Transaction Managment Flashcards
How layers has DBMS have
3
The inner most layer ( the kernel ) concerns itself with:
Hardware failure
Software failure
Consistency of the database
Transaction management is further subdivided into:
Transaction support
Concurrency control
Database recovery
Concurrency and recovery control required:
Are required to protect the database from inconsistencies and data loss
Concurrency allow DBMS’s what
Many DBMSs allow users to carry out operations on the database
Concurrency , If the operations are not controlled ,what?
database will become inconsistent
Recovery, mean
Eg.
Explain
Is the process of restoring the database to a correct state following a failure
Ex. A failure may be a result of:
A system crash
A software error
Whatever the reason, DBMS must be able to recover from the failure and restore the database to a consistent state
Consider an application program which processes the sale of stock
Contains a several database operations
Is a read write transaction data is being updated
What transaction
Transaction might be what
Is a logical unit of work on the database
Might be :
An entire computer application
A single command such as select , insert , update , delete
Transaction outcome
Successful:
Transaction completes successfully
Transaction is said to have committed
Transaction unsuccessful
Transaction does not complete successfully
Transaction is aborted
SQL COMMIT statement
Statement makes transaction updates permanent
Signifies a successful transaction
If it is decided that a committed transaction was a mistake :
Another transaction to reverse its effects must be performed
Revert to database backup files
SQL ROLLBACK statement
Statement undoes any transaction updates made since the start of the transaction
Signifies an unsuccessful transaction
Transaction properties
Atomicity
Consistency
Isolation
Durability
Reliability and consistency are compromised when:
Software and hardware failures occur
Must be maintained at all times
Explain Atomicity
All or nothing property
Either the entire transaction is performed or it is not performed at all
If one operation fails, all must be undone
Explain consistency
All transactions must preserve the consistency and integrity of the database.
Isolation mean
Transaction execute independently of one another
Durability
When transaction completes , the updates made by the transaction must be durable
Serialisability mean
Example
Ensures that concurrent execution of several transactions yields consistent results
Ex. T1 = T2 = T3 = result
T2 = T3 = T1 = result
T3 = T2 = T1 = result
What are high level database modules of DBMS Architecture (4)
Transaction
Concurrency
Recovery
Control
Transaction manager
It communicates with the scheduler, the module responsible for implementing a particular strategy for concurrency control
Recovery manager
Ensure that the database is restored to the state it was in at the start of the transaction
Buffer manager
Is responsible for the transfer of data between disk storage and main memory
Concurrency control
Is a process of managing simultaneous operations on the database without having them interfere with each other
What are the three potential problems caused by concurrency
Lost update
Uncommitted dependency
Inconsistent analysis
Explain lost update problem
Successfully completed update operation by one transaction can be overridden by another transaction
Lost update, how can we prevent this update from being lost?
The loss of T2s update could be avoided by preventing T1 from reading the account balance until after T2s update has been completed
Uncommitted dependency problem
occurs when one transaction is allowed to see the results of another transaction before it has committed
Inconsistent analysis problem
Occurs when a transaction reads several values but a second transaction updates some of them during the execution of the first transaction
Transactions that only read the database can also sometimes produce inconsistent
Serialisability means
Is a means of identifying those transactions which are guaranteed to ensure consistency
Schedule
A transaction consists of a sequence of reads and writes to the database
Serial Schedule
Schedule where the operations of each transaction are executed consecutively without any operations from other transactions
Nonserial schedule
A schedule where the operations from a set of concurrent transactions are interleaved
Transaction log
transaction log is a sequential record of all changes made to the database while the actual data is contained in a separate file.
Relational data model
Nb
