Introduction to Database Management- Wiley Pathways ISBN: 9780470101865 Answer t

Question

Introduction to Database Management- Wiley Pathways ISBN: 9780470101865

Answer the questions using complete sentences; I need to write an essay on these questions.

List and describe the elements of the acronym ACID. (see info pg 341)

List and describe four types of concurrency problems that result from processing concurrent transactions. (see info pg 333-334)

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10.1.4 UNDERSTANDING TRANSACTION SCOPE 341 10.1.3 Understanding Transaction Properties An abbreviation that you will commonly see when discussing transactions is ACID. This stands for atomicity, consistency, isolation, and durability. Let's look at each of these separately Atomicity: either all of the statements in the transaction are executed, or none of the statements are executed, An atomic transaction represents a complete unit of work A Consistency: the transaction preserves database consistency. If a transac- tion executes successfully and performs its database updates properly, the database is transformed from one consistent state to another consistent state A Isolation: a transaction must appear as though it executed by itself with out interference from any other concurrent transaction. In practice, how ever, many concurrent database transactions execute simultaneously. Durability: changes made to the database must be permanent. Even if and when there are subsequent tailures, the effects must persist in the database. One way this is ensured is through the transaction log and the recovery process after a lallure, as discussed earlier. Factors relating to isolation deserve a closer look. To ensure complete isola- tion, the DBMS would not be able to process any other transactions while the current transaction is running. This could have disastrous effects on perlormance while transactions build up in a queue, waiting for other transactions to com plete. For this reason, operations of one transaction are interleaved with opera tions of other transactions. However, the database system must guarantee the effects of database updates for every pair of concurrent transactions to be as though one transaction executed from beginning to end and then the second started, or vice versa. Transactions must execute independently, and intermedi- ary or partial results of incomplete transactions must be transparent to other transactions 10.1.4 Understanding Transaction Scope Transaction scope support and scope options are somewhat DBMS-specific, Two scope options, implicit and explicit transactions, are supported by most DBMSs to varying degrees. Another scope supported by some SQL management systems, including SQL Server, is autocommit transactions. Explicit Transactions Well start with explicit transactions because they're the easiest to understand. With an explicit transaction, the entire transaction life cycle is controlled

Explanation / Answer

ACID is a set of properties that you would like to apply when modifying a database.

1.Atomicity

The phrase "all or nothing" succinctly describes the first ACID property of atomicity. When an update occurs to a database, either all or none of the update becomes available to anyone beyond the user or application performing the update. 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:

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.

As an example, if two people are updating the same catalog item, it's not acceptable for one person's changes to be "clobbered" when the second person saves a different set of changes. Both users should be able to work in isolation, working as though he or she is the only user. Each set of changes must be isolated from those of the other users.

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 isolation1:

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:

As an example of what ACID might mean when a printer is involved in a transaction:

Durability: the printer can guarantee that it was not "printing" with empty cartridges.

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