Transaction:
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Transaction is a unit of program execution that access and possibly updates various data items.
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A Transaction must see a consistence data base.
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During transaction execution the database may be temporarily inconsistent.
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when the transaction complete successfully (is committed ), the database must be consistent.
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After transaction commits, the change it has made to the database persist. even if there are system failures.
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Multiple transaction can execute in parallel.
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Transaction boundaries: begin/end.
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There are many types of transaction e.g. read only transaction, read-write transaction.
Example:
Move $45 from checking to saving a/c
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To user, transaction appear as one activity as follows.
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To database, it is a series of activity as follows:
Read balance to checking a/c: Read(X).
Read balance if saving a/c: Read(Y).
Subtract $45 to Y.
Write new value of X back to disk.
write new value of Y back to disk.
Basic access operations of transaction:
A transaction can have two basic functions (operation):
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read_item(x): Read a database item named x into a program variable.
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write_item(x): Write the value of program variable x in to database item named x.
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Read_Set(x): All items that transaction reads.
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Write_Set(x): All items that transaction writes.
Transaction State:
There are five state of a transaction, that are
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Active:- The initial state , the transaction stays in this state while it is executing.
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Partially Committed:- After the final statement has been executed.
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Failed:- After discovery that normal execution can no longer proceed.
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Aborted:- After the transaction has been rolled back and the database restore to its state prior to the start of the transaction. Two options after it has been aborted:
Restart the transaction: can be done only no internal logical error.
Kill the transaction. -
Committed:- after successfully completion.
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