Database Recovery Techniques
In this chapter we discuss some of the techniques that can be used for database recovery from failures. In Section 21.1.4 we discussed the different causes of failure, such as system crashes and transaction errors. Also, in Section 21.2, we covered many of the concepts that are used by recovery processes, such as the system log and commit points.
This chapter presents additional concepts that are relevant to recovery protocols, and provides an overview of the various database recovery algorithms We start in Section 23.1 with an outline of a typical recovery procedure and a categorization of recovery algorithms, and then we discuss several recovery concepts, including write-ahead logging, in-place versus shadow updates, and the process of rolling back (undoing) the effect of an incomplete or failed transaction. In Section 23.2 we present recovery techniques based on deferred update, also known as the NO-UNDO/REDO technique, where the data on disk is not updated until after a transaction commits. In Section 23.3 we discuss recovery techniques based on immediate update, where data can be updated on disk during transaction execution; these include the UNDO/REDO and UNDO/NO-REDO algorithms. We discuss the technique known as shadowing or shadow paging, which can be categorized as a NO-UNDO/NO-REDO algorithm in Section 23.4. An example of a practical DBMS recovery scheme, called ARIES, is presented in Section 23.5. Recovery in multidatabases is briefly discussed in Section 23.6. Finally, techniques for recovery from catastrophic failure are discussed in Section 23.7. Section 23.8 summarizes the chapter.
Our emphasis is on conceptually describing several different approaches to recovery. For descriptions of recovery features in specific systems, the reader should consult the bibliographic notes at the end of the chapter and the online and printed user manuals for those systems. Recovery techniques are often intertwined with the concurrency control mechanisms. Certain recovery techniques are best used with specific concurrency control methods. We will discuss recovery concepts independently of concurrency control mechanisms, but we will discuss the circumstances under which a particular recovery mechanism is best used with a certain concurrency control protocol.
Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.