Oracle Architecture and User management

First, ORALCE system structure
1. Overview
Oracle's architecture is the composition of the database, the work process, and the organization and management of the data in the database, to understand the architecture of the Oracle database, you must understand the main components of Oracle and important concepts. The Oracle architecture includes a range of components, including instances, user processes, service processes, data files, and other files, such as parameter files, password files, and archive log files. As shown in the following:

Instances and databases are a core part of the Oracle database architecture and are the two most important concepts, and the main task for DBAs is to maintain instances and databases.
1) Example
An instance is a collection of background processes and memory that must be started to access data in the database.

When Oracle starts, a system global Zone (SGA) is assigned and a series of Oracle background processes are started
An instance can only open and use one database
Oracle is divided into single-process and multi-process instances, and is now generally a multi-process instance, multi-process is multi-user, different users use different processes to execute different parts of Oracle.

2) database
A database is a collection of data that is treated as a logical unit.
The Oracle database consists of operating system files that provide the actual physical storage for database information
Oracle databases include logical and physical structures.
Physical structure: Refers to a set of operating system files that store database information, each with a logical structure and a physical structure. A physical structure is a set of operating system files that comprise a database, consisting mainly of 3 types of files: Data files, control files, and redo log files.
Logical structure: Refers to the logical storage structure of database data, including: Table space, data segment, table, view, etc.

Ii. Oracle Storage Architecture
Oracle's storage structure is divided into physical and logical structures, both of which are independent and interlinked.

1. Physical Structure:
The physical structure is the operating system physical file used after the Oracle database was created, divided into the following two types:
1) Main documents
Data files: Data files are the files that are physically stored in the database, the characteristics of the data files: Each data file is associated with only one database, one table space can contain one or more data files, and a data file can belong to only one table space. The data file contains user or application data for the database, as well as metadata and data dictionaries.

Redo log file: It records all changes to the data and provides a data recovery mechanism that, if the database server crashes but does not lose any data files, the instance can use the information in these files to recover the database.

Control file: When the database reads, the data file is located according to the information of the control file. The control file stores the name and location of the data file and the redo log file. A database of at least two control files, oracle11g by default includes 3 control files, each control file content is the same, you can avoid because of a control file corruption caused the database cannot be started. The control file records the following key information: the location and size of the data file, the location and size of the redo log file, the database name and creation time, and the log sequence number. Control files are critical to the database. Without these files, you cannot open the data file to access the data in the database.

2) Other documents
Parameter file: Used to define the configuration at instance startup
Password file: Allow Sysdba, Sysoper, and sysasm to connect remotely to the instance and perform administrative tasks
Archive log files: Using these files and database backups, you can recover lost data files. That is, the archive log can restore the restored data file

2. Oracle Logical Structure
Oracle's logical components include table spaces, segments, extents, blocks, and schemas.
1) Table Space
The tablespace is the largest logical unit in the database, and the SYSTEM and Sysaux tablespace are the table spaces that must exist when the database is created, as well as temp,users.
Each tablespace is made up of one or more data files, and a data file can be associated with only one table space.
The size of the tablespace equals the sum of all the data file sizes that make up the tablespace.
Table space and data files are relative, if there is no data file, table space can not exist independently, if only table space does not have a data file is not good, like if the computer does not have a hard disk, you can no longer logically see the C disk D, where the hard disk is the equivalent of data files, and the C disk equivalent is the table space. We can make multiple data files into a table space, and then store the data in the table space, the table space and data files are corresponding, a table space can be composed of one or more data files, but a data file can only be in one table space.

The role of Table spaces:
For different users to assign different table space, different schema objects are assigned different table spaces, which facilitates the management of user actions and schema objects.
Different data files can be created on different disks, which facilitates managing disk space, improving I/O performance, backing up and recovering data, and more.

System tablespace: Data that holds internal tables and data dictionaries for Oracle systems, such as indicate, column names, user names, and so on.
Sysaux table space: As a secondary tablespace for the system, it is used to store data used by various database tools such as Enterprise Manager, Staspack, Logminer, and so on. Used to store various patterns of object data, such as intelligent Proxy User Dbsnmp, data Mining user ODM and so on.
Users table space: As a table space for users, you can create various objects on this table space, such as creating tables, indexes, and so on.
Temp tablespace: A special table space for temporary data, for example: when sorting is required, the system will temporarily store the sorted data in that tablespace, and once the sorting process is complete, the space occupied by the sort data can be released, called a temporary tablespace.

Oracle Architecture and User management