The document describes the architecture of Oracle databases, highlighting the roles of user processes, server processes, and various memory structures such as the System Global Area (SGA) and Program Global Area (PGA). It outlines key components like the shared pool, database buffer cache, and redo log buffer, as well as the function of background processes. Additionally, it explains the organization of databases and the concept of sessions connecting applications to these databases.

1. ORACLE ARCHITECTURE

2. User
Process
Server
Process
dd
Database Instances
SGA
Shared
pool
Buffer
cache
Redo
Log buffer
Java tool Large
tool
Background Processes
Data files
Redo log
files
Control
Files
Database
PGA

3. PGA
Server
process
User
process
Connection
established
SGA
Oracle Server
Session created

4. USER PROCESS: A process that runs on the client side
and interacts with the Oracle database server.
SERVER PROCESS: Oracle Database
creates server processes to handle the
requests of user processes connected to an
instance.
PGA (Program Global Area): A memory
region that contains data and control
information for a server process.

5. Instances (ASM and Database separate): An instance is a collection of processes and
memory structures that work together to manage a database. ASM (Automatic
Storage Management) is a volume manager and file system for Oracle databases
System Global Area (SGA) forms the part of the system memory (RAM)
shared by all the processes belonging to a single Oracle database instance.
The SGA contains all information necessary for the instance operation.
Shared pool is the area of the System Global Area (SGA)
that contains structures such as the data dictionary cache
and the shared SQL area.
Database buffer cache, also called the buffer cache,
is a memory area in the system global area (SGA) of the
database instance. It stores copies of data blocks that
are read from data files.

6. The Redo log buffer is the part of the System Global Area (SGA) that
holds information about changes made to the database. Each of these
changes generates a 'redo entry'. Redo entries are needed to
reconstruct these changes during the recovery process.
Large pool to provide large memory allocations for: Session memory for
the shared server and the Oracle XA interface (used where transactions
interact with more than one database)
Background processes: These are special processes that are started
automatically by Oracle when a database instance is started. They
perform various background tasks such as I/O, recovery, and memory
management.
A database is an organized collection of structured
information, or data, typically stored electronically in a
computer system.
A session represents the connection between an
application and the relational database that stores its
persistent objects.

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