Purpose and Function of the Oracle Process Monitor (PMON)

The PMON (Process Monitor) process in Oracle databases plays a crucial role in maintaining the health, stability, and efficiency of the Oracle database environment. As an integral component of the Oracle database's background processes, PMON is designed to perform several critical tasks that ensure the smooth operation of the database system. Firstly, PMON is responsible for performing process recovery when a user or a background process terminates abnormally. In the event of an unexpected failure, such as a user's session disconnecting abruptly due to network issues or a background process crashing, PMON steps in to clean up the resources that were allocated to the failed process. This cleanup includes freeing up memory, rolling back uncommitted transactions, and releasing locks held by the terminated process, thereby maintaining the integrity of the database and preventing potential data corruption or deadlocks.
Furthermore, PMON also plays a pivotal role in managing Oracle's dynamic service registration with the Oracle Listener. It periodically checks the status of the listener and registers information about the database instance and services with the listener. This automatic registration ensures that clients can discover and connect to available services without manual configuration, facilitating easier administration and connectivity. In addition to these primary functions, PMON is involved in monitoring the dispatchers and shared servers in an Oracle database configured for the shared server mode. It ensures that these components are functioning correctly and restarts them if they fail, thereby supporting Oracle's scalability features and ensuring that the database can efficiently handle multiple concurrent user connections.
PMON's activities are not limited to error recovery and process monitoring; it also contributes to the overall performance of the database by helping to clean up idle sessions. By terminating sessions that have been inactive beyond a certain threshold, PMON helps to free up system resources, thus optimizing the database's performance and resource utilization. In summary, the PMON process in Oracle databases is a fundamental background process that ensures the database operates smoothly by performing process recovery, managing service registrations, monitoring shared components, and cleaning up idle sessions. Its functions are critical to the database's resilience, scalability, and efficiency, making PMON an indispensable component of the Oracle database architecture.

The Process Monitor (PMON) process does the same sort of thing for user processes that SMON does for the instance--it takes care of various cleanup operations. Whenever a user process is abnormally terminated, PMON does the following:

1. Releases any locks that may have been held by the user process
2. Removes the process ID from the list of active processes
3. Rolls back any open transactions for the process

If the multi-threaded server option is being used, PMON will also periodically check to be sure that the neccessary dispatcher processes are running.

The process monitor (PMON) performs process recovery when a user process fails. PMON is responsible for

1. cleaning up the database buffer cache and
2. freeing resources that the user process was using.

For example, it resets the status of the active transaction table, releases locks, and removes the process ID from the list of active processes. PMON periodically checks the status of dispatcher and server processes, and restarts any that have stopped running (but not any that Oracle has terminated intentionally). PMON also registers information about the instance and dispatcher processes with the network listener.
If a user process terminates abnormally, PMON is responsible for cleaning up any of the resources left behind (such as memory) and for releasing any locks held by the failed process.
Like SMON, PMON checks regularly to see whether it is needed and can be called if another process detects the need for it.

Click the Quiz link below to take a short quiz and review what you have learned about PMON, SMON, and the SGA.
Pmon Smon Sga -Quiz