Database Systems: The Complete Book- P10

Database Systems: The Complete Book- P10: Database Systems and Database Design and Application courses offered at the junior, senior and graduate levels in Computer Science departments. Written by well-known computer scientists, this introduction to database systems offers a comprehensive approach, focusing on database design, database use, and implementation of database applications and database management systems
Nội dung trích xuất từ tài liệu:
Database Systems: The Complete Book- P10 880 CHAPTER 17. COPING WITH SYSTEM FAILURES 17.1. ISSUES AND MODELS FOR RESILIENT OPERATION 881 - The buffer may or may not be copied to disk immediately; that decision is ISthe Correctness Principle Believable? the responsibility of the buffer manager in general. As we shall soon see, one of the principal steps of using a log to assure resilience in the face of system Given that a database transaction could be an ad-hoc modification com- errors is forcing the buffer manager to write the block in a buffer back to disk mand issued a t a terminal, perhaps by someone who doesnt understand at appropriate times. However, in order to reduce the number of disk 1/Os, the implicit constraints in the mind of the database designer, is it plausible database systems can and will allow a change to exist only in volatile main- to assume all transactions take the database from a consistent state to an- other consistent state? Explicit constraints are enforced by the database, memory storage, at least for certain periods of time and under the proper set of conditions. so any transaction that violates them will be rejected by the system and not change the database at all. As for implicit constraints, one cannot In order to study the details of logging algorithms and other transaction- characterize them exactly under any circumstances. Our position, justi- management algorithms, nre need a notation that describes all the operations fying the correctness principle, is that if someone is given authority to that molre data between address spaces. The primitives we shall use are: modify the database, then they also have the authority to judge what the implicit constraints are. 1. INPUT (X) : Copy the disk block containing database element X to a mem- ory buffer. 2. READ (X ,t ) : Copy the database element X to the transactions local vari- There is a converse to the correctness principle that forms the motivation able t. llore precisely, if the block containing database element X is not for both the logging techniques discussed in this chapter and the concurrency in a memory buffer then first execute INPUT(X). Kext, assign the value of control mechanisms discussed in Chapter 18. This converse involves two points: X to local variable t. 1. A transaction is atornzc; that is, it must be executed as a whole or not 3. WRITE(X, t) : Copy the value of local variabIe t to database element X in at all. If only part of a transaction executes, then there is a good chance a memory buffer. XIore precisely. if the block containing database element that the resulting database state will not be consistent. IY is not in a memory buffer then execute INPUT(X). Next, copy the value 2. Transactions that execute simultaneously are likely to lead to an incon- of t to X in the buffer. sistent state unless we take steps to control their interactions, as we shall in Chapter 18. 4. OUTPUT(X): Copy the block containing . from its buffer to disk. I ...