Java™ Application Development on Linux - Dator

21.2 Expanding to EJBs481One further word. This is by no means an established doctrine, but itseems to be common practice to keep entity beans hidden behind session beans.In other words, it seems to be commonplace to not allow clients (Web applications,Java applications) to communicate directly with entity beans, but ratherto have session beans perform all entity bean operations. This is probably becausethe idea of session beans is to decouple business process from implementationdetails, and entity beans, no matter how much they hide the underlyingdatabase, are still rather tightly coupled to the implementation of the datastorage.Why you would use them. Essentially, they allow you to write your dataaccess layer exactly once, and then reuse it any number times without worryingabout capacity and management. Also, by trying to keep the most often useddata in fast memory, they can, when running on a powerful container or clusterof containers, keep many client operations running much faster than a strictwrite-read update back end.21.2.2.3Message BeansWhat they are. We’re going to gloss over message beans in our book, butyou should know what they are. A message bean represents an interface to amessage queue. An entity bean represents a transaction where completion ofthe call tells you the operation has fully completed. A message queue, on theother hand, is an interface where you are given the firm promise that systemwill get to this when it can. The client doesn’t know where or when the messagewill be handled. It doesn’t know if it was successful or not. But it does knowthat it is the problem of the queue reader.There are many such operations in large enterprises. And there are manyproducts that implement such functionality. Microsoft offers Message QueueServer (MSMQ). IBM has MQSeries. Message queues are common with operationsthat take time, or need to be batched, or require human intervention.Why you would use them. Let’s go back to our shopping cart idea. Managementmight want a daily sales report. You could send each and every itempurchased to a message bean. Then a process to read that queue might bekicked off once a day and the report produced without hitting the database thatsupports all the current Web users. That’s a good example of how a messagequeue (and thus a message bean) might be used. Again, we’re not going to workwith message beans here.