barcode generator vb.net Understanding the Nature of Uncommitted Reads in Software

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Understanding the Nature of Uncommitted Reads
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Uncommitted Read, or dirty read, lets your transaction read any data that is currently on a data page, whether or not that data has been committed. For example, another user might have a transaction in progress that has updated data, and even though it is holding exclusive locks on the data, your transaction can read it anyway. The other user might then decide to roll back his or her transaction, so logically those changes were never made. If the system is a single-user system and everyone is queued up to access it, the changes would never have been visible to other users. In a multiuser
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system, however, you read the changes and possibly take action on them. Although this scenario is not desirable, with Uncommitted Read you will not get stuck waiting for a lock, nor will your reads issue share locks (described below) that might affect others. When using Uncommitted Read, you give up assurances of having strongly consistent data so that you can have the benefit of high concurrency in the system without users locking each other out. So when should you choose Uncommitted Read Clearly, you do not want to choose it for financial transactions in which every number must balance. But it might be fine for certain decision-support analyses for example, when you look at sales trends for which complete precision is not necessary and the tradeoff in higher concurrency makes it worthwhile.
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Contrasting Committed with Uncommitted Reads
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Committed Read is SQL Server s default isolation level. It ensures that an operation will never read data that another application has changed but not yet committed. (That is, it will never read data that logically never existed.) With Committed Read, if a transaction is updating data and consequently has exclusive locks on data rows, your transaction must wait for those locks to be released before you can use that data (whether you are reading or modifying). Also, your transaction must put share locks (at a minimum) on the data that will be visited, which means that data is potentially unavailable to others to use. A share lock does not prevent others from reading the data, but it makes them wait to update the data. Share locks can be released after the data has been sent to the calling client they do not have to be held for the duration of the transaction.
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TIP: Although you can never read uncommitted data, if a transaction running with Committed Read isolation subsequently revisits the same data, that data might have changed or new rows might suddenly appear that meet the criteria of the original query. Rows that appear in this way are called phantoms.
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The Repeatable Read isolation level adds to the properties of Committed Read by ensuring that if a transaction revisits data or if a query is reissued, the data will not have changed. In other words, issuing the same query twice within a transaction will not pick up any changes to data values made by another user s transaction. Preventing nonrepeatable reads from appearing is a desirable safeguard. But there is no free lunch. The cost of this extra safeguard is that all the shared locks in a transaction must be held until the completion (COMMIT or ROLLBACK) of the transaction. (Exclusive locks must always be held until the end of a transaction, no matter what the isolation level, so that a transaction can be rolled back if necessary. If the locks were released sooner, it might be impossible to undo the work.) No other user can modify the data visited by your transaction as long as your transaction is outstanding. Obviously, this can seriously reduce concurrency and degrade performance. If transactions are not kept short or if applications are not written to be aware of such potential lock contention issues, SQL Server can appear to hang when it is simply waiting for locks to be released.
TIP: You can control how long SQL Server waits for a lock to be released by using the session option LOCK_TIMEOUT.
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