Give full details of a mechanism for concurrency control that can be used to ensure that the
types of problem discussed in Question 1 (a) cannot occur. Show how the mechanism
prevents the problems illustrated from occurring. Discuss how the concurrency control
mechanism interacts with the transaction mechanism.
Answers
Answer:
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Explanation:
Transaction:
A transaction is a unit of program execution that accesses and possibly updates various data items.
1)A transaction is the DBMS’s abstract view of a user program: a sequence of reads and writes
2)A transaction must see a consistent database
3)During transaction execution the database may be temporarily inconsistent
4) A sequence of many actions which are considered to be one atomic unit of work
5)When the transaction completes successfully (is committed), the database must be consistent ?
6)After a transaction commits, the changes it has made to the database persist, even if there are system failures ? Multiple transactions can execute in parallel
7) Two main issues to deal with:
a)Failures of various kinds, such as hardware failures and system crashes
b) Concurrent execution of multiple transactions
Concurrent Executions
Whenever multiple users or programs access a data base concurrently, the problem of concurrency control arises. The problem is to synchronize concurrent interactions so that each reads consistent data from the data base, writes consistent data, and is ultimately processed to completion.Concurrency control in DBMS, other transactional objects, and related distributed applications ensures that database transactions are performed concurrently without violating the data integrity of the respective databases. Thus concurrency control is an essential element for correctness in any system where two database transactions or more, executed with time overlap, can access the same data.
Advantages are: a) increased processor and disk utilization, leading to better transaction throughput: one transaction can be using the CPU while another is reading from or writing to the disk
b) reduced average response time for transactions: short transactions need not wait behind long ones.
Concurrency control schemes – mechanisms to achieve isolation; that is, to control the interaction among the concurrent transactions in order to prevent them from destroying the consistency of the database
Schedules
Schedule – a sequences of instructions that specify the chronological order in which instructions of concurrent transactions are executed
a)a schedule for a set of transactions must consist of all instructions of those transactions
b) must preserve the order in which the instructions appear in each individual...
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