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Tuesday, September 23, 2014

What is concurrency control with respect to databases?

Concurrency control is a technique applied in the fields of operating systems, databases, computer programming, and multi processing etc. for ensuring that concurrent operations produce the correct results in as less time as possible. Both the hardware and the software parts of the computer are made up of smaller modules and components where each of them is designed and programmed respectively to work correctly according to some consistency rules. There is concurrent interaction between these components through messages, shared data, which can lead to a result which is in violation of those rules.
The basic idea of concurrency control is to provide methodologies, theories and rules for enforcing consistency in the whole system. Implementation of concurrency control reduces the performance because we apply some constraints on the components which does have the effect of reducing the overall speed. However one thing that should be taken care of is to achieve consistency with as much efficiency as possible and without reducing the performance below minimum levels.
The drawbacks of concurrency control include additional complexity and generation of more overhead in using a concurrent algorithm. The concurrent algorithms generate more overhead when compared to their sequential algorithm counterparts. If the concurrency control mechanism fails, it can lead to torn read and write operations and can corrupt the data.
In this article we talk about concurrency with respect to the databases. Concurrency control is implemented in DBMS, distributed applications etc. for ensuring that the concurrent data transactions are accomplished without causing damage to the data integrity. Distributed applications include cloud computing and grid computing. Concurrency control is also used in some other transactional objects.
It is an essential part of the systems where two or more transactions overlap over the same time instant and can operate on the same data. This happens in almost any general purpose database management system.
Since the advent of database systems, research has been going on related to this concept. Serializability theory is the best established theory that helps define the concept of concurrency control.  This theory also lets us in designing as well as analyzing the concurrency control methods as well as mechanism as effectively as possible.  There is another theory that does not emphasize upon concurrency control over the abstract data types but rather over atomic transactions. However this theory though having a wider scope and more refined, it adds more complexity to the system. Both the theories have their advantages and disadvantages. Merging these two theories might help because they are complementary to some extent.
For ensuring proper concurrency control and correct execution of the transactions, only the serializable transactions and schedules are generated by the system and executed. In some cases, the serializability might be relaxed intentionally by the system for increasing the performance. But, this is done only in those cases where it won’t generate incorrect output.
There are many cases when the transactions fail. Here the system needs to have a recoverability property for recovering from the damage. A good database system also ensures that the result of the transactions that have committed is not lost if the system is switched off accidentally or crashes. On the other hand it also ensures that the incomplete results of the aborted transactions are erased and the actions are rolled back. The ACID rules (mentioned below) characterize the transactions:
- Atomicity: Each thread consists of a single transaction.
- Consistency: This characteristic depends largely on user.
- Isolation: Every transaction should be executed in isolation i.e., should not interfere with others.
 - Durability: The results of the committed actions should persist.
 Nowadays as, database systems are becoming more distributed, the focus is more upon the distribution of the concurrency control mechanism.  

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