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Atomic Transaction In Operating System


All the synchronization techniques we have studied are essentially low level as like semaphores. They require the programmer to involved with all the details of mutual exclusion, critical region management, deadlock prevention, and crash recovery. An abstraction exists there  and is widely used in distributed systems. We will call it an atomic transaction, or simply transaction.


 The term atomic action is also widely used. The mutual exclusion of critical section ensures that the critical sections are executed automatically. That is, if two critical sections are executed concurrently, the result is equivalent to their sequential execution in some unknown order.


  In many cases we would like to make sure that a critical section forms a single logical unit of work that either is performed in its entirely or is not performed at all. An example is funds transfer, in which one account is debited and another is credited. Clearly, it is essential for data consistency either that both the credit and debit occur or that neither occur.


A collection of instructions or operations that performs a single logical function is called transection. A major issue in processing transaction is the preservation of automicity despit the the possiblity of failures within the computer system. When a transaction starts executing, its first action is to execute wait() or down(). After the transaction either commits or abort , it executes signal() or up(). Although this scheme ensures that atomicity of all concurrently executing transactions, it is neverthless too restrictive.


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