process/session/connection的一些問題

process/session/connection的一些問題[@more@]

記得在一個群裡曾經討論過一個問題：是先有session還是先有connection呢？當時也沒有定論。回答問題的要點在於：什麼是session，什麼是connection，兩者的存在是以什麼為標識的，兩者直接有怎樣的依存關係。
首先看下三者的概念。以下所有概念來源於11gconcept。
Process
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All connected Oracle Database users must run two modules of code to access an Oracle Database instance.

Application or Oracle tool: A database user runs a database application (such as a precompiler program) or an Oracle tool (such as SQL*Plus), which issues SQL statements to an Oracle database.

Oracle database server code: Each user has some Oracle database code executing on his or her behalf, which interprets and processes the application's SQL statements.

These code modules are run by processes. A process is a "thread of control" or a mechanism in an operating system that can run a series of steps. (Some operating systems use the terms job or task.) A process normally has its own private memory area in which it runs.

The processes in an Oracle Database system can be categorized into two major groups:

User processes run the application or Oracle tool code.

Oracle Database processes run the Oracle database server code. They include server processes and background processes.[/php]
process其實就是程式程式碼的執行時版。可以按照作者意圖去自動執行特定的任務。Oracle安裝process所執行任務的來源分為user process和server process.
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Connections and Sessions
Connection and session are closely related to user process but are very different in meaning.

A connection is a communication pathway between a user process and an Oracle Database instance. A communication pathway is established using available interprocess communication mechanisms (on a computer that runs both the user process and Oracle Database) or network software (when different computers run the database application and Oracle Database, and communicate through a network).

A session is a specific connection of a user to an Oracle Database instance through a user process. For example, when a user starts SQL*Plus, the user must provide a valid user name and password, and then a session is established for that user. A session lasts from the time the user connects until the time the user disconnects or exits the database application.

Multiple sessions can be created and exist concurrently for a single Oracle Database user using the same user name. For example, a user with the user name/password of SCOTT/TIGER can connect to the same Oracle Database instance several times.

In configurations without the shared server, Oracle Database creates a server process on behalf of each user session. However, with the shared server, many user sessions can share a single server process.
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connection其實就是通訊鏈路，對oracle講就是user process和instance間的通訊鏈路，可以使用IPC或者TCP之類的網路機制。
session則是一個資料庫使用者在user process和instance之間所建立的一個具體連結.session存在的前提是建立了連線。建立了連結的同時，session已經存在了。當user資訊錯誤時，user process向instance傳送連結請求，instance驗證user資訊，發現user資訊錯誤，返回錯誤資訊。這個過程只是兩者的通訊，試圖建鏈，還根本沒建立成功，這時候既不存在connection也不存在session。從網路模型上理解二者之間的關係，應該會比較容易點。似乎建立了連結的同時，session也跟著出現了。
舉個例子，打電話。但我們想和對方通話的時候，我們首先要撥號，當對方按下接聽鍵時，雙方直接就建立一條通訊鏈路，這時雙方就可以談話了，session其實就是指從會話準備就緒開始到會話結束，會話結束了就要斷開鏈路。
session可以有active和inactive的狀態，當按下接聽鍵時，鏈路建立了，當我們說hello時，會話開始了，當我們交談時是active的，當我們沉默時是inactive的，當我們說bye的時候，會話結束了，當我們掛機的時候，鏈路斷了。
所以到底是現有session還是先有connection的問題也就清楚了，其實他們是同時的，只是對同一事件同一現象同一事務在不同層次上的描述，從底層通訊看，是叫connection，從上層user process和instance看，是兩者的session已經開始了。當我們敲exit時，會話中斷，底層拆鏈。
如果非要分個先後的話，那session是要放在connection之間的。
至於server process和要討論的問題沒什麼關係。
以上只是一些原理上的猜測，還是有兩個不確定點的，一是connection和session建立的標準是什麼，這個是至關重要的，二是oracle處理的二者的方法是什麼。事實上oracle建立connection後是要去建立session資訊，也就是say hello to user process，所以session建立似乎是要在connection後的。
以上只是我的一些猜想，歡迎大家拍磚。