Oracle Architecture Overview

An Oracle server:
• Is a database management system that provides an open, comprehensive, integrated approach to information management
• Consists of an Oracle Instance and an Oracle database
Oracle Instance
An Oracle Instance:
• Is a means to access an Oracle database
• Always opens one and only one database
• Consists of memory and background process structures
Establishing a Connection and Creating a Session
Connecting to an Oracle Instance:
• Establishing a user connection
• Creating a session
Oracle Database
An Oracle database:
• Is a collection of data that is treated as a unit
• Consists of three file types
Physical Structure
The physical structure includes three types of files:
• Control files
• Datafiles
• Redo log files
Memory Structure
Oracle’s memory structure consists of two memory areas known as:
• System Global Area (SGA): Allocated at instance startup, and is a fundamental component of
an Oracle Instance
• Program Global Area (PGA): Allocated when the server process is started
System Global Area
• The SGA consists of several memory structures:
– Shared Pool
– Database Buffer Cache
– Redo Log Buffer
– Other structures (for example, lock and latch management, statistical data)
• There are two additional memory structures that can be configured within the SGA:
– Large Pool
– Java Pool
• SGA is dynamic
• Sized by the SGA_MAX_SIZE parameter
• Allocated and tracked in granules by SGA components
– Contiguous virtual memory allocation
– Granule size based on total estimated SGA_MAX_SIZE
Shared Pool
• Used to store:
– Most recently executed SQL statements
– Most recently used data definitions
• It consists of two key performance-related memory structures:
– Library Cache
– Data Dictionary Cache
• Sized by the parameter SHARED_POOL_SIZE
ALTER SYSTEM SET
SHARED_POOL_SIZE = 64M;

Library Cache
• Stores information about the most recently used SQL and PL/SQL statements
• Enables the sharing of commonly used statements
• Is managed by a least recently used (LRU) algorithm
• Consists of two structures:
– Shared SQL area
– Shared PL/SQL area
• Size determined by the Shared Pool sizing
Data Dictionary Cache
• A collection of the most recently used definitions in the database
• Includes information about database files, tables, indexes, columns, users, privileges, and other
database objects
• During the parse phase, the server process looks at the data dictionary for information to
resolve object names and validate access
• Caching data dictionary information into memory improves response time on queries and DML
• Size determined by the Shared Pool sizing
Database Buffer Cache
• Stores copies of data blocks that have been retrieved from the datafiles
• Enables great performance gains when you obtain and update data
• Managed through an LRU algorithm
• DB_BLOCK_SIZE determines primary block size

Database Buffer Cache
• Consists of independent sub-caches:
– DB_CACHE_SIZE
– DB_KEEP_CACHE_SIZE
– DB_RECYCLE_CACHE_SIZE
• Can be dynamically resized
ALTER SYSTEM SET DB_CACHE_SIZE = 96M;
• DB_CACHE_ADVICE set to gather statistics for predicting different cache size behavior
• Statistics displayed by V$DB_CACHE_ADVICE
Redo Log Buffer
• Records all changes made to the database data blocks
• Primary purpose is recovery
• Changes recorded within are called redo entries
• Redo entries contain information to reconstruct or redo changes
• Size defined by LOG_BUFFER
Large Pool
• An optional area of memory in the SGA
• Relieves the burden placed on the Shared Pool
• Used for:
– Session memory (UGA) for the Shared Server
– I/O server processes
– Backup and restore operations or RMAN
– Parallel execution message buffers
– PARALLEL_AUTOMATIC_TUNING set to TRUE
• Does not use an LRU list
• Sized by LARGE_POOL_SIZE

Java Pool
• Services parsing requirements for Java commands
• Required if installing and using Java
• Sized by JAVA_POOL_SIZE parameter

Program Global Area
• Memory reserved for each user process connecting to an Oracle database
• Allocated when a process is created
• Deallocated when the process is terminated
• Used by only one process

Process Structure
Oracle takes advantage of various types of processes:
• User process: Started at the time a database user requests connection to the Oracle server
• Server process: Connects to the Oracle Instance and is started when a user establishes a
session
• Background processes: Started when an Oracle Instance is started

User Process
• A program that requests interaction with the Oracle server
• Must first establish a connection
• Does not interact directly with the Oracle server

Server Process
• A program that directly interacts with the Oracle server
• Fulfills calls generated and returns results
• Can be Dedicated or Shared Server

Background Processes
Maintains and enforces relationships between physical and memory structures
• Mandatory background processes:
– DBWn - PMON - CKPT
– LGWR - SMON
• Optional background processes:
– ARCn - LMDn - RECO
– CJQ0 - LMON - Snnn
– Dnnn - Pnnn
– LCKn - QMNn

Database Writer (DBWn)
DBWn writes when:
• Checkpoint occurs
• Dirty buffers reach threshold
• There are no free buffers
• Timeout occurs
• RAC ping request is made
• Tablespace OFFLINE
• Tablespace READ ONLY
• Table DROP or TRUNCATE
• Tablespace BEGIN BACKUP

Log Writer (LGWR)
LGWR writes:
• At commit
• When one-third full
• When there is 1 MB of redo
• Every three seconds
• Before DBWn writes

System Monitor (SMON)
Responsibilities:
• Instance recovery
– Rolls forward changes in redo logs
– Opens database for
user access
– Rolls back uncommitted transactions
• Coalesces free space
• Deallocates temporary segments

Process Monitor (PMON)
Cleans up after failed processes by:
• Rolling back the transaction
• Releasing locks
• Releasing other resources
• Restarting dead dispatchers

Checkpoint (CKPT)
Responsible for:
• Signaling DBWn at checkpoints
• Updating datafile headers with checkpoint information
• Updating control files with checkpoint information

Archiver (ARCn)
• Optional background process
• Automatically archives online redo logs when ARCHIVELOG mode is set
• Preserves the record of all changes made to the database

Logical Structure
• Dictates how the physcal space of a database is used
• Hierarchy consisting of tablespaces, segments, extents, and blocks

Processing SQL Statements
• Connect to an instance using:
– User process
– Server process
• The Oracle server components that are used depend on the type of SQL statement:
– Queries return rows
– DML statements log changes
– Commit ensures transaction recovery
• Some Oracle server components do not participate in SQL statement processing

Hope you might have understand the basics of Oracle architecture.....i will be keep posting about Oracle... wait for the further updates.