PMON : THE PROCESS MONITOR

1.PMON is responsible for cleaning up after abnormally terminated connections.

What  If your dedicated server “fails” or killed for some reason ?? PMON is the process responsible for fixing and releasing your resources.PMON will perform the below steps :

• Initiate rollback of uncommitted work
• Release locks
• Free SGA resources allocated to the failed process

2.PMON is responsible for monitoring other background processes and restarting them if necessary.

• What if a shared server or a dispatcher fails ?? PMON is the solution it will step in and restart another dispatcher.
• What if LGWR or DBWR fails ?? In this case PMON will terminate the instance as LGWR and DBWR are very critical background processes.So PMON will terminate the instance and let normal recovery fix the data.

3. PMON registers the instance with the Oracle TNS Listener: When an instance starts up,the PMON process polls for the well known port address 1521,unless directed otherwise,to check whether  or not a listener is up and running.

• What if listener is running but not on port 1521: In this case mechanism is same , except that listener address needs to be explicitly specified by the LOCAL_LISTENER parameter setting.
• What if listener is not started: PMON will periodicall attempts to contact it to register itself. 
• What if listener is started:If the listener is running when database is started ,PMON communicates with the listener and passes to it relevant parameters,such as the service name and load metrics of the instance.