1. Basic Oracle Architecture
Presented to HIOUG
21-Jun-2005
By Byron Bush

2. Workshop Objectives
Identify high-level architectural components of an Oracle Database
Describe the function of each of these components
Identify Oracle Database internal data structures
Define the mechanisms used when processing Insert/Update/Delete statements

3. Most people call it a “Database”
Technically an Oracle Database is broken into two high-level components
INSTANCE – Non-persistent, memory-based processes and structures
DATABASE – Persistent, disk-based data and control files

4. So Why Differentiate?
Most Oracle installations consist of only a single “Instance” and single “Database”…
Instance
Database

5. Oracle RAC is Different
Oracle “Real Application Clusters” allow multiple “Instances” to interact with a single “Database” to provide high availability…
Instance 1
Database
Instance 3
Instance 2

6. “Instance” a Little Closer
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

7. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

8. “Database” a Little Closer
An Oracle Database…
Is a collection of data that is treated as a unit
Consists of three file types

9. Picture of a “Database”
Oracle Database
Data Files
Control Files
Redo Log Files
Parameter File
Archived Log Files
Password File

10. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

11. Memory Structure
Oracle’s memory structure consists of two memory areas known as:
System Global Area (SGA): Allocated at instance start up, and is a fundamental component of an Oracle Instance
Program Global Area (PGA): Allocated when the server process is started

12. System Global Area The SGA consists of several memory structures:
Shared Pool
Database Buffer Cache
Redo Log Buffer
Other structures
There are two additional memory structures that can be configured within the SGA:
Large Pool
Java Pool

13. System Global Area
The size of the SGA is determined by the parameters that set the sizes of the various pools; these parameters are dynamic
The SGA_MAX_SIZE parameter sets the maximum size of the SGA (so you can limit it) and is not a dynamic parameter
Thanks Ned!

14. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

15. 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

16. 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 is determined by the Shared Pool sizing

17. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

18. 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

19. Database Buffer Cache
Stores copies of data blocks that have been retrieved from the data files
Enables great performance gains when you obtain and update data
Managed through an LRU algorithm
DB_BLOCK_SIZE determines primary block size

20. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

21. 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

22. Large Pool An optional area of memory in the SGA
Relieves the burden place on the Shared Pool
Used for:
Session memory for the Shared Server
I/O server processes
Backup and restore operations for RMAN
Does not use an LRU list
Sized by LARGE_POOL_SIZE
Can be dynamically resized

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

24. Oracle “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

25. User Process
A program that requests interaction with the Oracle server
Must first establish a connection
Does not interact directly with the Oracle server
User Process
Server Process
Connection Established

26. Server Process
A program that directly interacts with the Oracle server
Fulfills calls generated and returns results
Can be dedicated or shared server

27. Picture of an “Instance”
Library Cache
Data Dictionary Cache
Shared Pool
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Memory Structures
Back- Ground Processes

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

29. Database Writer (DBWn)
Instance
Shared Pool
Library Cache
Data Dictionary Cache
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
DBWn writes when:
Checkpoint occurs
Dirty buffers reach threshold
There are no free buffers
Timeout occurs
Tablespace OFFLINE
Tablespace READ ONLY
Table DROP or TRUNCATE
Tablespace BEGIN BACKUP
Oracle Database
Data Files
Control Files
Redo Log Files

30. Log Writer (LGWR) LGWR writes: At commit When one-third full
Instance
Shared Pool
Library Cache
Data Dictionary Cache
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
LGWR writes:
At commit
When one-third full
When there is 1 MB of redo
Every three seconds
Before DBWn writes
Oracle Database
Data Files
Control Files
Redo Log Files

31. System Monitor (SMON) Responsibilities: Instance recovery
Shared Pool
Library Cache
Data Dictionary Cache
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Responsibilities:
Instance recovery
Rolls forward changes in online redo log files
Opens database for user access
Rolls back uncommitted transactions
Coalesces free space
Deallocates temporary segments
Oracle Database
Data Files
Control Files
Redo Log Files

32. Process Monitor (PMON)
Instance
Shared Pool
Library Cache
Data Dictionary Cache
SGA
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Cleans up after failed processes by:
Rolling back the transaction
Releasing locks
Releasing other resources
Restarting dead dispactchers
Database Buffer Cache
Oracle Database
Data Files
Control Files
Redo Log Files

