Important Computer Awareness Materials for IBPS Exams 2017 (Day-7)

Important Computer Awareness for IBPS Exams 2017 (Day-7) – DBMS Part-7:
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Database Management Systems (DBMS) – PART 7

ACID Properties in DBMS:

A transaction is a single logical unit of work which accesses and possibly modifies the contents of a database. Transactions access data using read and write operations.
In order to maintain consistency in a database, before and after transaction, certain properties are followed. These are called ACID properties.
A- Atomicity
C- Consistency
I- Isolation
D- Durability
1. Atomicity:
This update to the database is called a transaction and it either commits or aborts. This means that only a fragment of the update cannot be placed into the database, should a problem occur with either the hardware or the software involved. Features to consider for atomicity:
·        a transaction is a unit of operation – either all the transaction’s actions are completed or none are
·        atomicity is maintained in the presence of deadlocks
·        atomicity is maintained in the presence of database software failures
·        atomicity is maintained in the presence of application software failures
·        atomicity is maintained in the presence of CPU failures
·        atomicity is maintained in the presence of disk failures
·        atomicity can be turned off at the system level
·        atomicity can be turned off at the session level
2. Consistency:
Consistency is the ACID property that ensures that any changes to values in an instance are consistent with changes to other values in the same instance. A consistency constraint is a predicate on data which serves as a precondition, post-condition, and transformation condition on any transaction.
3. Isolation:
The isolation portion of the ACID Properties is needed when there are concurrent transactions. Concurrent transactions are transactions that occur at the same time, such as shared multiple users accessing shared objects. This situation is illustrated at the top of the figure as activities occurring over time. The safeguards used by a DBMS to prevent conflicts between concurrent transactions are a concept referred to as isolation.
Example:
An important concept to understanding isolation through transactions is serializability. Transactions are serializable when the effect on the database is the same whether the transactions are executed in serial order or in an interleaved fashion. As you can see at the top of the figure, Transactions 1 through Transaction 3 are executing concurrently over time. The effect on the DBMS is that the transactions may execute in serial order based on consistency and isolation requirements. If you look at the bottom of the figure, you can see several ways in which these transactions may execute. It is important to note that a serialized execution does not imply the first transactions will automatically be the ones that will terminate before other transactions in the serial order.
Degrees of Isolation:
·        degree 0 – a transaction does not overwrite data updated by another user or process (“dirty data”) of other transactions
·        degree 1 – degree 0 plus a transaction does not commit any writes until it completes all its writes (until the end of transaction)
·        degree 2 – degree 1 plus a transaction does not read dirty data from other transactions
·        degree 3 – degree 2 plus other transactions do not dirty data read by a transaction before the transaction commits.
4. Durability:
 Maintaining updates of committed transactions is critical. These updates must never be lost. The ACID property of durability addresses this need. Durability refers to the ability of the system to recover committed transaction updates if either the system or the storage media fails. Features to consider for durability:
·        recovery to the most recent successful commit after a database software failure
·        recovery to the most recent successful commit after an application software failure
·        recovery to the most recent successful commit after a CPU failure
·        recovery to the most recent successful backup after a disk failure
·        recovery to the most recent successful commit after a data disk failure 
ONE LINER:
1. Transaction Management component takes care of:Atomicity
2. Railway Reservation System comes under:Isolation
3. Concurrency Control Component takes care of:Isolation
4. Recovery Management Component takes care of:Durability
5. Durability talks about:Data secure