Most optional background processes are specific to tasks or features. 

大多數(shù)可選后臺(tái)進(jìn)程都為了某些任務(wù)和特性而指定。

下面介紹幾個(gè)常見的可選后臺(tái)進(jìn)程：

Archiver Processes (ARCn) 

The archiver processes (ARCn)copy online redo log files to offline storage after a redo log switch occurs. These processes can also collect transaction redo data and transmit it to standby databasedestinations. ARCnprocesses exist onlywhen the database is in  ARCHIVELOG modeand automatic archiving is enabled.

歸檔進(jìn)程在log 切換的時(shí)候拷貝redo log file到離線存儲(chǔ)上面。進(jìn)程同樣可以收集事務(wù)的redo 數(shù)據(jù)傳輸?shù)絺鋷斓哪夸浬厦?。arcn進(jìn)程只有在歸檔模式并且自動(dòng)歸檔開啟的時(shí)候存在。

Job Queue Processes (CJQ0 and Jnnn)

Oracle Database uses job queue processesto run user jobs, often in batch mode. A jobis a user-defined task scheduled to run one or more times. For example, you can use a job queue to schedule a long-running update in the background. Given a start date and a time interval, the job queue processes attempt to run the job at the next occurrence of the interval.

oracle數(shù)據(jù)庫使用job queue 進(jìn)程去運(yùn)行用戶的jobs，經(jīng)常在批量模式下。任務(wù)是用戶自定義的預(yù)訂運(yùn)行一次或者多次的作業(yè)。比如說，可以使用一個(gè)job隊(duì)列去安排在后臺(tái)運(yùn)行的一個(gè)長(zhǎng)時(shí)間運(yùn)行的更新任務(wù)。給定一個(gè)開始時(shí)間和間隔時(shí)間，任務(wù)隊(duì)列進(jìn)程就會(huì)去嘗試在下一個(gè)間隔的時(shí)間點(diǎn)上面運(yùn)行job。

Oracle Database manages job queue processes dynamically, thereby enabling job queue clients to use more job queue processes when required. The database releases resources used by the new processes when they are idle.

oracle數(shù)據(jù)庫動(dòng)態(tài)的管理job隊(duì)列進(jìn)程，所以允許job隊(duì)列客戶端使用更多的job queue 進(jìn)程。數(shù)據(jù)庫會(huì)在它們空閑的時(shí)候釋放資源給新的進(jìn)程。

Dynamic job queue processes can run a large number of jobs concurrently at a given interval. The sequence of events is as follows:

動(dòng)態(tài)job 隊(duì)列進(jìn)程可以并發(fā)的運(yùn)行大量的job在一個(gè)給定的間隔里面。事件順序如下所示：
1. The job coordinator process (CJQ0)is automatically started and stopped as needed by Oracle Scheduler (see "Oracle Scheduler"on page 18-19). The coordinator process periodically selects jobs that need to be run from the system JOB$table. New jobs selected are ordered by time.
2. The coordinator process dynamically spawns job queue slave processes (Jnnn)to run the jobs.
3. The job queue process runs one of the jobs that was selected by the CJQ0 process for execution. Each job queue process runs one job at a time to completion.
4. After the process finishes execution of a singlejob, it polls for more jobs. If no jobs are scheduled for execution, then it enters a sleep state, from which it wakes up at periodic intervals and polls for more jobs. If the process does not find any new jobs, then it terminates after a preset interval.

1. cjq0在oracle調(diào)度器需要的時(shí)候自動(dòng)開啟。調(diào)度進(jìn)程定期的在job$表中選擇事務(wù)來運(yùn)行。新的事務(wù)選擇按照時(shí)間排列。
2. 調(diào)度進(jìn)程動(dòng)態(tài)的生成job queue slave process(jnnn）去運(yùn)行任務(wù)。
3. job隊(duì)列進(jìn)程運(yùn)行的任務(wù)由cjq0來選擇。每個(gè)job queue在一個(gè)時(shí)間點(diǎn)上面只能運(yùn)行一個(gè)job。
4. 在進(jìn)程完成執(zhí)行單個(gè)job后，它被分配更多的任務(wù)。如果沒有任務(wù)被安排去執(zhí)行，那么它們會(huì)進(jìn)入沉睡狀態(tài)。并且定期的蘇醒去檢查是否有job。如果沒有找到，那他們它們會(huì)進(jìn)入下一個(gè)沉睡周期。

The initialization parameter JOB_QUEUE_PROCESSESrepresents the maximum number of job queue processes that can concurrently run on an instance. However, clients should not assume that all job queue processes are available for job execution.

初始化參數(shù)job_queue_processes指定了job queue線程在實(shí)例中最大的并發(fā)運(yùn)行的數(shù)量。

Flashback Data Archiver Process (FBDA)

The flashback data archiver process (FBDA)archives historical rows of tracked tables into Flashback Data Archives. When a transaction containing DML on a tracked table commits, this process stores the pre-image of the rows into the Flashback Data Archive. It also keeps metadata on the current rows.

閃回?cái)?shù)據(jù)歸檔進(jìn)程，歸檔被追蹤的表的歷史行到閃回?cái)?shù)據(jù)歸檔文件里面。當(dāng)在被追蹤的表上面發(fā)生了dml事務(wù)的時(shí)候，這個(gè)進(jìn)程存儲(chǔ)行的前鏡像到閃回?cái)?shù)據(jù)歸檔里面。它同樣保存當(dāng)前行的元數(shù)據(jù)。

FBDA automatically manages the flashback data archive for space, organization, and retention. Additionally, the process keeps track of how far the archiving of tracked transactions has occurred.

FBDA自動(dòng)的管理閃回?cái)?shù)據(jù)歸檔文件的空間，組織結(jié)構(gòu)和滯留時(shí)間。

Space Management Coordinator Process (SMCO)

The SMCO process coordinates the execution of various space management related tasks, such as proactive space allocation and space reclamation. SMCO dynamically spawns slave processes (Wnnn) to implement the task.

smco進(jìn)程調(diào)度執(zhí)行的多種空間管理相關(guān)的任務(wù)，比如說提前的空間分配和回收。smco自動(dòng)的生成從屬進(jìn)程wnnn去完成任務(wù)。