Question

Tanenbaum, Ch 7 Question #1:

For active replication to work in general, it is necessary that all operations be carried out in the same order at each replica. Is this ordering always necessary?

Tanenbaum, Ch 8

Question #1: For each of the following applications, do you think that at least once semantics or at most once semantics is best? Discuss.

Question #2: Explain how the write-ahead log in distributes transactions can be used to recover from failures.

Tanenbaum, Ch 9

Question #1: What is wrong in implementing a nonce as a time stamp?

Answer 1

Question- CH7 OL Answer : - No, the ordering is not necessary always.consider read operations that operate on non-modified data of commutative write operations. In principle, such situations allow ordering to be different at different replicas. However, it can be hard or impossible to detect whether, for example, to write operations are commutative. Answer ch8 al. (since the applications are not mentioned in Q., I am Answering with those applications.) for Reading and writing files from a file server and compiling a program, at least once is best. There is no harm trying over and over. → For Remote banking it is best to give it only one try. If that fails, the user will have to intervene to clean up the mess. Answer cho Q2: The log contains a record for each read and write operation that took place within the transaction, when a fails occurs, the log can be replayed to the last recorded operation. Replaying the log is effectively the opposite from rolling back, which happens when the transaction needed to be abouted Answer Chg at: Although a timestamp is used only once, it is far from being random. Implementations of security protocols exist that use timestamps as nonces, and which have been successfully attacked by exploitting the non randomness of the иби te8.