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: Understanding the Fundamentals of Relational Databases
is an acronym that stands for Atomicity, Consistency, Isolation, and Durability. It is a set of fundamental principles that govern the behavior of relational database transactions. compliance ensures that data is reliably stored, retrieved, modified, and deleted in a consistent manner, preserving the integrity of the database even in the face of flures or concurrent access.
Atomicity refers to the indivisibility of a transaction. A transaction is either executed fully or not at all. If any error occurs during the execution, the transaction is rolled back to its original state, thus ensuring that the state of the database remns consistent. Atomicity is a crucial property of that ensures data integrity.
Consistency refers to the preservation of correctness in the database. A transaction must leave the database in a valid state, adhering to a set of predefined constrnts such as keys, relationships, and data types. Consistency is enforced by the database management system (DBMS), which validates all transactions agnst the schema and rules defined for the database.
Isolation refers to the independent execution of concurrent transactions. Multiple transactions can be executed simultaneously, but they must not interfere with each other. Isolation is implemented through locking mechanis that prevent conflicting access to data. Transactions are executed in isolation until they are committed or rolled back, ensuring that the database remns consistent even under heavy load.
Durability refers to the permanency of the database state. Once a transaction is committed, the changes made to the database must be durable, meaning they are persisted to non-volatile storage such as a hard disk. Transactions that are rolled back do not have any lasting effects on the database, but committed transactions must survive any subsequent flures.
compliance is an essential requirement for a reliable and robust database system. It provides a set of safeguards agnst data corruption, loss, or inconsistency due to various reasons such as software bugs, hardware flures, or human errors. However, compliance comes at a cost in terms of performance and scalability, as locking and validation operations can cause contention and delay in high-concurrency scenarios.
compliance is not a universal requirement for all database systems, and some alternative models such as NoSQL databases or distributed systems may sacrifice properties for performance, scalability, or flexibility. However, for applications that require strict data consistency and reliability, -compliant databases remn the gold standard for transactional processing.
