Term
| what is an ATOMIC TRANSACTION |
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Definition
| all transactions are performed or none are |
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Term
| what is a CONCURRENT TRANSACTION |
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Definition
| two transactions are processed at the same time but NOT simultaneously. They are not simultaneous because the CPU of the machine processing the DB can only execute one instruction at a time |
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Term
what is an EXPLICIT LOCK?
what is an IMPLICIT LOCK ? |
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Definition
explicit- locks placed by command
implicit-locks placed by the DBMS |
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Term
what is an EXCLUSIVE LOCK?
what is a SHARED LOCK |
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Definition
exclusive- locks an item from ANY other access.Can't read or write.
shared - locks an item from change but can still be read |
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Term
| what is TWO PHASED LOCKING? |
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Definition
| transactions are allowed to obtain locks as necessary but once the first lock is released no other lock can be obtained. |
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Term
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Definition
| A condition that can occur during concurrent processing in which each of two (or more) transactions is waiting to access the data that the other has locked. |
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Term
| How to prevent a DEAD LOCK |
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Definition
1. require users to issue all lock requests at the same time.
2.require all applications to lock resources in the same order. |
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Term
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Definition
| Cancel one of the transactions and remove it's changes from the database. |
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Term
| What is a LOG, why is it important? |
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Definition
a file containing a record of database changes. The log contains before images and after images.
The log is required for rollback and roll forward. |
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Term
| what is a check point, what is the advantage of taking frequent check points? |
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Definition
A point of synchronization between the database and log.
The more checkpoints you have the less processing you need to recover. |
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Term
| What is RESOURCE LOCKING? |
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Definition
| the process of allocating a database resource to a particular transaction in a concurrent processing system |
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Term
What is a before image?
What is an after image? |
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Definition
A record of a database entity BEFORE a change
A record of a database entity AFTER a change |
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Term
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Definition
| A copy of the database files that can be used to restore the database to some previous consistent state. |
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Term
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Definition
| a special type of stored procedure that is invoked bu the DBMS when a specified condition occurs. |
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Term
Define:
BEFORE trigger.
AFTER trigger.
INSTEAD OF trigger |
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Definition
triggers that are executed BEFORE a specified database action.
triggers that are executed AFTER a specified database action.
triggers that are executed INSTEAD OF a specified database action.(normally used to update data). |
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Term
| What is a STORED PROCEDURE? |
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Definition
| A collection of SQL statements stored as a file that can be invoked by a single command. |
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Term
| What is ROLL FORWARD, describe the process. |
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Definition
Used when there is physical damage.
recvoring database bu applying after-images to a saved copy of the database to bring it to a checkpoint.
STEPS:
1. replace physical property
2. put previously saved data back on
3. roll forward
4. take after images of completed transactions
5 read the after images in chronological order |
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Term
| What is ROLL BACK, describe the process |
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Definition
used when there is a power outage of interruption
before imags are applied to the DB to return to an earlier checkpoint.
STEPS:
1.look in reverse order to find incomplete transactions. (starts with out a finish)
2. reapply the before images in reverse order until you are back to where you where before the transaction
*thus replacing the incomplete transaction |
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