What is a transaction?
a single logical unit of work that must be entirely completed or aborted.
Name the transaction properties
- Atomicity
- Consistency
- Isolation
- Durability
define Atomicity.
all database operations (sql requests) of a transaction must be entirely completed or entirely aborted.
Define Consistency
it must take the database from one consistent state to another
Define Isolation
- it must not interfere with other concurrent transactions.
- Data used during execution of a transaction cannot be used by a second transaction until the first one is completed.
Define Durability
Once completed the changes the transaction made to the data are durable, even in the event of system failure.
What is serializability
- ensures that the schedule for the concurrent execution of several translations should yield consistent results.
What is a transaction log.
keeps track of all transactions in the database.
- recovery requirements triggered by Rollback Statement.
- Program’s abnormal termination.
- System Failure.
what is Concurrency Control.
A method used to manage simultaneous operations without conflicts.
Three Main problems associated with Concurrency Control
- Lost update
- Uncommitted data.
- Inconsistency Retrievals
Define lost update.
- Occurs in two concurrent transactions when : same data element is updated and one of the updates is lost.
Define Uncommitted data.
- occurs when 2 transactions are executed concurrently, first transaction is rolled back after the second transaction has accessed uncommitted data.
Define Inconsistency Retrievals
- occurs when a transaction accesses data before and after one or more other transactions finish working with such data.
