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• A transaction is an executing program that forms a logical unit of database processing. A transaction 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; otherwise it is known as a read-write transaction.
• A transaction is a sequence of read and write operations on data items that logically functions as one unit of work
• It should either be done entirely or not at all
• If it succeeds, the effects of write operations persist (commit); if it fails, no effects of write operations persist (abort)
• These guarantees are made despite concurrent activity in the system, and despite failures that may occur

ACID Properties of Transaction

Transactions should possess several properties, often called the ACID properties; they should be enforced by the concurrency control and recovery methods of the DBMS. The following are the ACID properties:

• Atomicity:- A transaction is an atomic unit of processing; it should either be performed in its entirety or not performed at all.
• Consistency preservation:- A transaction should be consistency preserving, meaning that if it is completely executed from beginning to end without interference from other transactions, it should take the database from one consistent state to another.
• Isolation:- A transaction should appear as though it is being executed in isolation from other transactions, even though many transactions are executing concurrently. That is, the execution of a transaction should not be interfered with by any other transactions executing concurrently.
• Durability or permanency:- The changes applied to the database by a committed transaction must persist in the database. These changes must not be lost because of any failure.

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