Note_Tech

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DBA - Process: Client Processes

When a user runs an application e.g., SQL*Plus, the operating system creates a client process / user process to run the user application.
- 当用户使用应用时, 系统创建

Client processes vs Oracle processes:
- Oracle processes:
  - can read from and write to the SGA. 可以读写 SGA
  - must run on the same host as the database host. 同一主机
- client process
  - cannot read from and write to the SGA. 不能直接读写
  - can run on a host other than the database host. 可以是不同主机.
Example:
- a user on a client host starts SQL*Plus, and then connects over the network to database sample on a different host when the database instance is not started
  - On the client host: a search of the processes shows only the sqlplus client process. 用户主机只显示 app 进程
  - On the database host: search of the processes hows a server process with a nonlocal connection, but no client process. DB 主机显示服务器进程, 不显示客户进程.

database connection
- a physical communication pathway between a client process and a database instance.
a connection occurs
- Typically between a client process and a server process or dispatcher,
- can also between a client process and Oracle Connection Manager (CMAN).
database session:
- a logical entity in the database instance memory that represents the state of a current user login to a database.
  - e.g.: when a user is authenticated by the database with a password, a session is established for this user.
A session lasts from the time the user is authenticated by the database until the time the user disconnects or exits the database application.
connection vs session
- - A single connection can have 0, 1, or more sessions established on it. 连接可以由多个会话
- The sessions are independent: a commit in one session does not affect transactions in other sessions.会话间独立.

User vs session
Multiple sessions can exist concurrently for a single database user.
In dedicated server connections:
- the database creates a server process on behalf of each connection.独享连接: 服务器进程即连接.
- Only the client process that causes the dedicated server to be created uses it. 用户进程触发独享服务器进程的创建; 也只有用户能专用
In a shared server connection
- many client processes access a single shared server process. 单一共享服务器进程
Example:

process_session_connection01

user hr can have multiple connections to a database. 单一用户可以由多个连接

process_session_connection

user hr can have single connections to a database with multiple sessions. 单一用户可以有多个会话

SELECT sid, serial#, paddr FROM V$SESSION WHERE username = user;

client_process_lab01

database operation
- session activity between two points in time
- defined by the end users or application code.
simple database operation:
- either a single SQL statement
- or a single PL/SQL procedure or function
composite database operation:
- a set of single or composite operations
User can divide a large set of tasks into database operations, and subdivide operations into phases.
- By configuring the job as a database operation, you can identify and tune the expensive steps in the job.
Each execution of a database operation is uniquely identified by a pair of attributes:
- operation name and execution ID.
One session can start or stop a database operation in a different session by specifying its session ID and serial number.
Two occurrences of the same database operation can execute at the same time using the same name but different execution IDs.
Each execution of a database operation with the same name can contain different statements.

Target	Identified
database operation	`operation name` + `execution ID`
session	`session ID` + `serial number`

总结:
- db operation can:
  - same name but diff statements
  - same name but diff ID
  - be divided