How does a DBMS handle concurrent access to data? What issues can arise and how are they addressed?

How does a DBMS ensure data integrity and consistency?

Which of the following is a function of the DBMS?Storing dataProviding multi-users access controlData IntegrityAll of the above

Select the purpose of concurrency control.To enforce IsolationTo preserve database consistencyTo resolve read-write and write-write conflictsTo make data more readable to the users

2 Concurrency Control TechniquesThese rules can be enforced by the lock manager module of the DBMS. Between thelock_item(X) and unlock_item(X) operations in transaction T, T is said to hold thelock on item X. At most one transaction can hold the lock on a particular item.Thus no two transactions can access the same item concurrently.Shared/Exclusive (or Read/Write) Locks. The preceding binary lockingscheme is too restrictive for database items because at most, one transaction canhold a lock on a given item. We should allow several transactions to access the sameitem X if they all access X for reading purposes only. This is because read operationson the same item by different transactions are not conflicting (see Section 21.4.1).However, if a transaction is to write an item X, it must have exclusive access to X. Forthis purpose, a different type of lock called a multiple-mode lock is used. In thisscheme—called shared/exclusive or read/write locks—there are three lockingoperations: read_lock(X), write_lock(X), and unlock(X). A lock associated with anitem X, LOCK(X), now has three possible states: read-locked, write-locked, orunlocked. A read-locked item is also called share-locked because other transactionsare allowed to read the item, whereas a write-locked item is called exclusive-lockedbecause a single transaction exclusively holds the lock on the item.One method for implementing the preceding operations on a read/write lock is tokeep track of the number of transactions that hold a shared (read) lock on an itemin the lock table. Each record in the lock table will have four fields: <Data_item_name,LOCK, No_of_reads, Locking_transaction(s)>. Again, to save space, the system needs tomaintain lock records only for locked items in the lock table. The value (state) ofLOCK is either read-locked or write-locked, suitably coded (if we assume no recordsare kept in the lock table for unlocked items). If LOCK(X)=write-locked, the value oflocking_transaction(s) is a single transaction that holds the exclusive (write) lockon X. If LOCK(X)=read-locked, the value of locking transaction(s) is a list of one ormore transactions that hold the shared (read) lock on X. The three operationsread_lock(X), write_lock(X), and unlock(X) are described in Figure 22.2. 2 As before,each of the three locking operations should be considered indivisible; no interleav-ing should be allowed once one of the operations is started until either the opera-tion terminates by granting the lock or the transaction is placed in a waiting queuefor the item.When we use the shared/exclusive locking scheme, the system must enforce the fol-lowing rules:1. A transaction T must issue the operation read_lock(X) or write_lock(X) beforeany read_item(X) operation is performed in T.2. A transaction T must issue the operation write_lock(X) before anywrite_item(X) operation is performed in T.2These algorithms do not allow upgrading or downgrading of locks, as described later in this section. Thereader can extend the algorithms to allow these additional operations.read_lock(X):B: if LOCK(X) = “unlocked”then begin LOCK(X) ← “read-locked”;no_of_reads(X) ← 1endelse if LOCK(X) = “read-locked”then no_of_reads(X) ← no_of_reads(X) + 1else beginwait (until LOCK(X) = “unlocked”and the lock manager wakes up the transaction);go to Bend;write_lock(X):B: if LOCK(X) = “unlocked”then LOCK(X) ← “write-locked”else beginwait (until LOCK(X) = “unlocked”and the lock manager wakes up the transaction);go to Bend;unlock (X):if LOCK(X) = “write-locked”then begin LOCK(X) ← “unlocked”;wakeup one of the waiting transactions, if anyendelse it LOCK(X) = “read-locked”then beginno_of_reads(X) ← no_of_reads(X) −1;if no_of_reads(X) = 0then begin LOCK(X) = “unlocked”;wakeup one of the waiting transactions, if anyendend;22.1 Two-Phase Locking Techniques for Concurrency Control 781Figure 22.2Locking and unlockingoperations for two-mode (read-write orshared-exclusive)locks.3. A transaction T must issue the operation unlock(X) after all read_item(X) andwrite_item(X) operations are completed in T.34. A transaction T will not issue a read_lock(X) operation if it already holds aread (shared) lock or a write (exclusive) lock on item X. This rule may berelaxed, as we discuss shortly.3This rule may be relaxed to allow a transaction to unlock an item, then lock it again later.782 Chapter 22 Concurrency Control Techniques5. A transaction T will not issue a write_lock(X) operation if it already holds aread (shared) lock or write (exclusive) lock on item X. This rule may also berelaxed, as we discuss shortly.6. A transaction T will not issue an unlock(X) operation unless it already holdsa read (shared) lock or a write (exclusive) lock on item X

Which is one function of a database management system (DBMS)?OptionsEnsuring usabilityDeciding what to do with legacy systemsPreventing errors arising, while enabling multiple, simultaneous usersIdentifying what a user needs