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IsolationLevel Enumeration..???

Posted on 2004-07-31
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I don't understand about all of property of IsolationLevel Enumeration.....I see it on MSDN but...I can't understand....so can you explain for me about these property and Examples about these property.....
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Question by:sonbinary
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by:gregoryyoung
ID: 11688653
what exactly do you not understand about it ? MSDN has a fairly clear explanation on the differences here http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfSystemDataIsolationLevelClassTopic.asp. Perhaps a more specific question on what needs clarification ?
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by:sonbinary
ID: 11691782
Yes....I want more clarification for those explain.....I want a compare with those property so I can understand its....Can you give me some example case adn situation for that property..???
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gregoryyoung earned 500 total points
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These are mostly the ANSI defined isolation levels

Chaos
 The pending changes from more highly isolated transactions cannot be overwritten.
this is fairly obvious

ReadCommitted
 Shared locks are held while the data is being read to avoid dirty reads, but the data can be changed before the end of the transaction, resulting in non-repeatable reads or phantom data.
You only see committed data

ReadUncommitted
 A dirty read is possible, meaning that no shared locks are issued and no exclusive locks are honored.
you read the data whether or NOT it has been committed

RepeatableRead
 Locks are placed on all data that is used in a query, preventing other users from updating the data. Prevents non-repeatable reads but phantom rows are still possible.
places a Write lock on the row being modified so that no other database user can update the row in the database (aka pessimistic locking during your transaction)

Serializable
 A range lock is placed on the DataSet, preventing other users from updating or inserting rows into the dataset until the transaction is complete.
this would be used during a process such as serialization so that when you are serializing out the data noone else can change the data ... this allows you to make whats called a snapshot of the data. This is needed due to the fact that after I write out table1 if you insert into table2 which has a p->c relationship with table1 and then insert the parent into table1 when I get to table2 my output will be erroneous as the relationship constraint would not have been met


Unspecified
 A different isolation level than the one specified is being used, but the level cannot be determined.
I do not think that this needs much more explanation .....

further explanation is given here
http://media.datadirect.com/download/docs/odbc/odbcref/locking.html
http://www.winnetmag.com/SQLServer/Article/ArticleID/5336/5336.html
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