Concurrency Control With Locking Methods

A lock guarantees exclusive use of a data item to a current transaction. In other words, transaction T2 does not have access to a data item that is currently being used by transaction T1.

A transaction acquires a lock prior to data access; the lock is released (unlocked) when the transaction is completed so that another transaction can lock the data item for its exclusive use. This series of locking actions assumes that there is a likelihood of concurrent transactions attempting to manipulate the same data item at the same time. The use of locks based on the assumption that conflict between transactions is likely to occur is often referred to as pessimistic locking.

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Concurrency Control With Time Stamping

The time stamping approach to scheduling concurrent transactions assigns a global, unique time stamp to each transaction. The time stamp value produces an explicit order in which transactions are submitted to the DBMS. Time stamps must have two properties: uniqueness and monotonicity. Uniqueness ensures that no equal time stamp values can exist, and monotonicity ensures that time stamp values always increase.

All database operations (Read and Write) within the same transaction must have the same time stamp. The DBMS executes conflicting operations in time stamp order, thereby ensuring serializability of the transactions. If two transactions conflict, one is stopped, rolled back, rescheduled, and assigned a new time stamp value.

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Concurrency Control With Optimistic Method

The optimistic approach is based on the assumption that the majority of the database operations do not conflict. The optimistic approach requires neither locking nor time stamping techniques. Instead, a transaction is executed without restrictions until it is committed. Using an optimistic approach, each transaction moves through two or three phases, referred to as read, validation, and write.

• During the read phase, the transaction reads the database, executes the needed computations, and makes the updates to a private copy of the database values. All update operations of the transaction are recorded in a temporary update file, which is not accessed by the remaining transactions.

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Deadlocks in DBMS

A deadlock occurs when two transactions wait indefinitely for each other to unlock data. For example, a deadlock occurs when two transactions, T1 and T2, exist in the following mode:

T1 = access data items X and Y

T2 = access data items Y and X

If T1 has not unlocked data item Y, T2 cannot begin; if T2 has not unlocked data item X, T1 cannot continue.

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Types of Locks

There are different types of Locks.

Binary Locks

A binary lock has only two states: locked (1) or unlocked (0).

If an object—that is, a database, table, page, or row—is locked by a transaction, no other transaction can use that object. If an object is unlocked, any transaction can lock the object for its use. Every database operation requires that the affected object be locked. As a rule, a transaction must unlock the object after its termination. Therefore, every transaction requires a lock and unlock operation for each data item that is accessed. Such operations are automatically managed and scheduled by the DBMS; the user does not need to be concerned about locking or unlocking data items. (Every DBMS has a default locking mechanism. If the end user wants to override the default, the LOCK TABLE and other SQL commands are available for that purpose.)  

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Advantages of Two Phase Locking

Two-Phase Locking to Ensure Serializability:

Two-phase locking defines how transactions acquire and relinquish locks.

Two-phase locking guarantees serializability, but it does not prevent deadlocks. The two phases are:

1. A growing phase, in which a transaction acquires all required locks without unlocking any data. Once all locks have been acquired, the transaction is in its locked point.

 2. A shrinking phase, in which a transaction releases all locks and cannot obtain any new lock.

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The Evolution of Distributed Database Management Systems

A distributed database management system (DDBMS) governs the storage and processing of logically related data over interconnected computer systems in which both data and processing are distributed among several sites.

The use of a centralized database required that corporate data be stored in a single central site, usually a mainframe computer. Data access was provided through dumb terminals. The centralized approach worked well to fill the structured information needs of corporations, but it fell short when quickly moving events required faster response times and equally quick access to information. The slow progression from information request to approval to specialist to user simply did not serve decision makers well in a dynamic environment.

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The Advantages and Disadvantages of DDBMS

There are several Advantages and disadvantages of DDBMS.

Advantages of DDBMS are as follows:

1. Data are located near the greatest demand site. The data in a distributed database system are dispersed to match business requirements which reduce the cost of data access.

2. Faster data access. End users often work with only a locally stored subset of the company’s data.

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