Oracle Server Description

What is Oracle Server ?

Oracle server includes an Oracle Database and Oracle Instance.

Oracle Database -

                                    Oracle database includes several different types of files.

                                            1. Data Files

                                            2. Control Files
                                            3. Redo Log Files

                                            4. Archive Log Files

                                            5. Parameter File

                                            6. Password File

                                            7. Alert Log Files

                                            8. Trace Log Files

The Oracle Server also accesses Parameter Files and Password Files. An Oracle database consists of files.

Sometimes there are referred to as operating systems 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 are used ot recover the database in the vent of application program failure, instance failures and other minor failures.
• The archived redo log files are used to recover the database if a disk fails.
• The required parameter file that is used to specify parameters for configuring an oracle instance when it starts up.
• The optional password file authenticates special users of the database - these are termed privileged users and include database administrators.
• 'Alerts' and 'Trace Log Files' - these file store information about errors and actions taken that affect the configuration of the database.

Oracle Instance -

                                    An oracle instance consists of two different sets of components -

                                    1. Background Processes

                                    2. Memory Structures

Background Processes -

                                    SMON, PMON, DBW0, LGWR, CKPT, RECO, D000 and others.

These Processes perform input/output and monitor other oracle processes to provide good performance and database reliability.

Memory Structures -

                                When an instance starts up, a memory structure called the System Global Area (SGA) is allocated.

At this point the background processes also starts. An oracle instance provides access to one and only one oracle 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 variable for a single user process.

Connecting to an Oracle Instance - Creating a session -

Connecting to an oracle instance

        - Establishing a user connection

        - Creating a session

System  User connect to an oracle database through SQLPlus or through an application program like Internet Developer Suite. This connection enables users to execute SQL statements. The User process communicates with the oracle server through a server process. The User process executes on the client computer. The Server Process executes on the server computer, and actually executes SQL Statements submitted by the system user. 
The figure shows a one-to-one correspondence between te user and server process. This is called a 'Dedicated Server' connection, An alternate configuration is to use a 'Shared Server' where more than one user process shares a Server Process.
Sessions - When a User connects to an Oracle server, this termed a session. The 'User Global Area' is session Memory.

Note - If a system users attempts to connect and the Oracle server is not running, the system user receives the "Oracle Not Available" error message.

Thanks

DB Tech Solutions