Database: Oracle Server Architecture (overview)

Oracle server consists of the following core components:

    1) database(s) &

    2) instance(s)



1) database consists of:

    1) datafiles (.dbf files) <- information (data) will be stored in flat files on the disk

    2) redo logs (.rdo & .arc files)

            Redo Logs hold recordings of every change made to datafiles; useful for fine-grained 

            recovery of specific changes. Oracle uses several of these files so that when it gets done 

            writing to the last file in the series, it begins overwriting to the first online redo log file. A

            set of redo log files is called the redo log

    3) control files (.ctl files)

            contain parameters (to configure how the database starts), and password files (to

            manage authentication)



2)  Instances provide the capability of accessing stored data. Each instance consists of:



    1) Program Global Area (PGA)

            When single-threaded servers are used, the Program Global Area (PGA) contains data

            (such as user’s session variables and arrays) for use only by a single process. Its

            contents is not visible to other processes

            eg., parse information from queries, database cursors



    2) System Global Area (SGA)

            The Shared System Global Area (SGA) contains user data and control information for a

            single oracle instance. Its contents are visible/accessible to several user applications

            and system processes



    3)  system processes and user applications

    4) db buffer cache, shared pool, redo log buffer etc.,



Datafiles:

    user data, system data, index, configuration & logs



Database structure

    logical & physical structures



logical:

    tables, tablespaces, segments, extents & oracle blocks



physical:

    raw files on disk



shared pool:

    contains data dictionary and SQL statements submitted for processing by users. None of the user data will be stored here