This document provides an overview of the Oracle database architecture. It describes the major components of Oracle's architecture, including the memory structures like the system global area and program global area, background processes, and the logical and physical storage structures. The key components are the database buffer cache, redo log buffer, shared pool, processes, tablespaces, data files, and redo log files.

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Exploring the Oracle Database Architecture

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Objectives
After completing this lesson, you should be able to:
• List the major architectural components of Oracle
Database
• Explain the memory structures
• Describe the background processes
• Correlate the logical and physical storage structures

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Oracle Database
The Oracle relational database management system
(RDBMS) provides an open, comprehensive, integrated
approach to information management

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Connecting to a Server
Client Middle tier Server
Multitier architecture shown

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Database
Data files
Online redo
log files
Control
files
Oracle Database Architecture: Overview
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
PMONSMON Others
Server
process
PGA
Archived
log files
User
process
Instance
RECO
ARCn
SGA
DBWn
Redo log
buffer
LGWRCKPT

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Connecting to the Database
• Connection: Communication between a user process
and an instance
• Session: Specific connection of a user to an instance
through a user process
SQL> Select …
Session
Connection
User
User
process
Server
process
Session

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Interacting with an Oracle Database
User
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
PMONSMON Others
Instance
RECOARCn
SGA
DBWn
Redo log
buffer
LGWRCKPT
User
process
Server
process

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Database
Oracle Database Server Structures
PMONSMON Others
Data files
Online redo
log files
Control
files
Instance
RECOARCnDBWn LGWRCKPT
Storage structures
User
process
Server
process
Memory structures
Processes
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Oracle Database
Memory Structures
DB structures
Memory
- Process
- Storage
SGA
Database buffer
cache
Redo log
buffer
Java
pool
Streams
pool
Shared pool
Large pool
PGA PGA PGA
Background
process
Server
process 1
Server
process 2
Shared
SQL area
Library
cache
Data dictionary
cache
Other
I/O buffer
Response
queue
Request
queue
Free
memory

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Database Buffer Cache
• Is part of the SGA
• Holds copies of data blocks that are read from data files
• Is shared by all concurrent users
PMONSMON Others
Instance
RECOARCnDBWn LGWRCKPT
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Redo Log Buffer
• Is a circular buffer in the SGA
• Holds information about changes made to the database
• Contains redo entries that have the information to redo
changes made by operations such as DML and DDL
PMONSMON Others
Instance
RECOARCnDBWn LGWRCKPT
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Shared Pool
• Is a portion of the SGA
• Contains:
– Library cache
— Shared SQL area
– Data dictionary cache
– Control structures
PMONSMON Others
Instance
RECOARCnDBWn LGWRCKPT
Shared
SQL area
Library
cache
Data dictionary
cache
Other
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Allocation and Reuse of Memory
in the Shared Pool
• Server process checks the shared pool to see if a
shared SQL area already exists for an identical
statement.
• Server process allocates a private SQL area on behalf of
the session.
Server
process
Shared
pool
Shared
SQL area
Library
cache
Data dictionary
cache
Other

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Large Pool
Provides large memory allocations for:
• Session memory for the shared server and the Oracle
XA interface
• I/O server processes
• Oracle Database backup and restore operations
Large pool
I/O buffer
Response
queue
Request
queue
Free
memory
Java
pool
Large pool
Shared pool
Database
buffer
cache
Redo log
buffer
Streams
pool

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Java Pool
and Streams Pool
• Java pool memory is used in server memory for all
session-specific Java code and data in the JVM.
• Streams pool memory is used exclusively by Oracle
Streams to:
– Store buffered queue messages
– Provide memory for Oracle Streams processes
Java pool Streams pool

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Process Architecture
• User process
– Is started when a database user or a batch process
connects to Oracle Database
• Database processes
– Server process: Connects to the Oracle instance and is
started when a user establishes a session
– Background processes: Are started when an Oracle
instance is started
DB structures
- Memory
 Process
- Storage
PMONSMON Others
Instance
RECOARCnDBWn LGWRCKPT
PGA
User
process
Server
process Background processes
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Process Structures
Server n
processes
SGA
Oracle
background
processes
PMON SMON OthersRECO ARCnDBWn LGWRCKPT
Server ServerServerServer Server
Database
buffer
cache
Shared pool
Data dictionary
cache
Library
cache
SGA
Redo log
buffer