33. Checkpoint (CKPT) Responsible for: Signaling DBWn at checkpoints
Instance
Shared Pool
Library Cache
Data Dictionary Cache
SGA
Database Buffer Cache
Redo Log Buffer
Java Pool
Large Pool
PMON
SMON
DBWR
LGWR
CKPT
Others
Responsible for:
Signaling DBWn at checkpoints
Updating datafile headers with checkpoint information
Updating control files with checkpoint information
Shared Pool
Oracle Database
Data Files
Control Files
Redo Log Files

34. Archiver (ARCn) Optional background process
Automatically archives online redo log files when ARCHIVELOG mode is set
Preserves the record of all changes made to the database
Oracle Database
Data Files
Control Files
Redo Log Files
Archived Logs
ARCn

35. Oracle Data Storage
Oracle keeps all system and user data in two basic storage containers:
Tablespace… a logical entity known only to Oracle
Data Files… physical files that may (typically) be seen from the operating system

36. Tablespaces Can belong to only one database at a time
Consist of one or more data files
Are further divided into logical units of store

37. Data Files Can belong to only one tablespace and one database
Are a repository for schema object data
Database
Tablespace
Data File
Data File

38. 2 Types of Tablespace
Tablespaces required by Oracle for normal operations
Tablespaces that contain data and indexes that support your applications

39. Required Tablespaces System – Contains the Oracle Data Dictionary
Sysaux - New to 10g; supports historic monitoring / tuning
Temporary – Used for disk-based sorting of data (select...from…order by)
Undo – Used for transaction consistency during Insert / Update / Delete statements

40. SQL Statements (IUDS) Insert Update Delete Select
What happens when someone connects to the database and issues one of these statements?

41. Connect to Oracle – What Happens
Server Process
User Process
Connect User/Pwd
Are “User” definition and privileges in SGA “Dictionary Cache”?
If not, try to fetch from the “System” tablespace
If there, validate Pwd and privileges to login
Allow connection if all is right; otherwise decline connection

42. “Select” – What Happens First?
From MyTab
Order by 1;
Is this statement in the “Library Cache”?
If statement cached then it has been recently been “parsed” and “executed”
If not cached then “parse” the statement
If cached then skip parsing stage and execute statement

43. “Select” – Parsing the Statement
From MyTab
Order by 1;
Does “MyTab” exist?
Does user have Privs to select from “MyTab”
What columns are in “MyTab”
What is the first column in “MyTab”

44. “Select” – Executing the Statement
From MyTab
Order by 1;
Is “MyTab” data in the SGA “Buffer Cache”?
If not, fetch data into Buffer Cache
Sort data in “MyTab” by the first column
If can sort records in memory then do so
If cannot sort in memory then use “Temporary” tablespace as disk-based staging area
Return records to client process

45. “I/U/D” – What Happens First?
Insert (a,b,c)
Into MyTab;
Is this statement in the “Library Cache”?
If statement cached then it has been recently been “parsed” and “executed”
If not cached then “parse” the statement
If cached then skip parsing stage and execute statement

46. “I/U/D” – Executing the Statement
Insert (a,b,c)
Into MyTab;
Validate values to be inserted
Execute the statement
Keep DB version of the record in “Undo” tablespace until Commit or Rollback
Record changes in SGA “Redo Log Buffer”
Change records in SGA “Buffer Cache”
DBWn writes changed records to data file(s) as part of buffer cache management

47. “I/U/D” – Executing the Statement
Insert (a,b,c)
Into MyTab;
Commit; or
Rollback;
If “Commit” then…
LGWn writes “Redo Log Buffer” entries to “Redo Logs”
Undo entries are invalidated
If “Rollback” then…
Migrate DB version of record in Undo back to tablespace/data file

48. Archivelog Mode
If Instance in “Archivelog” mode Redo Log Files are “Archived” to Archive Logs; which may be used to recover your database in the case of disaster
Oracle Database
Data Files
Control Files
Redo Log Files
Archived Logs
ARCn

49. Summary
What we called a “Database” is really an “Instance” and a “Database.”
An Oracle “Instance” is not persistent. It consists of the System Global Area (SGA) and a collection of processes.
An Oracle “Database” is persistent. It consists of a collection of “Required” and “User” tablespaces (with corresponding data files) along with other supporting files.
Nearly every component of the “Instance” and “Database” are mobilized to execute SQL statements.