1 ) a transaction is a logical unit of work each transaction is sequ.pdf
- 1. 1 ) a transaction is a logical unit of work each transaction is sequence of logical related commands that acomplish one task and transforms the database from one consistent state into another .transactions are atomic in other words sysbase iq executes all the statements within the transaction as unit at the end of each transaction changes can be commited to make them permanent for any reason if transactions do not process properly then some or all other intermediate changes can be undone or rolled back and the user application controls under which the changes are commited or rolled back . 4) transaction has four properties ie atomicity ,consistency , isolation and durability ie atomicity - requires that all parts of transaction must be completed or transactions is aborted and this property ensures that the database remains in a consistent state consistency- indicates the permenance of database in the consistency state . isolation - means that the data required by an executing transaction cannot be accessed by any other transactions until the first transaction finishes this propert ensures the data consistency for concurrently executing transactions. durability - indicates that the database will be in permanent consistent state after the execution of transaction 5) serialiazability- means that the series of concurrent transactions will yield the same result as if they were executed one after another . 2) consistent databse state is one in which all data integrity constraints are satisfied to achieve a consistence database state a transaction must take the database from one consistent state to another . 3) the databse 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 the database exists that the referenced attributes exists in the selected tables , that the attribute data type are correct and so on unfortunately the dbms is not designed to guarantee that the syntacticaly correct transaction actually represent the real world events . 6) transaction log is a special dbms table that contains description of all the database transactions executed by the dbms . the database transaction log plays a crucial role in maintaining database concurrency control and integrity .the information stored in the log is used by the dbms to recover the database after the transaction is aborted or after system failure the transaction log is stored in different hard disks or in different media tape to prevent the failure caused by a media error . 7) the scheduler is the dbms concept that establish the order in which concurrent database operations are executed . the schedular interleaves the execution of the database operations to ensure the serialiazability of transactions in other woords the schedular guarantees that the
- 2. execution of concurrent transactions will yield the same results as though the transactions were executed one after the another .the scheduler is important because it is dbms component that will ensure transaction serializability and it also allows the concurrent execution of transactions giving end users impression that they are dbms users only. 8) a lock is a mechanism used in concurrency control to guarantee the exclusive use of data element to the transaction that owns the lock for eg if the data element is x is currently locked by the transaction t1 and transaction t2 will not have the access to the data element x untill t1 releases its lock .and a data item can only be in two states ie locked or unlocked . to access data element x t1 must request a lock to the dbms and if the data element x is not in use then the dbms will lock x to be used by t1 exclusively no other transaction will have access to x while t1 is executed . Solution 1 ) a transaction is a logical unit of work each transaction is sequence of logical related commands that acomplish one task and transforms the database from one consistent state into another .transactions are atomic in other words sysbase iq executes all the statements within the transaction as unit at the end of each transaction changes can be commited to make them permanent for any reason if transactions do not process properly then some or all other intermediate changes can be undone or rolled back and the user application controls under which the changes are commited or rolled back . 4) transaction has four properties ie atomicity ,consistency , isolation and durability ie atomicity - requires that all parts of transaction must be completed or transactions is aborted and this property ensures that the database remains in a consistent state consistency- indicates the permenance of database in the consistency state . isolation - means that the data required by an executing transaction cannot be accessed by any other transactions until the first transaction finishes this propert ensures the data consistency for concurrently executing transactions. durability - indicates that the database will be in permanent consistent state after the execution of transaction 5) serialiazability- means that the series of concurrent transactions will yield the same result as if they were executed one after another . 2) consistent databse state is one in which all data integrity constraints are satisfied to achieve a consistence database state a transaction must take the database from one consistent state to another . 3) the databse is designed to verify the syntactic accuracy of the database commands given by
- 3. the user to be executed by the dbms the dbms will check the database exists that the referenced attributes exists in the selected tables , that the attribute data type are correct and so on unfortunately the dbms is not designed to guarantee that the syntacticaly correct transaction actually represent the real world events . 6) transaction log is a special dbms table that contains description of all the database transactions executed by the dbms . the database transaction log plays a crucial role in maintaining database concurrency control and integrity .the information stored in the log is used by the dbms to recover the database after the transaction is aborted or after system failure the transaction log is stored in different hard disks or in different media tape to prevent the failure caused by a media error . 7) the scheduler is the dbms concept that establish the order in which concurrent database operations are executed . the schedular interleaves the execution of the database operations to ensure the serialiazability of transactions in other woords the schedular guarantees that the execution of concurrent transactions will yield the same results as though the transactions were executed one after the another .the scheduler is important because it is dbms component that will ensure transaction serializability and it also allows the concurrent execution of transactions giving end users impression that they are dbms users only. 8) a lock is a mechanism used in concurrency control to guarantee the exclusive use of data element to the transaction that owns the lock for eg if the data element is x is currently locked by the transaction t1 and transaction t2 will not have the access to the data element x untill t1 releases its lock .and a data item can only be in two states ie locked or unlocked . to access data element x t1 must request a lock to the dbms and if the data element x is not in use then the dbms will lock x to be used by t1 exclusively no other transaction will have access to x while t1 is executed .