In multiversion timestamp ordering, read operations can access tentative versions of objects. Give
an example to show how cascading aborts can happen if all read operations are allowed to proceed
immediately.
What will be an ideal response?
The answer to Exercise 13.21gives the read rule for multiversion timestamp ordering. read operations are
delayed to ensure recoverability. In fact this delay also prevents dirty reads and cascading aborts.
Suppose now that read operations are not delayed, but that commits are delayed as follows to ensure
recoverability. If a transaction, T has observed one of U’s tentative objects, then T’s commit is delayed until
U commits or aborts (see page 503). Cascading aborts can occur when U aborts because T has done a dirty
read and will have to abort as well.
To find an example, look for an answer to Exercise 13.19where a read operation was delayed (i.e. (c) or (d)).
Consider the diagram for (c) in Exercise 13.2. With delays, T’s x := read(i) is delayed until U commits or
aborts. Now consider allowing T’s x := read(i) to use U’s tentative version immediately. Then consider the
situation in which T asks to commit and U subsequently aborts. Note that T’s commit request is delayed until
the outcome of U is known, so the situation is recoverable, but T has performed a ‘dirty read’ and must be
aborted. Cascading aborts can now arise because some other transactions may have observed T’s tentative
objects.
You might also like to view...
The compiler will implicitly create a default constructor if:
a. The class does not contain any data members. b. The programmer specifically requests that the compiler do so. c. The class does not define any constructors. d. The class already defines a default constructor.
Answer the following statements true (T) or false (F)
1. Frequent key changes are usually desirable to limit the amount of data compromised if an attacker learns the key. 2. For link encryption manual delivery is awkward. 3. Each user must share a unique key with the key distribution center for purposes of key distribution. 4. Typically the session key is used for the duration of a logical connection, such as a frame relay connection or transport connection, and then it is permanently stored. 5. Master keys can be distributed in some noncryptographic way such as physical delivery.
________ is the process of specifying the appearance of cells or overall layout of a worksheet
A) Formatting B) Designing C) Selecting a theme D) Applying a style
AES has been the federally approved standard for nonclassified data since 2002.
Answer the following statement true (T) or false (F)