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A database server is the key to information management.

In general, a server reliably manages a large amount of data in a multiuser environment so that users can concurrently access the same data. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.

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Database and Instance

An Oracle database server consists of a database and at least one database instance, commonly referred to as simply an instance. Because an instance and a database are so closely connected, the term Oracle database is sometimes used to refer to both instance and database.

In the strictest sense the terms have the following meanings:

• DatabaseA database is a set of files, located on disk, that store data. These files can exist independently of a database instance.
• Database instanceAn instance is a set of memory structures that manage database files. The instance consists of a shared memory area, called the system global area (SGA), and a set of background processes. An instance can exist independently of database files.

The following figure shows a database and its instance. For each user connection to the instance, a client process runs the application. Each client process is associated with its own server process. The server process has its own private session memory, known as the program global area (PGA).

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Figure 1-1 Oracle Instance and Database
Description of “Figure 1-1 Oracle Instance and Database”

Although in the strict sense an Oracle database is a set of physical structures (files and memory structures), applications can interact with multiple logical databases inside a single physical database, or a single logical database distributed across multiple physical databases.

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This section contains the following topics:

• Multitenant Architecture
• Sharding Architecture

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To connect to Oracle Database as a privileged user over a nonsecure connection, you must be authenticated by a password file.

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When using password file authentication, the database uses a password file to keep track of database user names that have been granted the SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM administrative privilege. This form of authentication is discussed in “Using Password File Authentication”.

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Local Connections and Secure Remote Connections

You can connect to Oracle Database as a privileged user over a local connection or a secure remote connection.

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You can connect in two ways:

• If the database has a password file and you have been granted a system privilege, then you can connect and be authenticated by a password file.
• If the server is not using a password file, or if you have not been granted a system privilege and are therefore not in the password file, then you can use operating system authentication. On most operating systems, authentication for database administrators involves placing the operating system username of the database administrator in a special group.For example, users in the OSDBA group are granted the SYSDBA administrative privilege. Similarly, the OSOPER group is used to grant SYSOPER administrative privilege to users, the OSBACKUPDBA group is used to grant SYSBACKUP administrative privilege to users, the OSDGDBA group is used to grant SYSDG administrative privilege to users, the OSKMDBA group is used to grant SYSKM administrative privilege to users, and the OSRACDBA group is used to grant SYSRAC administrative privilege to users.

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Database administrators can be authenticated with account passwords, operating system (OS) authentication, password files, or strong authentication with a directory-based authentication service, such as Oracle Internet Directory.

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• About Authentication Methods for Database Administrators
  There are several ways to authenticate database administrators.
• Nonsecure Remote Connections
  To connect to Oracle Database as a privileged user over a nonsecure connection, you must be authenticated by a password file.
• Local Connections and Secure Remote Connections
  You can connect to Oracle Database as a privileged user over a local connection or a secure remote connection.

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About Authentication Methods for Database Administrators

There are several ways to authenticate database administrators.

Oracle database can authenticate database administrators through the data dictionary, (using an account password) like other users. Keep in mind that database passwords are case-sensitive. See Oracle Database Security Guide for more information about case-sensitive database passwords.

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In addition to normal data dictionary authentication, the following methods are available for authenticating database administrators with the SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM privilege:

• Operating system (OS) authentication
• Password file including Kerberos and SSL authentication services
• Strong authentication with a directory-based authentication service, such as Oracle Internet Directory

These methods are required to authenticate a database administrator when the database is not started or otherwise unavailable. (They can also be used when the database is available.)

The remainder of this section focuses on operating system authentication and password file authentication. See Oracle Database Security Guide for information about authenticating database administrators with directory-based authentication services.

Your choice is influenced by whether you intend to administer your database locally on the same system where the database resides, or whether you intend to administer many different databases from a single remote client. The following figure illustrates the choices you have for database administrator authentication schemes.

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Administrative privileges that are required for an administrator to perform basic database operations are granted through special system privileges.

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These privileges are:

• SYSDBA
• SYSOPER
• SYSBACKUP
• SYSDG
• SYSKM
• SYSRAC

Excluding the SYSRAC privilege, grant these privileges to users depending upon the level of authorization they require. The SYSRAC privilege cannot be granted to users because it is used only by the Oracle agent of Oracle Clusterware to connect to the database using operating system authentication.

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Starting with Oracle Database 12c Release 1 (12.1), the SYSBACKUP, SYSDG, and SYSKM administrative privileges are available. Starting with Oracle Database 12c Release 2 (12.2), the SYSRAC administrative privilege is available. Each new administrative privilege grants the minimum required privileges to complete tasks in each area of administration. The new administrative privileges enable you to avoid granting SYSDBA administrative privilege for many common tasks.

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As a DBA, you often perform special operations such as shutting down or starting up a database. Because only a DBA should perform these operations, the database administrator user names require a secure authentication scheme.

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• Administrative Privileges
  Administrative privileges that are required for an administrator to perform basic database operations are granted through special system privileges.
• Operations Authorized by Administrative Privileges
  Each administrative privilege authorizes a specific set of operations.

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• Authentication Methods for Database Administrators
  Database administrators can be authenticated with account passwords, operating system (OS) authentication, password files, or strong authentication with a directory-based authentication service, such as Oracle Internet Directory.
• Using Operating System Authentication
  Membership in special operating system groups enables a DBA to authenticate to the database through the operating system rather than with a database user name and password. This is known as operating system authentication.

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• Using Password File Authentication
  You can use password file authentication for an Oracle database instance and for an Oracle Automatic Storage Management (Oracle ASM) instance. The password file for an Oracle database is called a database password file, and the password file for Oracle ASM is called an Oracle ASM password file.

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When you create an Oracle database, the user SYSTEM is also automatically created and granted the DBA role.

