Transaction Processing
Content
Transaction processing system
The concept of transaction provides a mechanism for describing logical units of database processing. Transaction processing systems are systems with large databases and hundreds of concurrent users that are executing database transactions. Examples of such systems include systems for reservations, banking, credit card processing, stock markets, supermarket checkout, and other similar systems. They require high availability and fast response time for hundreds of concurrent users.
Definition of Transaction A transaction is a logical unit of database processing that includes one or more database access operations—these can include insertion, deletion, modification, or retrieval operations. The database operations that form a transaction can either be embedded within an application program or they can be specified interactively via a high-level query language such as SQL. One way of specifying the transaction boundaries is by specifying explicit begin transaction and end transaction statements in an application program; in this case, all database access operations between the two are considered as forming one transaction. A single application program may contain more than one transaction if it contains several transaction boundaries. If the database operations in a transaction do not update the database but only retrieve data, the transaction is called a read-only transaction. Single User versus Multi-user Systems: A DBMS is single user if at most one user at a time can use the system and it’s a multi-user if many users can use the system and they can access the database concurrently. Single users are restricted to some microcomputer and other dbms are multi user. For example, an airlines reservation system is used by hundreds of travel agents and reservation clerks concurrently. Database systems used in banks, insurance agencies, stock exchanges, supermarkets, and many other applications are multi user systems. In these systems, hundreds or thousands of users are typically operating on the database by submitting transactions concurrently to the system. Multiple users can access databases—and use computer systems—simultaneously because of the concept of multiprogramming, which allows the operating system of the computer to execute multiple programs—or processes—at the same time. A
single central processing unit (CPU) can only execute at most one process at a time.
Properties of Transaction (ACID Properties) :
Transactions provide several important properties, referred to as the ACID properties: • Atomicity: A transaction is either successful or unsuccessful. Either all of the operations that make up a transaction take effect or none take effect. A successful transaction is said to commit. An unsuccessful transaction is said to abort. Any operations performed by an aborted transaction are undone (rolled back) so that its effects are not visible. • Consistency: A transaction transforms distributed data from one consistent state to another. The application program is responsible for ensuring consistency. • Isolation: Each transaction appears to run independently of other transactions that are running concurrently. The effects of a transaction are not visible to others until the transaction completes (commits or aborts). The transactions appear to be serialized, with two or more transactions acting as though one completed before the other began, even though they are executed concurrently. • Durability: Also known as permanence, this property ensures that once completed, the effects of a transaction are permanent. A subsequent failure (such as abnormal program termination, communications failure, or hardware failure) does not cause the effects to be undone.
The concept of transaction provides a mechanism for describing logical units of database processing. Transaction processing systems are systems with large databases and hundreds of concurrent users that are executing database transactions. Examples of such systems include systems for reservations, banking, credit card processing, stock markets, supermarket checkout, and other similar systems. They require high availability and fast response time for hundreds of concurrent users.
Definition of Transaction A transaction is a logical unit of database processing that includes one or more database access operations—these can include insertion, deletion, modification, or retrieval operations. The database operations that form a transaction can either be embedded within an application program or they can be specified interactively via a high-level query language such as SQL. One way of specifying the transaction boundaries is by specifying explicit begin transaction and end transaction statements in an application program; in this case, all database access operations between the two are considered as forming one transaction. A single application program may contain more than one transaction if it contains several transaction boundaries. If the database operations in a transaction do not update the database but only retrieve data, the transaction is called a read-only transaction. Single User versus Multi-user Systems: A DBMS is single user if at most one user at a time can use the system and it’s a multi-user if many users can use the system and they can access the database concurrently. Single users are restricted to some microcomputer and other dbms are multi user. For example, an airlines reservation system is used by hundreds of travel agents and reservation clerks concurrently. Database systems used in banks, insurance agencies, stock exchanges, supermarkets, and many other applications are multi user systems. In these systems, hundreds or thousands of users are typically operating on the database by submitting transactions concurrently to the system. Multiple users can access databases—and use computer systems—simultaneously because of the concept of multiprogramming, which allows the operating system of the computer to execute multiple programs—or processes—at the same time. A
single central processing unit (CPU) can only execute at most one process at a time.
Properties of Transaction (ACID Properties) :
Transactions provide several important properties, referred to as the ACID properties: • Atomicity: A transaction is either successful or unsuccessful. Either all of the operations that make up a transaction take effect or none take effect. A successful transaction is said to commit. An unsuccessful transaction is said to abort. Any operations performed by an aborted transaction are undone (rolled back) so that its effects are not visible. • Consistency: A transaction transforms distributed data from one consistent state to another. The application program is responsible for ensuring consistency. • Isolation: Each transaction appears to run independently of other transactions that are running concurrently. The effects of a transaction are not visible to others until the transaction completes (commits or aborts). The transactions appear to be serialized, with two or more transactions acting as though one completed before the other began, even though they are executed concurrently. • Durability: Also known as permanence, this property ensures that once completed, the effects of a transaction are permanent. A subsequent failure (such as abnormal program termination, communications failure, or hardware failure) does not cause the effects to be undone.
