Oracle Database Process Structure

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:

• Parse and run SQL statements issued through the application

• Read 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)

• Return 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 on the background processes.

Some background processes are created automatically when an instance is started, whereas others are started as required.