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信息检索代写 Question 1. Choose concept(s) from below that are useful in discretionary access control. Explain their usefulness. (a) roles, (b) security classes,

Question 1.

Choose concept(s) from below that are useful in discretionary access control. Explain their usefulness. (a) roles, (b) security classes, (c) views, (d) statistical inference attack


Using the notations above, describe two protocols that realize (1) and (2):

(1) User B signs document d2 with timestamp t2, and sends secretly to user A. User A checks whether the sent document has a valid signature of B.

(2) Both users A and B sign on document d3, and d3 is encrypted by some keys. User C should not see document d3, but C needs to check whether the signs by A and B are both valid.

Question 3. 信息检索代写

Below (a)-(c) are schedules with symbols S(shared lock), X(exclusive lock), U(x) (unlock), Ab (abort), and Cm (commit). T1, T2, T3 are transactions.

(a) T1:X(A), T1:R(A), T2:X(B), T1:W(A), T1:U(A), T2:W(B), T2: U(B), T1:Co, T2:Co

(b) T1:X(B), T2:X(A), T1:R(B),T2:R(A), T2:S(B), T1:S(A),T2:R(B), T1:R(A), T1:W(B), T1:U(A), T1:U(B), T1:Co, T2:W(A), T2:U(A), T2:U(B), Co

(c) T1:X(A), T1:R(A), T1:W(A), T1:U(A), T2:X(A), T1:S(B), T2:W(A), T1:R(B), T2:U(A), T2:Co, T1:X(A), T1:W(A), T1:Co

(1) For each schedule of (a)-(c) above, choose all the properties from below that hold in the schedule. Also, briefly explain your answers.

1. RW-conflict, 2. WR-conflict, 3. WW-conflict, 4. conflict-serializable, 5. two-phase locking, 6. causing a deadlock, 7. avoiding cascading aborts, 8. view-serializable, 9. strict, 10. recoverable.

Here, a schedule is said to be strict if a value written by a transaction T is not read or overwritten by other transactions until T either aborts or commits. In a recoverable schedule, transactions commit only after all transactions whose changes they read commit.

(2) For statements (i) and (ii) below, argue whether each statement is true or not. Also show examples or counter-examples of schedules.

(i) Recoverable schedules are avoiding cascading aborts.

(ii) Two-phase locking protocol produces conflict-serializable schedules.

Question 4. 信息检索代写

When inserting a value to a B+-index tree, node splits can happen. To keep consistency, nodes need to be locked. There are two algorithms such that (A) starts locking from the root node, and (B) starts locking from a leaf node.

1. Briefly explain node insertion algorithms (A) and (B).

2. Discuss advantages and disadvantages of (A) and (B).

Question 5. 信息检索代写

1. Write the answers to the following XPath queries applied on d1.

(a) //car[option]/@color

(b) //*[@grade]/text()

(c) /*/*/option/ancestor::*/attribute::*

(d) /*/*/*/preceding-sibling::option

(e) //*[name()=’year’]/ancestor::*/name() 

2. Write an XPath query which returns the following:

(a) Query which is equivalent to the query of 1.(c) and the query contains a qualifier.

(b) Return the maker of ‘car’ elements that have a ‘year’ element or ‘year’ attribute in its descendants, and the ‘year’ is greater 2016.

(c) Find ‘car’ elements that have two or more ‘option’ elements.

(d) Find elements q such that q has NO ancestor that has element name ‘special’.

(e) Simplest query which returns “Shuttle”, but the query must not include any element or attribute names in document d1.


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