Overview of the Oracle Instance

An Oracle database server consists of an Oracle database and an Oracle instance. Every time a database is started, a system global area (SGA) is allocated and Oracle background processes are started. The combination of the background processes and memory buffers is called an Oracle instance.

Real Application Clusters: Multiple Instance Systems

Some hardware architectures (for example, shared disk systems) enable multiple computers to share access to data, software, or peripheral devices. Real Application Clusters (RAC) takes advantage of such architecture by running multiple instances that share a single physical database. In most applications, RAC enables access to a single database by users on multiple computers with increased performance.
An Oracle database server uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.

Instance Memory Structures

Oracle creates and uses memory structures to complete several jobs. For example, memory stores program code being run and data shared among users. Two basic memory structures are associated with Oracle: the system global area and the program global area. The following subsections explain each in detail.

System Global Area

The System Global Area (SGA) is a shared memory region that contains data and control information for one Oracle instance. Oracle allocates the SGA when an instance starts and deallocates it when the instance shuts down. Each instance has its own SGA.
Users currently connected to an Oracle database share the data in the SGA. For optimal performance, the entire SGA should be as large as possible (while still fitting in real memory) to store as much data in memory as possible and to minimize disk I/O.
The information stored in the SGA is divided into several types of memory structures, including the database buffers, redo log buffer, and the shared pool.

Database Buffer Cache of the SGA

Database buffers store the most recently used blocks of data. The set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks. Because the most recently (and often, the most frequently) used data is kept in memory, less disk I/O is necessary, and performance is improved.

Redo Log Buffer of the SGA

The redo log buffer stores redo entries—a log of changes made to the database. The redo entries stored in the redo log buffers are written to an online redo log, which is used if database recovery is necessary. The size of the redo log is static.

Shared Pool of the SGA

The shared pool contains shared memory constructs, such as shared SQL areas. A shared SQL area is required to process every unique SQL statement submitted to a database. A shared SQL area contains information such as the parse tree and execution plan for the corresponding statement. A single shared SQL area is used by multiple applications that issue the same statement, leaving more shared memory for other uses.

Statement Handles or Cursors

A cursor is a handle or name for a private SQL area in which a parsed statement and other information for processing the statement are kept. (Oracle Call Interface, OCI, refers to these as statement handles.) Although most Oracle users rely on automatic cursor handling of Oracle utilities, the programmatic interfaces offer application designers more control over cursors.
For example, in precompiler application development, a cursor is a named resource available to a program and can be used specifically to parse SQL statements embedded within the application. Application developers can code an application so it controls the phases of SQL statement execution and thus improves application performance.

Program Global Area

The Program Global Area (PGA) is a memory buffer that contains data and control information for a server process. A PGA is created by Oracle when a server process is started. The information in a PGA depends on the Oracle configuration.

Oracle Background Processes

An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.
The architectural features discussed in this section enable the Oracle database to support:

• Many users concurrently accessing a single database
• The high performance required by concurrent multiuser, multiapplication database systems

Oracle creates a set of background processes for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. They asynchronously perform I/O and monitor other Oracle process to provide increased parallelism for better performance and reliability.
There are numerous background processes, and each Oracle instance can use several background processes.

Process Architecture

A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job or task. A process generally has its own private memory area in which it runs.
An Oracle database server has two general types of processes: user processes and Oracle processes.

User (Client) Processes

User processes are created and maintained to run the software code of an application program (such as an OCI or OCCI program) or an Oracle tool (such as Enterprise Manager). User processes also manage communication with the server process through the program interface, which is described in a later section.

Oracle Processes

Oracle processes are invoked by other processes to perform functions on behalf of the invoking process.
Oracle creates server processes to handle requests from connected user processes. A server process communicates with the user process and interacts with Oracle to carry out requests from the associated user process. For example, if a user queries some data not already in the database buffers of the SGA, then the associated server process reads the proper data blocks from the datafiles into the SGA.
Oracle can be configured to vary the number of user processes for each server process. In a dedicated server configuration, a server process handles requests for a single user process. A shared server configuration lets many user processes share a small number of server processes, minimizing the number of server processes and maximizing the use of available system resources.
On some systems, the user and server processes are separate, while on others they are combined into a single process. If a system uses the shared server or if the user and server processes run on different computers, then the user and server processes must be separate. Client/server systems separate the user and server processes and run them on different computers.