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tarting with this chapter, I ll switch gears and look at some complete case studies of event-based systems designed with decoupled parts. If you re like me and enjoy looking at code, you should find these case studies interesting, because they bring together many of the ideas and concepts introduced earlier. The first example is a small desktop system browser, conceptually similar to Microsoft Windows Explorer. The program, called SystemBrowser, consists of only one component and allows users to browse the file system and search for files. The main purpose of the example is to show how you might use decoupled classes to build a program with a user interface. The program demonstrates the following tasks: Using workers and coordinators to build a GUI program Managing the startup and shutdown phases using LifecycleCoordinators Saving and retrieving persistent user settings Updating a splash screen during initialization Using a coordinator to manage multithreading Unit-testing workers with dedicated test fixtures While you can develop all the system s design and concepts in any programming language that supports events, the implementation details will vary somewhat across operating systems and component platforms. I ll provide the complete source code in both C# and VB .NET.

Serializable transactions are more prone to deadlocks, a situation in which transactions are stuck waiting for each other to release locks over data objects Deadlocks lead to costly rollbacks of transactions In general, it s safest to stick with Oracle s default read-committed level of transaction isolation, although it isn t perfect Oracle recommends that you stick with the default read-committed level of isolation, which produces the maximum throughput with only a slight risk of running into the nonrepeatable-read and phantom-read anomalies The read-committed transaction level provides a good trade-off between data concurrency and data consistency Also, the throughput is much higher with this mode of isolation than with the purer serialization mode If getting a repeatable read is your objective in using a serializable isolation level, you can always use explicit locks in situations where that is necessary.

For standard OLTP applications, in particular, with their high-volume, concurrent, short-lived transactions that are unlikely to conflict with each other, this mode is ideal from a performance point of view Very few transactions in an OLTP database issue the same query multiple times, so phantom reads and nonrepeatable reads are rare Serializable modes of concurrency are more appropriate for databases with mostly read-only transactions that run for a long time..

A database may use one or more methods to implement concurrency of use These include locking mechanisms to guarantee exclusive use of a table by a transaction, time-stamping methods that enable serialization of transactions, and the validation-based scheduling of transactions Locking methods are called pessimistic because they assume that transactions will violate the serializable schedules unless they re prevented explicitly from doing so The time-stamping and validation methods, on the other hand, are called optimistic because they don t assume that transactions are bound to violate the serializable schedules As you might guess, locking methods cause more delays than the optimistic methods because they keep conflicting transactions waiting for access to the locked database objects On the positive side, however, locking methods don t have to abort transactions because they prevent potentially conflicting transactions from interacting with other transactions.

The program needs to support a two pane-window with a navigation pane on the left side and a content pane on the right. The navigation pane must have two different navigator controls: a Folders navigator and a Search navigator. Only one must be visible at a time. The Folders navigator shows a hierarchical list of folders on the C: drive. The Search navigator contains controls to search for files. The right pane must display information related to the selection in the left pane. Figure 11-1 shows what the program should look like when the Folders navigator is selected.

The optimistic methods usually have to abort transactions when they might violate a serializable schedule Time-stamping methods assign time stamps to each transaction and enforce serializability by ensuring that the transaction time stamps match the schedule for the transactions Validation methods maintain a record of transaction activity Before committing a transaction, the database validates the changed data against the changed items of all currently active transactions to eliminate any unserialized schedules Oracle uses a combination of the available methods It uses locks along with what is called the multiversion concurrency control system (a variation of the time-stamping method) to manage concurrency Oracle locks prevent destructive interaction between transactions that are trying to access the same resource The resource could be an application table or row, or it could be a shared system data structure in memory It could also be a data dictionary table or row.