Oracle Architecture

Description:
user process and server processes
when ever user connnect's to the oracle server, then it will create one session. that session is colled as user session. session is nothing but user and server prosesses transactons between is colled as a session.
Oracle instance:  An Oracle Instance consists of two different sets of components:
SGA:System Global Area
SGA(system global area or shared global area) contains memory structures and background processes.
Memory Structures
1.Dabase Buffer cache
2.Redolog Buffer
3.Shared pool
4.javapool and largepool

Background Processes
1.DBwn 2.ckpt 3.smon 4.pmon 5.arcn 6.lgwr

Memory structures:
1.Database Buffer Cache:

• The database buffer cache holds blocks of data from disk that have been recently read to satisfy a SELECT statement or that contain modified blocks that have been changed or added from a DMLstatement. 

2.Redo Log guffer

• The redo log buffer holds the most recent changes to the data blocks in the datafiles.
• When the redo log buffer is one-third full, or every three seconds, Oracle writes redo log records to the redo log files. the LGWR process will write the redo log records to the redo log files.
• A user’s committed transaction is not considered complete until the redo log entries have been successfully written to the redo log files.
• whenever user's perform's any DML Operations like update, delete that statemnts will be strored under redolog buffer. if the user gives any commit operations the log writer (LGWR) writes the statemts into redolog files.  

Shared pool:

• The shared pool contains two major sub caches: the library cache and the data dictionary cache.
• The shared pool is sized by the SHARED_POOL_SIZE initialization parameter. 

Lirary cache

• The library cache holds information about SQL and PL/SQL statements that are run against the database.
• In the library cache, because it is shared by all users, many different 
  database users can potentially share the same SQL statement.

• The second time an identical SQL statement is run, by the same user or a different user, the execution plan and parse tree are already computed, improving the execution time of the query or DML statement.

DATA Dictionary Cache:

• it is a part of sharedpool memory structure, data dictionary is a collection of database tables and views containing reference information about the database, its structures, and its users. Oracle accesses the data dictionary frequently during SQL statement parsing. 

The data dictionary has three primary uses:

• Oracle accesses the data dictionary to find information about users, schema objects, and storage structures.

• Oracle modifies the data dictionary every time that a data definition language (DDL) statement is issued.

• Any Oracle user can use the data dictionary as a read-only reference for information about the database.

• The data dictionary is always available when the database is open. It resides in the SYSTEM tablespace, which is always online.

Logical And Physical database structures