Oracle System Architecture

7/29/2019 Oracle System Architecture

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Neenu Prasad


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Oracle Database

A database is collection of data files plus programs that manipulate

those data files

Two types of information are stored in an Oracle database

User data, relevant to a particular application

System data, that the database needs to manage itself


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Oracle server

An Oracle server consists of :

1. Oracle databasethe collection of stored data, including log

and control files

2. Oracle Instancethe processes, including Oracle (system)

processes and user processes taken together, created for aspecific instance of the database operation.

Oracle server supports SQL to define and manipulate data,

PL/SQLto control the flow of SQL, to use variables, and to

provide error-handling procedures.

Oracle can also be accessed through general purposeprogramming languages such as C or JAVA.


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Oracle database

Components Database files contain all database data

Control files contain info to access and manipulate thedata

Redo Logs

record all the transactions on the database

Tablespaces logical divisions of a database

Segments set of database blocks

Extents

allocation of contiguous space on disk exp inbytes.


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Oracle Database Structure

Database files

An Oracle database consists of one or more database files.

These files contain data that can be accessed with the help of SQL.

A file can be associated with one and only one tablespace.

The database when created initially, has only one physical file.

The maximum number of files that can be opened at a time are 32by

default.

One or more physical files form a logical unit called tablespace.


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Cont.

Control files

A databases overall physical architecture is maintained by its control

files.

These record control information about all the files within the database.

A control file is associated with a single database

Control files are automatically modified by Oracle; a user cannot edit

them.

They are used to maintain internal consistency and guide recovery

operations


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Cont..

Redo Log files

A Redo Log contains all the transactions that have occurred againstthe database.

No activity in the database can take place without being recorded in Redo Logs.

The Redo Logs are necessary to protect against data loss.

Redo Logs are also called transaction logs.

Every database requires a minimum of two Redo Logs.

These are used in recovery operations to restore lost or damaged files.


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TABLESPACES

Tablespaces

A database is divided into logical divisions called tablespaces

A database may have one or more tablespaces

Each logical tablespace corresponds to one or more physical database files.

The typical tablespace present in an Oracle database are :


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Cont..

SYSTEMstores all information needed to manage itself

TEMPstores all temporary tables

TOOLSstores database objects needed to support different tools

USERinformation about users is stored

DATA & INDEXholds the actual data and the indexes

ROLLBACKwhere all undo information is stored


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Segments & Extents

Segments & Extents

All data in tablespace are stored in allocations of database space called segments.

For each table there is a table segment. For indexes so-called index segments are

allocated. The segment associated with a database object belongs to exactly one

tablespace.

An Oracle database requires upto five types of segments, viz: Data segments, Index

segments, Rollback segments, Temporary segments, Bootstrap segments

An extent is an allocation of contiguous database space expressed in bytes


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Oracle Instance

the set of processes that constitute an instance of

the servers operation is called an Oracle Instance

Consists of :

System Global Area

set of background processes.


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System Global Area(SGA)

Shared memory area

Oracle assigns an SGA area when an instance starts

SGA is divided into 3

1. Database buffer cache: This keeps the most recently

accessed data blocks from the database.

2. Redo log buffer, which is the buffer for the redo log file and

is used for recovery purposes.

3. Shared pool, which contains shared memory constructs

Eg: shared SQL areas,


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User processes

Each user process corresponds to the execution of some

application (for example, an Oracle Forms application) or

some tool


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Program global area (PGA)

This 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.


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Oracle Processes

Oracle creates server processes to handle requests from

connected user processes

The background processes are created for each instance

of Oracle; they perform I/O asynchronously and provide

parallelism for better performance and reliability.


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background processes

Database Writer (DBWR): Writes the modified blocks from

the buffer cache to the data files on disk.

Log writer (LGWR): Writes from the log buffer area to the on-

line disk log file.

Checkpoint (CKPT): Refers to an event at which all modified

buffers in the SGA since the last checkpoint are written to the

data files

System monitor (SMON): Performs instance recovery,

manages storage areas by making the space contiguous, and

recovers transactions skipped during recovery.


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Cont..

Process monitor (PMON): Performs process recovery

when a user process fails. It is also responsible for managing

the cache and other resources used by a user process.

Archiver (ARCH):Archives on-line log files to archival

storage (for example, tape) if configured to do so.

Recoverer process (RECO): Resolves distributed

transactions that are pending due to a network or systems

failure in a distributed database


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Dispatchers (Dnnn): In multithreaded server configurations,

route requests from connected user processes to available

shared server processes. There is one dispatcher per

standard communication protocol supported .

Lock processes (LCKn): Used for inter-instance locking

when Oracle runs in a parallel server mode.


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Oracle Startup and

Shutdown

Starting a database

1. Starting an instance of the database

2. Mounting a database

3. Opening a database


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Oracle Startup and

Shutdown Shutdown

1. Close the database.

2. Dismount the database.

3. Shut down the Oracle instance.


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Oracle System Architecture

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