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transaction and concurrency control

Transaction

In the context of DBMS, a transaction is an execution of a program that accesses a database

in the context pf transactional file server,a transaction is an execution of a sequence of client requests for file operations.

in some situation clients require a method for the server to execute a sequence of requests and to be atomic in the sense that:

1. they are free form the interference by operations being performed on the same subject on behalf of other clients

2. all the operations must be completed if one of them fails the whole should not have any effect on the object

A client’s banking transaction

Transaction T:

a.withdraw(100);

b.deposit(100);

c.withdraw(200);

b.deposit(200)

a simple client transaction specifying a series of related actions involving the bank accounts A, B and C. The first two actions transfer $100 from A to B and the second two transfer $200 from C to B. A client achieves a transfer operation by doing a withdrawal followed by a deposit.

->a transaction applies to recoverable objects and is intended to be atomic. It is often called an atomic transaction. There are two aspects to atomicity:

All or nothing: A transaction either completes successfully, in which case the effects of all of its operations are recorded in the objects, or (if it fails or is deliberately aborted) has no effect at all. This all-or-nothing effect has two further aspects of its own: * Failure atomicity: The effects are atomic even when the server crashes.

Durability: After a transaction has completed successfully, all its effects are saved in permanent storage. We use the term ‘permanent storage’ to refer to files held on disk or another permanent medium. Data saved in a file will survive if the server process crashes