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If servlets form the backbone of synchronous request processing in J2EE, the Java Message Service (JMS) does the same for asynchronous request processing Unlike servlets, messaging technology was not introduced by Java effort As middleware solutions, they have been around for a while, traditionally associated with the need to handle enterprise application integration that is, to easily and reliably connect applications from different organizations (ie, companies) However, in the past two years or so, messaging has undergone a rebirth, at least in terms of the Java and Web application community Messaging solutions such as JMS are seeing increased support by other application-level components For example, the J2EE specification describes how EJBs can be designed to integrate with JMS (ie, message-driven beans) This makes sense: Even though messaging clients and servlet clients are typically very different (one is a company; the other, a single user), they both want access to the same business logic Under the J2EE model, that logic is associated with EJBs, so it makes sense to enable them to handle both synchronous and asynchronous requests JMS acts as a thin API layer that exists on both the producer and the consumer sides of the communication As shown in Figure 6-5, the producer uses the API to communicate with a messaging system that conforms to the JMS while a consumer uses the JMS to access the transmitted messages JMS thus acts like JDBC Instead of abstracting database access, however, it abstracts messaging behavior As long as it conforms to the JMS, a messaging solution from any vendor can be integrated into your application scheme Figure 6-5 JMS messaging architecture
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Although we have been discussing request processing paradigms and design tradeoffs at a fairly abstract level, we can nevertheless identify some practical suggestions in terms of overall Web application design
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As we've seen, asynchronous communication is inherently faster than synchronous communication The bottom line is that parallelism is better exploited under an asynchronous model at the cost of complexity or flexibility if the return value (if any) has any meaning In 9, I'll discuss the Java Message Service the primary J2EE technology for building asynchronous solutions However, before you can even begin to consider how to use JMS, you must be able to identify asynchronous opportunities Toward that end, let's consider one common task that system designers often (mistakenly) assume can only be handled synchronously Generating Highly Dynamic Web Pages Highly dynamic Web pages are those where the data changes (or may change) frequently They're all over the place and include
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"Live" sports scores or sports event statistics (eg, baseball box scores, like those provided by the Yahoo or ESPN Web sites) Current weather conditions Flight status for a particular airline or airport Newswire stories Concert/event seating availability
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The problem is that readers of these pages know they change frequently and tend to request them more often (to get the latest information) This means that request-processing demands rise Another unfortunate thing is that data that's looked up or computed isn't shared between clients, which causes a lot of repetition resulting in a significant waste of server-side resources However, there's an asynchronous way to approach this problem Notice that our Web page delivery is reactive (and synchronous) when a client wants an updated page version, we have to generate a new copy (even if it contains the same data) However, suppose that we instead have a proactive application server that periodically generates a static page, called, say, http://wwwexamplecom/current_resultshtml Instead of dynamically fulfilling client requests on a demand basis, we can change our strategy and have all clients access a single static page Rather than dynamically generate HTML per request, we can generate dynamic HTML only when the data changes This is an approach that, by itself, scales beautifully Admittedly, this solution will not be acceptable in some situations where a response has to be the freshest possible or in cases where the data generated is user/request specific (ie, personalized data such as stock portfolios) Even in this latter case, a simple change in your page design can result in significant performance benefits Consider a Web page for an airline that lists the status of all flights Instead of designing the page so that users enter a number and get back a flight status,
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it may be possible to get away with publishing a single list showing the status of all flights and letting the user locate the one she wants In short, highly dynamic Web page generation can be made efficient by pregenerating a single static page This removes a tremendous burden on the application as the number of concurrent users rises If you stand back and look at the whole solution, you'll see that it's effectively asynchronous at the application level: A message (the data results) is periodically pushed out to the filesystem, and the client simply gets the latest copy of it
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