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Database Writer Process (DBWn)
Writes modified (dirty) buffers in the database buffer cache
to disk:
• Asynchronously while performing other processing
• Periodically to advance the checkpoint
Database buffer
cache
Database writer
process
Data files
DBWn

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LogWriter Process (LGWR)
• Writes the redo log buffer to a redo log file on disk
• Writes:
– When a user process commits a transaction
– When the redo log buffer is one-third full
– Before a DBWn process writes modified buffers to disk
Redo log buffer LogWriter process Redo log files
LGWR

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Checkpoint Process (CKPT)
• Records checkpoint information in
– Control file
– Each data file header
Checkpoint
process
Data files
Control fileCKPT

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System Monitor Process (SMON)
• Performs recovery at instance startup
• Cleans up unused temporary segments
Instance
Temporary
segment
System Monitor
process
SMON

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Process Monitor Process (PMON)
• Performs process recovery when a user process fails
– Cleans up the database buffer cache
– Frees resources that are used by the user process
• Monitors sessions for idle session timeout
• Dynamically registers database services with listeners
Process Monitor
process
Database buffer
cache
Failed user
process
UserPMON

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Recoverer Process
• Used with the distributed database configuration
• Automatically connects to other databases involved in
in-doubt distributed transactions
• Automatically resolves all in-doubt transactions
• Removes any rows that correspond to in-doubt
transactions
Recoverer process
in database A
In-doubt transaction
in database B
RECO

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Archiver Processes (ARCn)
• Copy redo log files to a designated storage device after
a log switch has occurred
• Can collect transaction redo data and transmit that data
to standby destinations
Archiver process Archive destinationCopies of redo log
files
ARCn

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Other Processes
• MMON: Performs manageability-related background
tasks
• MMNL: Performs frequent and lightweight
manageability-related tasks
• MMAN: Performs automatic memory management tasks
• CJQ0: Runs user jobs used in batch processing
• QMNx: Monitors the Streams Advanced Queuing
message queues

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Server Process and Database
Buffer Cache
Buffers:
• Pinned
• Clean
• Free or unused
• Dirty
Database
writer
process
Database
buffer
cache
SGA
Data files
DBWn
Server
process

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Database Storage Architecture
Online redo log files
Password file
Parameter file Archived redo log
files
Control files Data files
Alert log and trace files
Backup files
DB structures
- Memory
- Process
 Storage

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Logical and Physical Database Structures
Database
Logical Physical
Tablespace Data file
OS block
Segment
Extent
Oracle data
block
Schema

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Tablespaces and Data Files
• Tablespaces consist of one or more data files.
• Data files belong to only one tablespace.
USERS tablespace
Data file 1 Data file 2

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SYSTEM and SYSAUX Tablespaces
• The SYSTEM and SYSAUX tablespaces are mandatory
tablespaces that are created at the time of database
creation. They must be online.
• The SYSTEM tablespace is used for core functionality
(for example, data dictionary tables).
• The auxiliary SYSAUX tablespace is used for additional
database components (such as the Enterprise Manager
Repository).

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Segments, Extents, and Blocks
• Segments exist in a tablespace.
• Segments are collections of extents.
• Extents are collections of data blocks.
• Data blocks are mapped to disk blocks.
Segment Extents Data
blocks
Disk
blocks

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Database Architecture:
Summary of Structural Components
• Memory structures:
– System Global Area (SGA): Database buffer cache, redo
buffer, and various pools
– Program Global Area (PGA)
• Process structures:
– User process and server process
– Background processes: SMON, PMON, DBWn, CKPT,
LGWR, ARCn, and so on
• Storage structures:
– Logical: Database, schema, tablespace, segment, extent,
and Oracle block
– Physical: Data files, control files, and redo log files

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Summary
In this lesson, you should have learned how to:
• List the major architectural components of Oracle
Database
• Explain the memory structures
• Describe the background processes
• Correlate the logical and physical storage structures

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Practice 1: Overview
This is a paper practice with questions about:
• Database architecture
• Memory
• Processes
• File structures