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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)問題拓展閱讀：
- 數(shù)據(jù)庫的4種狀態(tài)是什么
數(shù)據(jù)庫的4種狀態(tài)是什么
Oracle10g數(shù)據(jù)庫的4種存儲形式
數(shù)據(jù)庫的存在一定需要有永久性存儲方式和介質(zhì)。Oracle自然也不例外，在Oracle10g中，有4種存儲形式，分別是操作系統(tǒng)文件，裸分區(qū)，自動存儲管理，集群系統(tǒng)OCFS（RAC）。下面分別說一下這4種存儲形式。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)運行模式（比如開發(fā)或者開發(fā)階段的測試環(huán)境）下最常用的形式。當大家安裝Oracle的時候，如果選用了操作系統(tǒng)文件的存儲形式，那么就會把Oracle的數(shù)據(jù)存儲在操作系統(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。用文件形式存儲數(shù)據(jù)，帶來的優(yōu)點就是易于數(shù)據(jù)庫移動。比如你把Oracle的數(shù)據(jù)文件直接copy到一臺電腦上，就可以很快復制出一個一樣的數(shù)據(jù)庫系統(tǒng)出來。在文件的讀寫的時候，操作系統(tǒng)通常會提供緩存形式進行讀寫，避免過多讀寫硬盤對磁盤造成很大傷害。所以用操作系統(tǒng)文件形式存儲數(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)庫直接寫在磁盤上，不再經(jīng)過操作系統(tǒng)這一層，Oracle專門來對這個分區(qū)進行讀寫。因為沒有了操作系統(tǒng)這一層，所以讀寫起來的速度和性能是質(zhì)一般的飛躍的，在某些對Oracle讀和穗寫非常頻繁的數(shù)據(jù)庫應(yīng)用中，采用裸分區(qū)形式存儲數(shù)據(jù)甚至可以提高30%以上的性能。當然，如喚者卜果數(shù)據(jù)存儲是采用的操作系統(tǒng)文件，如果在Unix下采用RAID對于文件讀寫操作頻繁的情況下也是一個可以提高性能的手段。
3 自動存儲管理。 自動存儲管理通常也被稱為A，是在Oracle 10g以后才提供的一種新的存儲形式。這種存儲形式貌似是前兩種存儲形式的折中。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。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，只不過這個時候的文件格式是Oracle自有的A形式。在Linux中和A有點相似的就是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的全稱是什么呢，就是Real Application Cluster，是一種在集群的環(huán)境下實現(xiàn)共享的一種數(shù)據(jù)庫存儲形式。在通常我們的數(shù)據(jù)庫，如果是單機版的數(shù)據(jù)庫，當數(shù)據(jù)文件磁盤不能訪問，那么這個數(shù)據(jù)庫就掛掉了。如果是集群共享數(shù)據(jù)庫的話，其中一種方法就是在某個節(jié)點存放數(shù)據(jù)文件，其余的數(shù)據(jù)庫都通過NFS或者Samba來進行共享，雖然數(shù)據(jù)庫的數(shù)據(jù)處理能力得到了很大的提高，不過如果這個存儲節(jié)點掛掉的話，同樣數(shù)據(jù)庫也就宕掉了。而OCFS則是一種無斷點恢復的存儲形式，在整體給人的感覺就是每臺數(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)又仿佛只是同一個文件系統(tǒng)，因為數(shù)據(jù)更新或者其他的操作都是很統(tǒng)一的。即使某臺電腦掛了，其余的服務(wù)器照樣運行，不會嫌碰出現(xiàn)數(shù)據(jù)庫的異常，而如果想新加入一臺服務(wù)器也是很簡單的事。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)存儲方式都介紹完了，我們最常用的自然是之一種形式，不過后面三種形式在商用環(huán)境下比較多。特別在大型商用對可靠性要去很高應(yīng)用中，第四種RAC形式的使用是比較多的。當然，這里只是很簡要的介紹了一下幾個文件系統(tǒng)的概念。如果要仔細了解的話，當然是要好好查閱Oracle技術(shù)手冊了。
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