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Public Class BuilderBinder Public _coordinator1 As Coordinator1 Public _worker1 As Worker1 Public _router As Router Public _coordinator2 As Coordinator2 Public _worker2 As Worker2 Public Sub Build() _coordinator1 = New Coordinator1 _worker1 = New Worker1 _router = New Router _coordinator2 = New Coordinator2 _worker2 = New Worker2 End Sub Public Sub Bind() AddHandler _coordinator1.OnDoSomething, AddressOf _router.DoSomething AddHandler _coordinator1.OnMethod1, AddressOf _worker1.Method1 AddHandler _coordinator1.OnMethod2, AddressOf _worker1.Method2 AddHandler _router.OnDoSomething, AddressOf _coordinator2.DoSomething AddHandler _router.OnNowDoThis, AddressOf _coordinator1.NowDoThis AddHandler _coordinator2.OnNowDoThis, AddressOf _router.NowDoThis AddHandler _coordinator2.OnMethod1, AddressOf _worker2.Method1 AddHandler _coordinator2.OnMethod2, AddressOf _worker2.Method2 End Sub End Class Public Class Worker1 Public Sub Method1() End Sub Public Sub Method2() End Sub End Class Public Class Worker2 Public Sub Method1() End Sub Public Sub Method2() End Sub End Class The Router implementation contains only inputs that are wired directly to outputs. Routers often have no other logic. If you opt to add logic to a Router, the logic should only be related to notification routing. For example, a Router serving as a bridge across a remote connection may use a high-speed connection by default, but fall back on an alternate and slower connection if problems arise with the fast connection.

Practical real-world databases need a compromise between concurrency access and serializable modes of operation. The key issue here is that by specifying a high degree of isolation, you can keep one transaction from affecting another, but at the cost of significant deterioration in database

performance. On the other hand, a low level of transaction isolation will introduce the data problems outlined earlier in the chapter, but it leads to better performance. A transaction running at a serializable isolation level will appear as if it s running in isolation it s as if all the other concurrent transactions run either before or after this transaction. Three of the four main ISO isolation levels allow for some deviation from the theoretical concept of serializable transactions. Table 8-1 shows the extent to which each of the four main levels of isolation suffers from the concurrency problems listed earlier. Note that a value of Yes in the table means that the particular problem is possible under that isolation level, and a value of No means that the problem isn t possible for that isolation level.

As you can see, the last isolation level in Table 8-1, serializable, avoids all concurrency problems, but unfortunately, it s not a practical option because it doesn t allow any concurrent use of the database. Oracle s default read-committed isolation level will get rid of the dirty-read and the lostupdate problems. You won t have the dirty-read problem because your queries will read only data that was committed at the beginning of the query, thereby avoiding reading data that may later be rolled back by a different transaction. In addition, you ll avoid the lost-update problem because transactions can t read data that s currently being modified until the updates have been completed.

Summary

Oracle automatically provides statement-level read consistency by default. That is, all data that a query sees comes from a single point in time. This means that a query will see consistent data when it begins. The query sees only data committed before it starts, and no data committed during the course of the query is visible to it. Queries in this context don t have to be SELECT statements. An INSERT with a subquery or an UPDATE or DELETE will also involve an implicit query, and they all return consistent data. Oracle can also provide transaction-level read consistency, though this is not the default. Oracle can use pre-change data images stored in undo segments to provide the transaction- and statementlevel read consistency.

Oracle s read-committed level of isolation provides protection against dirty reads and lost updates because queries read data only after the COMMIT statement is executed. The transactions are all consistent on a per-statement basis. Readers will not block writers of transactions, and vice versa. As you can see, Oracle s default read-committed isolation level doesn t guarantee you ll avoid the nonrepeatable-read and phantom-read problems. Oracle guarantees only statement-level, not transaction-level, read consistency. However, Oracle allows you to explicitly change the default read-committed isolation level by selecting the serializable isolation level as an alternative.