An Oracle instance has three broad classes of processes:
Server processes: These perform work based on a client’s request. We have already looked at dedicated and shared servers to some degree. These are the server processes.
a. Dedicated server, whereby you get a dedicated process on the server for your connection. There is a one-to-one mapping between a connection to the database and a server process or thread
b. Shared server, whereby many sessions share a pool of server processes spawned and managed by the Oracle instance. Your connection is to a database dispatcher, not to a dedicated server process created just for your connection.
Background processes: These are the processes that start up with the database and perform various maintenance tasks, such as writing blocks to disk, maintaining the online redo log, cleaning up aborted processes, and so on.
Slave processes: These are similar to background processes, but they are processes that perform extra work on behalf of either a background or a server process.
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