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This popular architecture is where EJB applications and their clients (web applications) are hosted in the same container, and typically the HTTP requests are load-balanced between multiple instances of application servers. If your application is stateless in nature, then you probably don t have to do anything except deploy the identical application on multiple servers, as illustrated in figure 13.4. If your application requires statefulness and you want to achieve session failover, then you must enable HTTP and EJB session state replication, which allows you to replicate HTTP session objects between two server instances. This is done so that if one instance fails, the client doesn t lose the session state. If your business logic is stateless in nature, this is something you don t have to worry about. Your application will work as expected when your sessions fail over to another container, assuming the identical application is deployed. If you are using stateful session beans to store session objects, then things will be a little trickier here. As mentioned in chapter 12, you should store the EJB

Figure 13.4 EJB(s) and web applications are collocated, and web applications use the local EJB interface to access the nearby EJBs.

import javax.servlet.*; public class LoginServlet extends HttpServlet { //home interface for the EJB PasswordBean private LoginHome loginHome = null; public void init(ServletConfig conf) throws ServletException { super.init(conf); Looks up the EJB home try { from the servlet init lookupLoginHome(); } catch (NamingException e) { e.printStackTrace (); } } public void doGet( HttpServletRequest req, HttpServletResponse res ) { try { String name = getUserName( req ); String password = getUserPassword( req ); Login loginBean = loginHome.create( name ); Boolean valid = loginBean.login( password ); }catch( Exception e ) { //handle exception } //perform further work (not shown) } public void doPost( HttpServletRequest req, HttpServletResponse res ) { doGet( req ); } private void lookupLoginHome() throws NamingException { Context ctx = new InitialContext(); try { if( loginHome == null ) loginHome = ( LoginHome )3 ctx.lookup( "servletAccess.loginHome" ); }

object in the HttpSession object. Check your vendor documentation before you start developing your applications, because it will avoid frantic refactoring of code the day before you release everything to production. You have to enable session replication for your application server; that way, when a session failover occurs, the client will be routed to another server instance where the HTTP session was replicated. The HttpSession object will be restored and the client can retrieve the EJB object to perform the necessary operations. Some vendors may not allow replication of local EJB stateful session beans, so check your vendor documentation. You may have to use the remote interface, even if the client and bean are collocated in the same Java EE container in order to enable session replication. What happens when multiple instances of your application are required in order to divide the web and EJB tiers into different containers Next we ll see how EJB containers support load balancing of stateless EJBs.

Figure 2-3. Dependency inversion between an application and a nonfunctional API Figure 2-3 illustrates the direction of dependency in OOP and how this changes in AOP. The left side shows that, in OOP, the application uses and thus depends on the nonfunctional service. The situation is the same with procedural programming.

Your clients may be located remotely from the EJB container. The clients may be JSF or JSP applications running in a separate container, or Swing/SWT clients running in an application client container. You must realize that when dividing EJBs and their clients into separate JVMs, you are imposing a condition that Remote Method Invocation (RMI) be used to communicate between them. If you plan to use this architecture, you ll have to use remote interfaces for your EJBs.

Figure 13.5 EJB and web applications are in separate JVMs. Clients access EJBs using the remote interface via RMI. The same EJB application can be deployed to multiple servers.

Using the method-level @ejb-relation tag shown in the recipe generates the following XML in the assembly descriptor section of the ejb-jar.xml file:

Most application servers support load balancing of requests between multiple application server instances. When using this architecture, the same application is deployed in all instances. Since the EJBs are stateless, the client can go to any of the servers being load-balanced, as seen in figure 13.5. If you want to use load balancing of stateless session beans deployed in a cluster, you need a basic understanding of RMI and remote invocation of an EJB. We briefly discussed RMI in chapter 1. Figure 13.6 shows how remote invocation of an EJB works.