is possible to combine the user
process and corresponding server process into a single process to reduce system
overhead. However, when the application and Oracle Database operate on
different computers, a user process always communicates with Oracle Database
through a separate server process.
Server
processes created on behalf of each user’s application can perform one or more
of the following tasks:
•
Parsing
and running SQL statements issued through the application
•
Reading
necessary data blocks from data files on disk into the shared database buffers
of the SGA, if the blocks are not already present in the SGA
•
Returning
results in such a way that the application can process the information
Background
Processes
To maximize
performance and accommodate many users, a multiprocess Oracle Database system
uses some additional Oracle Database processes called background processes. An
Oracle Database instance can have many background processes.
The
background processes commonly seen in non-RAC non-ASM environments can include
the following:
•
Database
Writer process (DBWn)
•
Log
Writer process (LGWR)
•
Checkpoint
process (CKPT)
•
System
Monitor process (SMON)
•
Process
Monitor process (PMON)
•
Recoverer
process (RECO)
•
Job
Queue processes
•
Archiver
processes (ARCn)
•
Queue
Monitor processes (QMNn)
Other
background processes may be found in more advanced configurations such as RAC.
See the V$BGPROCESS view for more information about the background processes.
On many operating systems, background processes are created automatically when an instance is started.
On many operating systems, background processes are created automatically when an instance is started.
