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b A view containing the names and addresses of all customers who have an account with the bank, but do not have a loan c A view containing the name and average account balance of every customer of the Rock Ridge branch 611 For each of the views that you de ned in Exercise 610, explain how updates would be performed (if they should be allowed at all) Hint: See the discussion of views in 3 612 In 3, we described the use of views to simplify access to the database by users who need to see only part of the database In this chapter, we described the use of views as a security mechanism Do these two purposes for views ever con ict Explain your answer 613 What is the purpose of having separate categories for index authorization and resource authorization 614 Database systems that store each relation in a separate operating-system le may use the operating system s security and authorization scheme, instead of de ning a special scheme themselves Discuss an advantage and a disadvantage of such an approach 615 What are two advantages of encrypting data stored in the database 616 Perhaps the most important data items in any database system are the passwords that control access to the database Suggest a scheme for the secure storage of passwords Be sure that your scheme allows the system to test passwords supplied by users who are attempting to log into the system

If you omit the privilege level, it defaults to 15 Of course, most commands are at level 15, with a few at level 1; therefore, to take advantage of this approach, you ll need to change the privilege level of various commands and assign a password to access that privilege level You need to configure three commands in order to set up command authorization:

In Section 172, we described the use of a two-level storage hierarchy The system stores the database in nonvolatile storage (disk), and brings blocks of data into main memory as needed Since main memory is typically much smaller than the entire database, it may be necessary to overwrite a block B1 in main memory when another block B2 needs to be brought into memory If B1 has been modi ed, B1 must be output prior to the input of B2 As discussed in Section 1151 in 11, this storage hierarchy is the standard operating system concept of virtual memory The rules for the output of log records limit the freedom of the system to output blocks of data If the input of block B2 causes block B1 to be chosen for output, all log records pertaining to data in B1 must be output to stable storage before B1 is output Thus, the sequence of actions by the system would be: Output log records to stable storage until all log records pertaining to block B1 have been output Output block B1 to disk Input block B2 from disk to main memory It is important that no writes to the block B1 be in progress while the system carries out this sequence of actions We can ensure that there are no writes in progress by using a special means of locking: Before a transaction performs a write on a data

Summary

aaa authentication Specifies where to find user accounts and/or password for authentication; this is not required if using enabled passwords for Privilege EXEC access aaa authorization command Enables command authorization and the use of the privileges defined in the privilege command

The McGraw Hill Companies, 2001

17

To change the privilege levels of commands, use the privilege command:

item, it must acquire an exclusive lock on the block in which the data item resides The lock can be released immediately after the update has been performed Before a block is output, the system obtains an exclusive lock on the block, to ensure that no transaction is updating the block It releases the lock once the block output has completed Locks that are held for a short duration are often called latches Latches are treated as distinct from locks used by the concurrency-control system As a result, they may be released without regard to any locking protocol, such as two-phase locking, required by the concurrency-control system To illustrate the need for the write-ahead logging requirement, consider our banking example with transactions T0 and T1 Suppose that the state of the log is <T0 start> <T0 , A, 1000, 950> and that transaction T0 issues a read(B) Assume that the block on which B resides is not in main memory, and that main memory is full Suppose that the block on which A resides is chosen to be output to disk If the system outputs this block to disk and then a crash occurs, the values in the database for accounts A, B, and C are $950, $2000, and $700, respectively This database state is inconsistent However, because of the WAL requirements, the log record <T0 , A, 1000, 950> must be output to stable storage prior to output of the block on which A resides The system can use the log record during recovery to bring the database back to a consistent state

ciscoasa(config)# privilege [show | clear | configure] level level [mode enable | configure] command command