Oracle 9i Database Server architecture

2.4.1 Process Structure
There are two major classes of processes in Oracle 9i network environments.
1. User process
A user process is a program that runs on the client's memory, such as "SQL Plus", "Enterprise Manager," running on the client. The user process is making an operational request to the server process.
2. Server process
The main server process is shown in table 2.5.
Table 2.5 Oracle 9i's primary background support process

Name	Main role
System monitoring Process (Smon)	Perform recovery work on database system startup to restore a failed database
Process monitoring process (Pmon)	Used to restore failed user processes
Database write process (DBWR)	Write the modified block contents back to the database
Log write process (LGWR)	Write log contents in memory to the log file
Archive process (ARCH)	Call this process to complete the log archive when the database server is running as an archive
Checkpoint process (CKPT)	Identify checkpoints to reduce the time required for database recovery
Recovery process (RECO)	For failure handling in a distributed database
Lock Process (LCKN)	Ensure data consistency in parallel server mode
Snapshot process (SNPN)	Make snapshot Refresh
Scheduling Process (DNNN)	Responsible for routing the user process to the available server process for processing

2.4.2 Memory Structure
1. System global Area (SGA)
The SGA is shown in Figure 2.47.

2. Program Global Area (PGA)
The PGA is a dedicated memory area allocated to a single user process in the database server memory and is private to the user process and cannot be shared.
Logical Structure of 2.4.3 database
The logical structure of an Oracle 9i database is primarily the composition of the database that is examined from the perspective of the database user, as shown in Figure 2.48. From the bottom up, the logical structure of the database has a total of 6 layers.

1. Data block
2. Data range (Extent)
3. Data segment (Segment)
4. Logical objects (Logic object)
5. Table Space (tablespace)
The Oracle 9i installation automatically establishes 9 default tablespaces, as shown in table 2.6.
Table 2.6 The default tablespace for Oracle 9i databases

Name	Main role
Cwmlite	For online analytical processing (OLAP)
Drsys	For storing information about workspace settings
EXAMPLE	Instance table space, storing instance information
INDEX	Index table space, storing database index information
SYSTEM	system table space, storage table space name, containing data files and other management information
TEMP	Temporary table spaces, storing temporary tables
TOOLS	Tool table space, database objects that are required to store database Tools software
Undotbs	Rollback table space to store database recovery information
USERS	User table space, storing user private information

6. Databases (database)
The storage structure of the 2.4.4 database
The storage structure of a database refers to how the logical structure is physically implemented, with a total of 3 layers, as shown in Figure 2.49.

1. Physical block
2. Physical files
Each physical file consists of several physical blocks, consisting mainly of data files, control files, and log file 3 classes.
Data file: Used to store all the data, in DBF as the extension.
Log file: Records all operations on the database, with log as the extension.
Control file: Records the control information of all files in the database, with the CTL as the extension.
Take the author's installation environment as an example, the main physical files of the database "global database name" for "myoracle.mynet" are stored under C:\oracle\oradata\myoracle, as shown in Figure 2.50.

2.4.5 The overall structure of the database server
The overall structure of the Oracle 9i database server is shown in Figure 2.51.