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In computer science, transaction processing is information processing that is divided into individual, indivisible operations, called transactions. Computer science (or computing science) is the study and the Science of the theoretical foundations of Information and Computation and their Each transaction must succeed or fail as a complete unit; it cannot remain in an intermediate state.

Contents

Description

Transaction processing is designed to maintain a computer system (typically, but not limited to, a database or some modern filesystems) in a known, consistent state, by ensuring that any operations carried out on the system that are interdependent are either all completed successfully or all canceled successfully. A Computer Database is a structured collection of records or data that is stored in a computer system In Computing, a file system (often also written as filesystem) is a method for storing and organizing Computer files and the data they contain to make

For example, consider a typical banking transaction that involves moving £500 from a customer's savings account to a customer's checking account. This transaction is a single operation in the eyes of the bank, but it involves at least two separate operations in computer terms: debiting the savings account by £500, and crediting the checking account by £500. If the debit operation succeeds but the credit does not (or vice versa), the books of the bank will not balance at the end of the day. There must therefore be a way to ensure that either both operations succeed or both fail, so that there is never any inconsistency in the bank's database as a whole. Transaction processing is designed to provide this.

Transaction processing allows multiple individual operations to be linked together automatically as a single, indivisible transaction. The transaction-processing system ensures that either all operations in a transaction are completed without error, or none of them are. If some of the operations are completed but errors occur when the others are attempted, the transaction-processing system “rolls back” all of the operations of the transaction (including the successful ones), thereby erasing all traces of the transaction and restoring the system to the consistent, known state that it was in before processing of the transaction began. If all operations of a transaction are completed successfully, the transaction is committed by the system, and all changes to the database are made permanent; the transaction cannot be rolled back once this is done. In the context of Computer science and Data management, commit refers to the idea of making a set of tentative changes permanent

Transaction processing guards against hardware and software errors that might leave a transaction partially completed, with the system left in an unknown, inconsistent state. If the computer system crashes in the middle of a transaction, the transaction processing system guarantees that all operations in any uncommitted (i. e. , not completely processed) transactions are cancelled.

Transactions are processed in a strict chronological order. If transaction n+1 intends to touch the same portion of the database as transaction n, transaction n+1 does not begin until transaction n is committed. Before any transaction is committed, all other transactions affecting the same part of the system must also be committed; there can be no “holes” in the sequence of preceding transactions.

Methodology

The basic principles of all transaction-processing systems are the same. However, the terminology may vary from one transaction-processing system to another, and the terms used below are not necessarily universal.

Rollback

Transaction-processing systems ensure database integrity by recording intermediate states of the database as it is modified, then using these records to restore the database to a known state if a transaction cannot be committed. For example, copies of information on the database prior to its modification by a transaction are set aside by the system before the transaction can make any modifications (this is sometimes called a before image). If any part of the transaction fails before it is committed, these copies are used to restore the database to the state it was in before the transaction began (rollback).

Rollforward

It is also possible to keep a separate journal of all modifications to a database (sometimes called after images); this is not required for rollback of failed transactions, but it is useful for updating the database in the event of a database failure, so some transaction-processing systems provide it. If the database fails entirely, it must be restored from the most recent back-up. The back-up will not reflect transactions committed since the back-up was made. However, once the database is restored, the journal of after images can be applied to the database (rollforward) to bring the database up to date. Any transactions in progress at the time of the failure can then be rolled back. The result is a database in a consistent, known state that includes the results of all transactions committed up to the moment of failure.

Deadlocks

In some cases, two transactions may, in the course of their processing, attempt to access the same portion of a database at the same time, in a way that prevents them from proceeding. For example, transaction A may access portion X of the database, and transaction B may access portion Y of the database. If, at that point, transaction A then tries to access portion Y of the database while transaction B tries to access portion X, a deadlock occurs, and neither transaction can move forward. Transaction-processing systems are designed to detect these deadlocks when they occur. Typically both transactions will be cancelled and rolled back, and then they will be started again in a different order, automatically, so that the deadlock doesn't occur again. Or sometimes, just one of the deadlocked transactions will be cancelled, rolled back, and automatically re-started after a short delay.

Deadlocks can also occur between three or more transactions. The more transactions involved, the more difficult they are to detect, to the point that transaction processing systems find there is a practical limit to the deadlocks they can detect.

ACID criteria (Atomicity, Consistency, Isolation, Durability)

Main article: ACID

Transaction processing has these benefits:

Implementations

Main article: Transaction processing system

Standard transaction-processing software, notably IBM's Information Management System, was first developed in the 1960s, and was often closely coupled to particular database management systems. A Transaction Processing System To be considered a transaction processing system the computer must pass the ACID test. International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology IBM Information Management System ( IMS) is a joint Hierarchical database and Information management system with extensive Transaction processing The 1960s decade refers to the years from the beginning of 1960 to the end of 1969 A database management system ( DBMS) is Computer software designed for the purpose of managing Databases DBMSes may use any of a variety of Data models Client-server computing implemented similar principles in the 1980s with mixed success. The client-server Software architecture model distinguishes client systems from server systems which communicate over a Computer network The 1980s was the decade spanning from January 1 1980 to December 31 1989. However, in more recent years, the distributed client-server model has become considerably more difficult to maintain. As the number of transactions grew in response to various online services (especially the Web), a single distributed database was not a practical solution. The World Wide Web (commonly shortened to the Web) is a system of interlinked Hypertext documents accessed via the Internet. In addition, most online systems consist of a whole suite of programs operating together, as opposed to a strict client-server model where the single server could handle the transaction processing. Today a number of transaction processing systems are available that work at the inter-program level and which scale to large systems, including mainframes. Mainframes (often colloquially referred to as Big Iron) are Computers used mainly by large organizations for critical applications typically bulk data

An important open industry standard is the X/Open Distributed Transaction Processing (DTP) (see JTA). The Java Transaction API ( JTA) is one of the Java EE APIs allowing distributed Transactions to be done across multiple XA resources However, proprietary transaction-processing environments such as IBM's CICS are still very popular, although CICS has evolved to include open industry standards as well. CICS ( Customer Information Control System) is a Transaction server that runs primarily on IBM mainframe systems under Z/OS and Z/VSE

See also

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