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ASAP Cars contains several components, some of which are in the client space and some in the server space. The client space includes components that run in the car dealer offices. The server space includes all the other components. Although this case study is relatively simple, it consists of nine components. To keep the implementation easy to follow, I ll use Visual Studio Enterprise Template Projects (ETPs) to organize the various projects. ETPs are really just containers of regular projects. ETPs show up in the Solution Explorer as nodes under the Solution node, with Project nodes inside them, as shown in Figure 13-4. In Figure 13-4, the Enterprise Template Project nodes are highlighted. Visual Studio allows you to nest these nodes recursively, and in the figure you can see that the Order Processor System template is nested inside the Server Side template. In the following sections, I ll discuss the design and implementation of each component.

Databases aren t very useful if a large number of users can t access and modify data simultaneously. Data concurrency refers to the capability of the database to handle this concurrent use by many users. To provide consistent results, the database also needs a mechanism within it that ensures users don t step on each other s changes. Data consistency refers to the ability of a user to get a meaningful and consistent view of the data, including all the changes made to it by other users.

Oracle uses special structures called undo segments to ensure data consistency. For example, when you re reading a set of data for a transaction, Oracle ensures that the data you read is transaction-set consistent; that is, it guarantees that the data you see reflects a single set of committed transactions. Oracle also provides read consistency of data, meaning that all the data selected by your queries comes from a single point in time. Oracle s undo segments are part of the undo tablespace mentioned earlier in this chapter. Oracle uses locking mechanisms to ensure data concurrency. By allowing one user to lock individual rows or entire tables, that user is guaranteed exclusive use of the table for updating purposes. An important feature of the Oracle locking mechanisms is that they are, for the most part, automatic. You don t need to concern yourself with the details of how to lock the objects you want to modify Oracle will take care of it for you behind the scenes. Oracle uses two basic modes of locking. The exclusive lock mode is used for updates, and the share lock mode is used for SELECT operations on tables. The share lock mode enables several users to simultaneously read the same rows in a table. The exclusive lock mode, because it involves updates to the table, can only be used by one user at any given time. Exclusive locks are almost always applied to the specific rows being updated, permitting simultaneous use of the database by several users. Oracle releases the locks it holds on the tables and other internal resources automatically after the issue of a COMMIT or ROLLBACK command. Oracle locking is complex, and you ll learn about it in detail in 8, along with how Oracle ensures data consistency and concurrency.

The database writer, as you saw earlier, is responsible for writing all modified buffers in the database buffer cache to the datafiles. Further, it has the responsibility of ensuring there is free space in the buffer cache so the server process can read in new data from the datafiles when necessary. The (log) write ahead protocol also requires that the redo records in the redo log buffer associated with the changed data in the data buffer cache are written to the redo logs before the changes are recorded in the datafiles. The importance of the redo log contents makes it imperative that Oracle write the contents of the redo log file to permanent storage before it writes the changes to the datafiles on disk. When users commit their transactions, the log writer process immediately writes only a single commit record to the redo log files. The entire set of records affected by the committed transaction may not be written simultaneously to the datafiles. This fast commit mechanism, along with the write ahead protocol, ensures that the database is not kept waiting for all the physical writes to be completed after each transaction. As you can well imagine, a huge OLTP database with numerous changes throughout the day cannot function optimally if it has to write to disk after every committed data change.

The client side of the system consists of a simple UI allowing dealers to enter new car orders. Figure 13-5 shows the order-entry UI.