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This is generally considered the most restrictive level of transaction isolation, but it provides the highest degree of isolation. A SERIALIZABLE transaction operates in an environment that makes it appear as if there are no other users modifying data in the database. Any row we read is assured to be the same upon a reread, and any query we execute is guaranteed to return the same results for the life of a transaction. For example, if we execute Select * from T; Begin dbms_lock.sleep( 60*60*24 ); end; Select * from T; the answers returned from T would be the same, even though we just slept for 24 hours (or we might get an ORA-01555, snapshot too old error, which is discussed in 8). The isolation level SERIALIZABLE assures us these two queries will always return the same results. Side effects (changes) made by other transactions are not visible to the query regardless of how long it has been running. In Oracle, a SERIALIZABLE transaction is implemented so that the read consistency we normally get at the statement level is extended to the transaction.

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Ensures that the associated input control is not empty. Validates that the user input is in a given range. Checks whether the user input matches a regular expression pattern. See 10 for a description of using .NET regular expressions. Compares the user input against a given value or another control. Validates the user input based on a custom, user-specified logic. Lists all validation error messages.

ONLY. It has all of the qualities of the SERIALIZABLE isolation level, but it prohibits modifications. It should be noted that the SYS user (or users connected as SYSDBA) can t have a READ ONLY or SERIALIZABLE transaction. SYS is special in this regard.

Instead of results being consistent with respect to the start of a statement, they are preordained at the time you begin the transaction. In other words, Oracle uses the undo segments to reconstruct the data as it existed when our transaction began, instead of just when our statement began. That s a pretty deep thought there: the database already knows the answer to any question you might ask it, before you ask it. This degree of isolation comes with a price, and that price is the following possible error: ERROR at line 1: ORA-08177: can't serialize access for this transaction You will get this message whenever you attempt to update a row that has changed since your transaction began.

Each validator has an ErrorMessage and a Text property that you can set in conjunction with displaying an error message. When the validator fails, it displays the content of the Text property at the place where it is inserted in your page markup. A typical scenario is ensuring that a given user input control has a value entered. Here, all you need to do is add your input control and a RequiredFieldValidator next to it, set its ControlToValidate property to the ID of the associated input control, and set the Text property to say *. This will show an asterisk when no value is entered in the input control at the time of submitting the page. Using another validator is just as easy, except you need to customize different properties. Table 14-7 summarizes these main properties for each validator.

Note Oracle attempts to do this purely at the row level, but you may receive an ORA-08177 error even when the

row you are interested in modifying has not been modified. The ORA-08177 error may happen due to some other row(s) being modified on the block that contains your row.

RequiredFieldValidator RangeValidator RegularExpressionValidator CompareValidator CustomValidator ValidationSummary

Oracle takes an optimistic approach to serialization it gambles on the fact that the data your transaction wants to update won t be updated by any other transaction. This is typically the way it happens, and usually the gamble pays off, especially in quick-transaction, OLTP-type systems. If no one else updates your data during your transaction, this isolation level, which will generally decrease concurrency in other systems, will provide the same degree of concurrency as it would without SERIALIZABLE transactions. The downside to this is that you may get the ORA-08177 error if the gamble doesn t pay off. If you think about it, however, it s worth the risk. If you re using SERIALIZABLE transactions, you shouldn t expect to update the same information as other transactions. If you do, you should use the SELECT ... FOR UPDATE as described in 1 Developing Successful Oracle Applications , and this will serialize the access. So, using an isolation level of SERIALIZABLE will be achievable and effective if you: Have a high probability of no one else modifying the same data. Need transaction-level read consistency. Will be doing short transactions (to help make the first bullet point a reality).