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 like shared disk systems, enables 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. RAC enables access to a single database by users on multiple computers with increased performance.
Instance Memory Structures: The basic memory structures are associated with Oracle: the system global area and the program global area.
System global area: SGA
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 de-allocate it when the instance shuts down. Each instance has its own SGA.
User currently connected to an Oracle database share the data in the SGA. 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: store the most recently used blocks of data. The buffer cache contains modified as well as unmodified blocks.
Redo Log Buffer: 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: 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 handler or name for a private SQL area in which a parsed statement and other information for processing the statement are kept.
Program Global Area(PGA)
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
Oracle creates a set of background process for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle program running for each user process. Each Oracle instance can use several background processes.
Process Architecture
A process is a ‘thread of control’ or a mechanism in an OS that can run a series of steps. 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) Process: user processes are created and maintained to run the software code of an application program. User processes also manage communication with the server process through the program interface.
Oracle process: 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.
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 process. On some systems, the user and server processes are separate, while on others, they are combined into a single process. Client/Server systems separate the user and server processes.