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毕业设计(论文)外文翻译

DATABASE SYSTEM CONCEPTS

Abraham Silberschatz

Yale University

Henry F. Korth

Lehigh University

S. Sudarshan

Indian Institute of Technology, Bombay

14.1 Transaction Concept

A transaction is a unit of program execution that accesses and possibly updates various data items. Usually, a transaction is initiated by a user program written in a high-level data-manipulation language (typically SQL), or programming language (for example, C , or Java), with embedded database accesses in JDBC or ODBC. A transaction is delimited by statements (or function calls) of the form begin transaction and end transaction. The transaction consists of all operations executed between the begin transaction and end transaction.

This collection of steps must appear to the user as a single, indivisible unit. Since a transaction is indivisible, it either executes in its entirety or not at all. Thus, if a transaction begins to execute but fails for whatever reason, any changes to the database that the transaction may have made must be undone. This requirement holds regardless of whether the transaction itself failed (for example, if it divided by zero), the operating system crashed, or the computer itself stopped operating. As we shall see, ensuring that this requirement is met is diffificult since some changes to the database may still be stored only in the main-memory variables of the transaction, while others may have been written to the database and stored on disk. This “all-or-none” property is referred to as atomicity.

Furthermore, since a transaction is a single unit, its actions cannot appear to be separated by other database operations not part of the transaction. While we wish to present this user-level impression of transactions, we know that reality is quite different. Even a single SQL statement involves many separate accesses to the database, and a transaction may consist of several SQL statements. Therefore, the database system must take special actions to ensure that transactions operate properly without interference from concurrently executing database statements. This property is referred to as isolation.

Even if the system ensures correct execution of a transaction, this serves little purpose if the system subsequently crashes and, as a result, the system “forgets” about the transaction. Thus, a transactionrsquo;s actions must persist across crashes. This property is referred to as durability.

Because of the above three properties, transactions are an ideal way of structuring interaction with a database. This leads us to impose a requirement on transactions themselves. A transaction must preserve database consistency—if a transaction is run atomically in isolation starting from a consistent database, the database must again be consistent at the end of the transaction. This consistency requirement goes beyond the data integrity constraints we have seen earlier (such as primary-key constraints, referential integrity, check constraints, and the like). Rather, transactions are expected to go beyond that to ensure preservation of those application-dependent consistency constraints that are too complex to state using the SQL constructs for data integrity. How this is done is the responsibility of the programmer who codes a transaction. This property is referred to as consistency.

grammer who codes a transaction. This property is referred to as consistency.To restate the above more concisely, we require that the database system maintain the following properties of the transactions:

bull; Atomicity. Either all operations of the transaction are reflected properly in the database, or none are.

bull; Consistency. Execution of a transaction in isolation (that is, with no other transaction executing concurrently) preserves the consistency of the database.

bull; Isolation. Even though multiple transactions may execute concurrently, the system guarantees that, for every pair of transactions Ti and Tj , it appears to Ti that either Tj finished execution before Ti started or Tj started execution after Ti finished. Thus, each transaction is unaware of other transactions executing concurrently in the system.

bull; Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.

These properties are often called the ACID properties; the acronym is derived from the fifirst letter of each of the four properties.

As we shall see later, ensuring the isolation property may have a signifificant adverse effect on system performance. For this reason, some applications compromise on the isolation property. We shall study these compromises after fifirst studying the strict enforcement of the ACID properties.

14.2 A Simple Transaction Model

Because SQL is a powerful and complex language, we begin our study of transactions with a simple database language that focuses on when data are moved from disk to main memory and from main memory to disk. In doing this, we ignore SQL insert and delete operations, and defer considering them until Section 15.8.The only actual operations on the data are restricted in our simple language to arithmetic operations. Later we shall discuss transac

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