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The SYSTEM user name is used to create additional tables and views that display administrative information, and internal tables and views used by various Oracle Database options and tools. Never use the SYSTEM schema to store tables of interest to non-administrative users.

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SYSBACKUP, SYSDG, SYSKM, and SYSRAC

When you create an Oracle database, the following users are automatically created to facilitate separation of duties for database administrators: SYSBACKUP, SYSDG, SYSKM, and SYSRAC.

These users separate duties in the following ways:

• SYSBACKUP facilitates Oracle Recovery Manager (RMAN) backup and recovery operations either from RMAN or SQL*Plus.
• SYSDG facilitates Data Guard operations. The user can perform operations either with Data Guard Broker or with the DGMGRL command-line interface.
• SYSKM facilitates Transparent Data Encryption keystore operations.
• SYSRAC facilitates Oracle Real Application Clusters (Oracle RAC) operations by connecting to the database by the Clusterware agent on behalf of Oracle RAC utilities such as SRVCTL.The SYSRAC administrative privilege cannot be granted to database users and is not supported in a password file. The SYSRAC administrative privilege is used only by the Oracle agent of Oracle Clusterware to connect to the database using operating system authentication.

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Each of these accounts provides a designated user for the new administrative privilege with the same name. Specifically, the SYSBACKUP account provides a designated user for the SYSBACKUP administrative privilege. The SYSDG account provides a designated user for the SYSDG administrative privilege. The SYSKM account provides a designated user for the SYSKM administrative privilege.

Create a user and grant to that user an appropriate administrative privilege to use when performing daily administrative tasks. Doing so enables you to manage each user account separately, and each user account can have a distinct password. Do not use the SYSBACKUP, SYSDG, or SYSKM user account for these purposes. These accounts are locked by default and should remain locked.

To use one of these administrative privileges, a user must exercise the privilege when connecting by specifying AS SYSBACKUP, AS SYSDG, or AS SYSKM. If the authentication succeeds, then the user is connected with a session in which the administrative privilege is enabled. In this case, the session user is the corresponding administrative user account. For example, if user bradmin connects with the AS SYSBACKUP administrative privilege, then the session user is SYSBACKUP.

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Administrative user accounts have special privileges required to administer areas of the database, such as the CREATE ANY TABLE or ALTER SESSION privilege, or EXECUTE privileges on packages owned by the SYS schema.

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The following administrative user accounts are automatically created when Oracle Database is installed:

• SYS
• SYSTEM
• SYSBACKUP
• SYSDG
• SYSKM
• SYSRAC

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\SYS

When you create an Oracle database, the user SYS is automatically created with all the privileges.

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All of the base tables and views for the database data dictionary are stored in the schema SYS. These base tables and views are critical for the operation of Oracle Database. To maintain the integrity of the data dictionary, tables in the SYS schema are manipulated only by the database. They should never be modified by any user or database administrator, and no one should create any tables in the schema of user SYS. (However, you can change the storage parameters of the data dictionary settings if necessary.)

Ensure that most database users are never able to connect to Oracle Database using the SYS account.

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In Oracle Database, each user must see a consistent view of the data, including visible changes made by a user’s own transactions and committed transactions of other users.

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For example, the database must prevent the lost update problem, which occurs when one transaction sees uncommitted changes made by another concurrent transaction.

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Oracle Database always enforces statement-level read consistency, which guarantees that the data that a single query returns is committed and consistent for a single point in time. Depending on the transaction isolation level, this point is the time at which the statement was opened or the time the transaction began. The Oracle Flashback Query feature enables you to specify this point in time explicitly.

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The database can also provide read consistency to all queries in a transaction, known as transaction-level read consistency. In this case, each statement in a transaction sees data from the same point in time, which is the time at which the transaction began.

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To perform many of the administrative duties for a database, you must be able to execute operating system commands.

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Depending on the operating system on which Oracle Database is running, you might need an operating system account or ID to gain access to the operating system. If so, your operating system account might require operating system privileges or access rights that other database users do not require (for example, to perform Oracle Database software installation). Although you do not need the Oracle Database files to be stored in your account, you should have access to them.

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Administrative User Accounts

Oracle Database provides several administrative user accounts that are associated with administrative privileges.

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• About Administrative User Accounts
  Administrative user accounts have special privileges required to administer areas of the database, such as the CREATE ANY TABLE or ALTER SESSION privilege, or EXECUTE privileges on packages owned by the SYS schema.
• SYS
  When you create an Oracle database, the user SYS is automatically created with all the privileges.
• SYSTEM
  When you create an Oracle database, the user SYSTEM is also automatically created and granted the DBA role.
• SYSBACKUP, SYSDG, SYSKM, and SYSRAC
  When you create an Oracle database, the following users are automatically created to facilitate separation of duties for database administrators: SYSBACKUP, SYSDG, SYSKM, and SYSRAC.
• The DBA Role
  A predefined DBA role is automatically created with every Oracle Database installation. This role contains most database system privileges. Therefore, the DBA role should be granted only to actual database administrators.

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A requirement of a multiuser RDBMS is the control of data concurrency, which is the simultaneous access of the same data by multiple users.

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Without concurrency controls, users could change data improperly, compromising data integrity. For example, one user could update a row while a different user simultaneously updates it.

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If multiple users access the same data, then one way of managing concurrency is to make users wait. However, the goal of a DBMS is to reduce wait time so it is either nonexistent or negligible. All SQL statements that modify data must proceed with as little interference as possible. Destructive interactions, which are interactions that incorrectly update data or alter underlying data structures, must be avoided.

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Oracle Database uses locks to control concurrent access to data. A lock is a mechanism that prevents destructive interaction between transactions accessing a shared resource. Locks help ensure data integrity while allowing maximum concurrent access to data.

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