Primary Architecture Components
Oracle server: An Oracle server includes an Oracle Instance and an Oracle database.
· An Oracle database includes several different types of files: datafiles, control files, redo log files and archive redo log files. The Oracle server also accesses parameter files and password files.
· This set of files has several purposes.
o One is to enable system users to process SQL statements.
o Another is to improve system performance.
o Still another is to ensure the database can be recovered if there is a software/hardware failure.
· The database server must manage large amounts of data in a multi-user environment.
· The server must manage concurrent access to the same data.
· The server must deliver high performance. This generally means fast response times.
Oracle instance: An Oracle Instance consists of two different sets of components:
· The first component set is the set of background processes (PMON, SMON, RECO, DBW0, LGWR, CKPT, D000 and others).
o These will be covered later in detail – each background process is a computer program.
o These processes perform input/output and monitor other Oracle processes to provide good performance and database reliability.
· The second component set includes the memory structures that comprise the Oracle instance.
o When an instance starts up, a memory structure called the System Global Area (SGA) is allocated.
o At this point the background processes also start.
· An Oracle Instance provides access to one and only one Oracle database.
Oracle database: An Oracle database consists of files.
· Sometimes these are referred to as operating system files, but they are actually database files that store the database information that a firm or organization needs in order to operate.
· The redo log files are used to recover the database in the event of application program failures, instance failures and other minor failures.
· The archived redo log files are used to recover the database if a disk fails.
· Other files not shown in the figure include:
o The required parameter file that is used to specify parameters for configuring an Oracle instance when it starts up.
o The optional password file authenticates special users of the database – these are termed privileged users and include database administrators.
o Alert and Trace Log Files – these files store information about errors and actions taken that affect the configuration of the database.
User and server processes: The processes shown in the figure are called user and server processes. These processes are used to manage the execution of SQL statements.
· A Shared Server Process can share memory and variable processing for multiple user processes.
· A Dedicated Server Process manages memory and variables for a single user process.
This figure from the Oracle Database Administration Guide provides another way of viewing the SGA.
Connecting to an Oracle Instance – Creating a Session
This figure from the Oracle Database Administration Guide provides another way of viewing the SGA.
Connecting to an Oracle Instance – Creating a Session
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