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1
Oracle Architectural
Components
Objectives
• Listing the structures involved in
connecting a user to an Oracle server
• Listing the stages in processing a query
• Listing the stages in processing a DML
statement
• Listing the stages in processing
COMMITS
1-2
The Oracle Server
Oracle server
Server
1-3
Application/
network
server
Users
Connecting to a Database
Client
User
process
User
1-4
Server
process
Server
Oracle server
User Process
• Runs on the client machine
• Is spawned when a tool or an application
is invoked
• Runs the tool or application (SQL*Plus,
Server Manager, Oracle Enterprise
Manager, Developer/2000)
• Includes the User Program Interface (UPI)
• Generates calls to the Oracle server
1-5
Server Process
• Runs on the server machine (host)
• Services a single user process in the
dedicated server configuration
• Uses an exclusive PGA
• Includes the Oracle Program Interface
(OPI)
• Processes calls generated by the client
• Returns results to the client
1-6
Oracle Instance
Instance
SGA
Background processes
An Oracle instance:
• Is a means to access an Oracle
database
• Always opens one and only one
database
1-7
Oracle Database
Control
files
Parameter
file
Password
file
Data files
Database
1-8
Redo log
files
Archived
log files
Other Key Physical Structures
Parameter
file
Password
file
1-9
Database
Archived
log files
Processing a Query
• Parse:
- Search for identical statement
-Check syntax, object names, privileges
- Lock objects used during parse
-Create and store execution plan
• Execute: identify rows selected
• Fetch: return rows to user process
1-10
Processing a Query (cont.)
SELECT *
FROM emp
ORDER BY ename;
Statement
Handle
Handle
User
process
Server
process
Results
OK
OK
Parse
1-11
Execute
Fetch
The Shared Pool
Shared pool
Library
cache
Data
dictionary
cache
• Size defined by SHARED_POOL_SIZE
• Library cache contains statement text,
parsed code, and an execution plan
• Data dictionary cache contains table
and column definitions and privileges
1-12
Database Buffer Cache
• Number of buffers defined by
DB_BLOCK_BUFFERS
• Size of a buffer based on DB_BLOCK_SIZE
• Stores the most recently used blocks
1-13
Program Global Area (PGA)
Server
process
PGA
• Not shared and not writable
• Contains
– Sort area
– Session information
– Cursor state
– Stack space
1-14
Processing a DML Statement
1. If data blocks not in the buffer, server
process reads them into the buffer
2. Server process places locks on rows to
be modified
3. The redo log buffer is modified with the
changed values
4. The data blocks are changed
5. Before image is recorded in the rollback
block
1-15
Processing a DML Statement (cont.)
UPDATE emp
SET sal=sal*1.1
WHERE empno=7369
Server
process
3
Instance
SGA
4
5
Shared pool
Library
cache
Database
buffer
cache
Redo log
buffer
Data
dictionary
cache
2
1
1-16
Data files
Control
files
Database
Redo log
files
Rollback Segment
Old
image
Table
New
image
Rollback
segment
DML
statement
1-17
Redo Log Buffer
• Size defined by LOG_BUFFER
• Records changes made through the
instance
• Used sequentially
• Circular buffer
1-18
Other Instance Processes
• Other required processes
- Database Write (DBW0)
- Log Writer (LGWR)
- Process Monitor (PMON)
- System Monitor (SMON)
- Checkpoint (CKPT)
• Archive process (ARC0) is optional; is
used in production database
1-19
Database Writer (DBWR)
Instance
SGA
Shared pool
Database
buffer
cache
DBW0 writes
when:
• There are many
dirty buffers
• There are few
free buffers
DBWR
• Timeout occurs
• Checkpoint
occurs
Data files
1-20
Control
files
Redo log
files
Log Writer (LGWR)
Instance
SGA
Shared pool
Redo log
buffer
LGWR writes
when:
• There is a commit
LGWR
• The redo log buffer
is one-third full
• There is more than
1MB of redo
• Before DBW0 writes
Data files
1-21
Control
files
Redo log
files
COMMIT Processing
1. Server process places a commit record
and system change number (SCN) in
redo log buffer.
2. LGWR writes redo log to redo log files.
3. User is informed COMMIT is complete
4. Server process records trans. Is
complete and locks can be released.
1-22
COMMIT Processing (cont.)
1
Instance
SGA
Server
process
4
Database
buffer
cache
Shared pool
Redo log
buffer
LGWR
3
2
User
process
Data files
Control
files
Database
1-23
Redo log
files
SMON: System Monitor
• Automatically recovers the instance
-Rolls forward changes in redo logs
-Opens the database for user access
-Rolls back uncommitted transactions
• Coalesces free spaces
• Deallocates temporary segments
1-24
PMON: Process Monitor
Cleans up after failed processes by:
• Rolling back the transaction
• Releasing locks
• Releasing other resources
1-25
Archiving
• Database archive mode
- NOARCHIVELOG for databases that do
not require recovery after disk failure
- ARCHIVELOG mode for production
•ARC0 process
• Automatically archives online redo
log files
• Preserves the record of all changes
made to the database
1-26
Summary
Instance
SGA
Shared pool
Server
process
PGA
User
process
DBWR
Control
files
Parameter
file
Password
file
LGWR
Redo log
files
Data files
Database
1-27
Archived
log files
Summary (cont.)
You should have learned:
• Explain the SGA, Instance, memory
structures
• Explain the database files
• Explain the primary background processes
(DBW0, LGWR, CKPT, PMON, SMON, ARC0)
•Explain SQL processing steps
•Explain the COMMIT process
1-28