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Generate DataMatrix in Java Tuning the JMS provider

Most of the performance issues surrounding MDBs are generally related to the underlying JMS providers. Some general tips that we provided in chapter 4 include choosing the right messaging models and persistence strategy, using the correctly sized messages, setting the lifetime of messages, and using appropriate filters and handling of poison messages. Check your vendor documentation for the JMS provider you are using for your applications.

Initializing resources Like stateless session beans, MDB instances are pooled, and initialization tasks are performed only once, for any given MDB instance. You can perform expensive tasks such as initialization of resources (opening database connections, sockets, files, etc.) in any method annotated with @PostConstruct. Methods marked with @PostConstruct will be invoked only once, immediately after creation. Similarly, close any resources in methods annotated with @PreDestroy. This will significantly improve the performance of your classes in general, and your MDBs in particular. Sizing the pool

MDBs are pooled, and the pool size is usually configured using vendor-specific deployment descriptors. Improper sizing of MDB pools may degrade perfor-

mance of your applications when large numbers of messages arrive and there is not a sufficient number of bean instances available to process those messages. Make sure you do some predictive analysis in determining the capacity your system will need to support (including spikes, and growth over the short term). Then run some tests that will show how your hardware, operating system, and application server will perform to meet these expectations. There are almost as many ways to configure hardware and operating systems as there are combinations of the two. Once you are happy with that configuration, make sure your application server pool for MDBs is sized as well. This concludes the discussion on EJB performance. It s time to focus on scalability and high availability, which are two other important aspects of enterprise applications.

High availability and scalability go hand in hand. Every application has some requirement for availability and scalability, and meeting those requirements begins when you start architecting the application. Availability requirements can vary widely. Some applications, say a human resources (HR) application, may have a requirement to be available only during normal business hours. On the other hand, an online bidding system may have a requirement to be available 24/7, year-round. Similarly, the scalability requirements for an HR application may not be as demanding as an online bidding system. The EJB 3 specification doesn t address clustering aspects for EJB applications. However, most application server vendors allow customers to cluster EJB containers to achieve the desired scalability and availability aspects required by today s

demanding enterprise applications. Before we dive into such architectural concerns, let s discuss a couple of concepts that will help you understand the basic aspects of EJB clustering:

Load balancing This is when multiple application server instances work as a group to balance the incoming request load across all the servers. This allows for scalability of applications. When your user base increases, you can simply add new server instances to the group. This is typically most useful when your application is stateless in nature. Failover Adding failover capability to your applications improves the availability of your applications. Clients typically get bound to a specific server instance by stateful applications. In the event of a catastrophic failure of a server, the client requests are routed to a separate server in a seamless manner, transparent to the user.

Clustering of EJB applications adds load balancing and failover capabilities to your applications. Covering all aspects of clustering is beyond the scope of this book. In this section we ll provide architectural primers, and focus on the knowledge you need to effectively build scalable, available EJB applications. There are several ways to deploy EJB applications. We ll discuss three commonly used approaches. We won t describe any configuration details related to clustering because clustering tends to be vendor specific.