Transaction
Is any action that reads from and/or writes to a database. A transaction may consist of a simple SELECT statement to generate a list of table contents; it may consist of series of INSERT statements to add rows to one or more tables.
TRANSACTION PROPERTIES
Each individual transaction must display atomicity, consistency, isolation, and durability. Let’s look briefly at each of the properties.
• Atomicity requires that all operations (SQL requests) of a transaction be completed; if not, the transaction is aborted. If a transaction T1 has four SQL requests, all four requests must be successfully completed; otherwise, the entire transaction is aborted. In other words. a transaction is treated as a single, indivisible, logical unit
work.
• Consistency indicates the permanence of the database’s consistent state (consistent database state is one in which all data integrity constraints are satisfied). A transaction takes a database from one consistent state to another consistent state. When a transaction is completed, the database must be in a consistent state; if any of the transaction parts violates an integrity constraint, the entire transaction is aborted.
• Isolation means that the data used during the execution of a transaction cannot be used by a second transaction until the first one is completed. In other words, if a transaction T1 is being executed and is using the data item X, that data item cannot be accessed by any other transaction (T2 ... Tn) until T1 ends. This property is particularly useful in multi-user database environments because several users can access and update the database at the same time.
• Durability ensures that once transaction changes are done (committed). they cannot be undone or lost, even in the event of a system failure.
• Serializability ensures that the schedule for the concurrent execution of the transactions yields consistent results. This property is important in multi-user and distributed databases, where multiple transactions are likely to be executed concurrently. Naturally, if only a single transaction is executed, serializability is not an issue.
THE TRANSACTION LOG
A DBMS uses a transaction log to keep track of all transactions that update the database. The information stored in this log is used by the DBMS for a recovery requirement triggered by a ROLLBACK statement, a program abnormal termination, or a system failure such as a network discrepancy or a disk crash.
While the DBMS executes transactions that modify the database, it also automatically updates the transaction log. The transaction log stores:
• A record for the beginning of the transaction.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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