1. What is concurrency control? State and explain the different issues that may arise in the absence of concurrency control.

2. What are the different techniques to achieve concurrency control? Summarize each of these concurrency control techniques.

1. Concurrency control is the process of managing simultaneous execution of transactions in a multiprocessing database system without having them interfere with one another. The transactions could mean different operation like queries, updates, inserts, deletes and so on.

Issues that may arise in the absence of concurrency control are:

a) The Lost update problem

b) The Temporary update (or dirty read) problem

c) The Incorrect summary problem

d) The Unrepeatable read problem

The Lost update problem: This problem occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of some database items incorrect.

The Temporary update (or dirty read) problem: This problem occurs when one transaction updates a database item and then the transaction fails for some reason. Meanwhile, the updated item is accessed (read) by another transaction before it is changed back (or rolled back) to its original value. This problem is also known as the dirty read problem.

The Incorrect summary problem: This problem occur if one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated.

The Unrepeatable Read Problem. This problem occur where a transaction T reads the same item twice and the item is changed by another transaction T′ between the two reads. Hence, T receives different values for its two reads of the same item.

2. The different techniques to achieve concurrency control are categorized into:

a) Lock-based protocol

b) Timestamp-based protocol

Lock-based protocol: Those database systems that are prepared with the concept of lock-based protocols employ a mechanism where any transaction cannot read or write data until it gains a suitable lock on it.

Timestamp-based Protocol: It is the most frequently used concurrency protocol is the timestamp based protocol. This protocol uses either system time or logical counter as a timestamp.