Components of an instance

An Oracle instance

Background Processes for an Instance

Database Writer (DBWR)

Log Writer (LGWR)

System Monitor (SMON)

Process Monitor (PMON)

Archiver (ARC)

Checkpoint (CKPT)

Recover (RECO)

Processes or Threads?

With all this talk about processes, you may be wondering whether Oracle actually uses threads or processes in the underlying operating system to implement these services.

For simplicity, throughout this book we use the term process generically to indicate a function that Oracle performs, such as DBWR or LGWR. Oracle on Windows uses one operating system process per instance; thus each "Oracle process" is actually a thread within the one Oracle process. Oracle on Unix uses a process-based architecture. All of the "processes" are actual operating system processes, not threads. Thus, on Unix DBWR, LGWR, and so on are specific operating system processes, while on Windows they are threads within a single process.

There are some exceptions, however. For instance, the shared server uses real threads on Windows and simulated threads on Unix.