INF3703 EXAM ANSWERS PACK

INF3703Answers to Review Questions 1. Explain the following statement: a transaction is a logical unit of work. A transaction is a logical unit of work that must be entirely completed of aborted; no intermediate states are accepted. In other words, a transaction, composed of several database requests, is treated by the DBMS as a unit of work in which all transaction steps must be fully completed if the transaction is to be accepted by the DBMS. Acceptance of an incomplete transaction will yield an inconsistent database state. To avoid such a state, the DBMS ensures that all of a transaction's database operations are completed before they are committed to the database. For example, a credit sale requires a minimum of three database operations: 1. An invoice is created for the sold product. 2. The product's inventory quantity on hand is reduced. 3. The customer accounts payable balance is increased by the amount listed on the invoice. If only parts 1 and 2 are completed, the database will be left in an inconsistent state. Unless all three parts (1, 2, and 3) are completed, the entire sales transaction is canceled. 2. What is a consistent database state, and how is it achieved? A consistent database state is one in which all data integrity constraints are satisfied. To achieve a consistent database state, a transaction must take the database from one consistent state to another. (See the answer to question 1.) 3. The DBMS does not guarantee that the semantic meaning of the transaction truly represents the real-world event. What are the possible consequences of that limitation? Give an example. The database is designed to verify the syntactic accuracy of the database commands given by the user to be executed by the DBMS. The DBMS will check that the database exists, that the referenced attributes exist in the selected tables, that the attribute data types are correct, and so on. Unfortunately, the DBMS is not designed to guarantee that the syntactically correct transaction accurately represents the realworld event. For example, if the end user sells 10 units of product (Crystal Vases), the DBMS cannot detect errors such as the operator entering 10 units of product (Crystal Glasses). The DBMS will execute the transaction, and the database will end up in a technically consistent state but in a realworld inconsistent state because the wrong product was updated. 4. List and discuss the five transaction properties. The five transaction properties are: Atomicity requires that all parts of a transaction must be completed or the transaction is aborted. This property ensures that the database will remain in a consistent state. Consistency Indicates the permanence of the database con EXAM ANSWERS PACK