Application Logic and Where to Deploy It in Java

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Application Logic and Where to Deploy It
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Application logic is often called "business logic" because it relates to the core business goals of an application For example, an application server might contain logic to calculate the total value of a user's electronic shopping cart Or it might provide automatic membership approval for a credit card Other kinds of application logic are more subtle or indirectly executed For example, an application server may contain logic that determines a list of books or CDs that best fit a user profile These items might then be displayed on the side of an application screen to entice the user during her session Recall that, at a bare minimum, a Web application consists of a client and a server separated by a network It's no surprise, then, that these are our main choices for where to deploy business logic So-called "fat client" solutions, such as Java applet-based applications, can package the logic inside the client itself This is often a good choice if your application is heavily CPU bound, works with many local files and resources, or requires some kind of local security Since Java applets can be rebuilt when necessary, there's no loss of control over the application However, fat clients are generally unpopular because most Web applications are actually I/O bound, either reading and writing data to a remote database or sitting idle waiting for user requests Fat clients can also require long downloading times, making the application seem bulky Also, although Java itself is portable, not everyone has it let alone the latest version so the assumption that applets will run anywhere doesn't always hold up Furthermore, packaging logic in an applet typically means that some sort of host application (like a browser) is required to launch Java for logic execution This means that such designs are less flexible and less amenable to more general application integration needs, making the logic available only to the interactive end user For example, B2Bstyle integrated applications connected via a fat client may not be possible the client is always remotely accessing business logic and can't interact with an applet or anything other than a remote resource over a network In short, fat clients have their place in Web-based applications, but they tend to be the exception for now Instead, most Web applications rely on a thin client approach In contrast to fat clients, thin clients position logic squarely on the server side, in either the application server or the database Making the logic available on the server side as a generic set of functionality accessible through multiple interfaces makes it usable not only by interactive Web clients but also by other integrating applications and technologies One of the nicest things about server-side application logic is that clients can pick and choose the functionality they want without being forced to deal with application baggage they don't need There are some disadvantages to server-side application logic The primary one is that resources such as CPU and memory can quickly become exhausted With server-side application logic, a provider is effectively supplying computing power to the hundreds, thousands, and perhaps millions of clients that use its application Thus, it faces the very scalability and performance challenges that are the subject of this book To scale effectively and perform consistently, a
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server-side application must be strategically partitioned into components and distributed across multiple application or database servers An interesting debate involves determining where to place the application logic In particular, should it be encoded in an application server or directly in the database As we'll see, modern databases have the capability to integrate with and execute powerful procedural languages (such as Oracle's PL/SQL or even Java), enabling execution even within the same address space of the database server itself The lure of encoding application logic in the database is that logic requiring many database calls will, by and large, execute faster no extra overhead is required to transfer intermediate data streams back and forth between the database and application server However, this also means that anyone who wants access to application logic needs to contact the database to execute it Certainly, this makes little sense for applications that rarely or never need persistent storage or for those that don't have the database-specific interfaces necessary for access
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