Oracle Processes

Three types of processes are part of the instance:
✓ Background processes are involved in running the Oracle software itself.
✓ Server processes negotiate the actions of the users.
✓ User processes commonly work outside the database server itself to run the application that accesses the database.

Background processes

• PMON : The process monitor  manages the system’s server processes. It cleans up failed processes by releasing resources and rolling back uncommitted data
• SMON : The system monitor is primarily responsible for instance recovery. If the database crashes and redo information must be read and applied, the SMON takes care of it. It also cleans and releases temporary space.
• DBWn : The database writer’s sole job is taking dirty blocks from the dirty list and writing them to disk. There can be up to 20 of them, hence the n. It starts as DBW0 and continues with DBW1, DBW2, and so on. After DBW9, it continues with DBWa through DBWj. An average system won’t see more than a few of these.
• LGWR : The log writer process flushes the redo log buffer. It writes the redo entries to disk and signals a completion.
• CKPT : The checkpoint process is responsible for initiating check points. A check point is when the system periodically dumps all the dirty buffers to disk. Most commonly, this occurs when the database receives a shutdown command. It also updates the data file headers and the control files with the
  check point information so the SMON know where to start recovery in the event of a system crash.

The above mentioned 5 processes are mandatory processes that are must required for any Oracle Instance to run properly. Any one process missing can bring down the whole instance.

The rest of the processes are optional processes. They may or may not be present in the system depending upon the additional functionality assigned to the DB instance.

• ARCn : Up to 30 archiver processes (0–9, a–t) are responsible for copying filled redo logs to the archived redo storage area. If your database isn’t running in archive mode, this process shuts down.
• CJQ0 : The job queue coordinator checks for scheduled tasks within the database. These jobs can be set up by the user or can be internal jobs for maintenance. When it finds a job that must be run it spawns the following goodie.
• J000 : A job queue process slave actually runs the job. There can be up to 1,000 of them (000–999).
• DIA0 : The diagnosability process resolves deadlock situations and investigates hanging issues.
• VKTM : The virtual keeper of time sounds like a fantasy game character but simply provides a time reference within the database.