Oracle Processes

Oracle background process (Photo credit: Wikipedia)

Server Processes

Oracle creates server processes to handle the requests of user processes connected to the instance. In some situations when the application and Oracle operate on the same machine, 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 operate on different machines, a user process always communicates with Oracle through a separate server process.

Server processes (or the server portion of combined user/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 datafiles 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 system uses some additional Oracle processes called background processes.

An Oracle instance can have many background processes; not all are always present. The background processes in an Oracle instance include the following:

• Database Writer (DBW0 or DBWn)
• Log Writer (LGWR)
• Checkpoint (CKPT)
• System Monitor (SMON)
• Process Monitor (PMON)
• Archiver (ARCn)
• Recoverer (RECO)
• Lock Manager Server (LMS) – Real Application Clusters only
• Queue Monitor (QMNn)
• Dispatcher (Dnnn)
• Server (Snnn)

On many operating systems, background processes are created automatically during instance startup.