In conclusion, understanding the fundamentals of is critical for anyone working with relational databases. The principles of atomicity, consistency, isolation, and durability ensure that transactions are executed reliably and consistently, preventing data corruption and loss. compliance is an essential requirement for applications that value data integrity over performance or scalability, and it remns a benchmark for transactional databases.
相關(guān)問(wèn)題拓展閱讀：
- 數(shù)據(jù)庫(kù)的4種狀態(tài)是什么
數(shù)據(jù)庫(kù)的4種狀態(tài)是什么
Oracle10g數(shù)據(jù)庫(kù)的4種存儲(chǔ)形式
數(shù)據(jù)庫(kù)的存在一定需要有永久性存儲(chǔ)方式和介質(zhì)。Oracle自然也不例外，在Oracle10g中，有4種存儲(chǔ)形式，分別是操作系統(tǒng)文件，裸分區(qū)，自動(dòng)存儲(chǔ)管理，集群系統(tǒng)OCFS（RAC）。下面分別說(shuō)一下這4種存儲(chǔ)形式。1 操作系統(tǒng)文件。 這種是大家最常用的方式了，也是非商業(yè)運(yùn)行模式（比如開(kāi)發(fā)或者開(kāi)發(fā)階段的測(cè)試環(huán)境）下最常用的形式。當(dāng)大家安裝Oracle的時(shí)候，如果選用了操作系統(tǒng)文件的存儲(chǔ)形式，那么就會(huì)把Oracle的數(shù)據(jù)存儲(chǔ)在操作系統(tǒng)中，以文件的形式存在。就好像我們玩某些單機(jī)版游戲，你的存檔就是操作系統(tǒng)的文件一樣。用文件形式存儲(chǔ)數(shù)據(jù)，帶來(lái)的優(yōu)點(diǎn)就是易于數(shù)據(jù)庫(kù)移動(dòng)。比如你把Oracle的數(shù)據(jù)文件直接copy到一臺(tái)電腦上，就可以很快復(fù)制出一個(gè)一樣的數(shù)據(jù)庫(kù)系統(tǒng)出來(lái)。在文件的讀寫(xiě)的時(shí)候，操作系統(tǒng)通常會(huì)提供緩存形式進(jìn)行讀寫(xiě)，避免過(guò)多讀寫(xiě)硬盤(pán)對(duì)磁盤(pán)造成很大傷害。所以用操作系統(tǒng)文件形式存儲(chǔ)數(shù)據(jù)，等于先天性獲得了操作系統(tǒng)的緩存功能。
2 裸分區(qū)。裸分區(qū)就是把數(shù)據(jù)庫(kù)直接寫(xiě)在磁盤(pán)上，不再經(jīng)過(guò)操作系統(tǒng)這一層，Oracle專(zhuān)門(mén)來(lái)對(duì)這個(gè)分區(qū)進(jìn)行讀寫(xiě)。因?yàn)闆](méi)有了操作系統(tǒng)這一層，所以讀寫(xiě)起來(lái)的速度和性能是質(zhì)一般的飛躍的，在某些對(duì)Oracle讀和穗寫(xiě)非常頻繁的數(shù)據(jù)庫(kù)應(yīng)用中，采用裸分區(qū)形式存儲(chǔ)數(shù)據(jù)甚至可以提高30%以上的性能。當(dāng)然，如喚者卜果數(shù)據(jù)存儲(chǔ)是采用的操作系統(tǒng)文件，如果在Unix下采用RAID對(duì)于文件讀寫(xiě)操作頻繁的情況下也是一個(gè)可以提高性能的手段。
3 自動(dòng)存儲(chǔ)管理。 自動(dòng)存儲(chǔ)管理通常也被稱(chēng)為A，是在Oracle 10g以后才提供的一種新的存儲(chǔ)形式。這種存儲(chǔ)形式貌似是前兩種存儲(chǔ)形式的折中。既不是用的host操作系統(tǒng)的文件系統(tǒng)，也不是裸分區(qū)的形式直接由Oracle讀寫(xiě)。而是可以看作為一種Oracle特有的文件系統(tǒng)形式。在A上可以存儲(chǔ)數(shù)據(jù)文件，控制文件，日志文件等等，在A中，這些也是以文件的形式存在的，只不過(guò)這個(gè)時(shí)候的文件格式是Oracle自有的A形式。在Linux中和A有點(diǎn)相似的就是LVM（邏輯Volume管理），也是可以看作是一種自有的新的文件系統(tǒng)形式。
4 集群系統(tǒng)OCFS（RAC）。RAC的全稱(chēng)是什么呢，就是Real Application Cluster，是一種在集群的環(huán)境下實(shí)現(xiàn)共享的一種數(shù)據(jù)庫(kù)存儲(chǔ)形式。在通常我們的數(shù)據(jù)庫(kù)，如果是單機(jī)版的數(shù)據(jù)庫(kù)，當(dāng)數(shù)據(jù)文件磁盤(pán)不能訪問(wèn)，那么這個(gè)數(shù)據(jù)庫(kù)就掛掉了。如果是集群共享數(shù)據(jù)庫(kù)的話，其中一種方法就是在某個(gè)節(jié)點(diǎn)存放數(shù)據(jù)文件，其余的數(shù)據(jù)庫(kù)都通過(guò)NFS或者Samba來(lái)進(jìn)行共享，雖然數(shù)據(jù)庫(kù)的數(shù)據(jù)處理能力得到了很大的提高，不過(guò)如果這個(gè)存儲(chǔ)節(jié)點(diǎn)掛掉的話，同樣數(shù)據(jù)庫(kù)也就宕掉了。而OCFS則是一種無(wú)斷點(diǎn)恢復(fù)的存儲(chǔ)形式，在整體給人的感覺(jué)就是每臺(tái)數(shù)據(jù)服務(wù)器都有自己的文件系統(tǒng)，但是這些文件系統(tǒng)又仿佛只是同一個(gè)文件系統(tǒng)，因?yàn)閿?shù)據(jù)更新或者其他的操作都是很統(tǒng)一的。即使某臺(tái)電腦掛了，其余的服務(wù)器照樣運(yùn)行，不會(huì)嫌碰出現(xiàn)數(shù)據(jù)庫(kù)的異常，而如果想新加入一臺(tái)服務(wù)器也是很簡(jiǎn)單的事。特別適合于商用的7X24這種服務(wù)形式。
Oracle10g中的4種數(shù)據(jù)存儲(chǔ)方式都介紹完了，我們最常用的自然是之一種形式，不過(guò)后面三種形式在商用環(huán)境下比較多。特別在大型商用對(duì)可靠性要去很高應(yīng)用中，第四種RAC形式的使用是比較多的。當(dāng)然，這里只是很簡(jiǎn)要的介紹了一下幾個(gè)文件系統(tǒng)的概念。如果要仔細(xì)了解的話，當(dāng)然是要好好查閱Oracle技術(shù)手冊(cè)了。
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