Is Tomcat enterprise ready?

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n a Java EE environment, specialized containers are designated to handle component isolation and resource sharing. These containers are, in turn, managed by the application server. Containers provide an isolated context wherein Java EE components are deployed and execute. Containers provide an abstraction layer between the components, ensuring that they seldom interact directly. Instead, components can use the protocols and APIs of the container to interface with each other and with other application services. This layer of abstraction enables containers to provide ancillary services required by the component, such as pooling, transaction management, and state management.

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Web applications vs. enterprise applications                     Forsome, the confusion over Tomcat's definition points to the deeperquestion of what differentiates an enterprise application from a Webapplication. Traditionally, a Java enterprise application is defined asa combination of the following components and technologies:
                     

• EAR files
• Java Servlets
• JavaServer Pages or JavaServer Faces
• Enterprise JavaBeans (EJB)
• Java Authentication and Authorization Service (JAAS)
• J2EE Connector Architecture
• JavaBeans Activation Framework (JAF)
• JavaMail
• Java Message Service (JMS)
• Java Persistence API (JPA)
• Java Transaction API (JTA)
• The Java Management Extensions (JMX) API
• Java API for XML Processing (JAXP)
• The Java API for XML-based RPC (JAX-RPC)
• The Java Architecture for XML Binding (JAXB)
• The SOAP with Attachments API for Java (SAAJ)
• Java Database Connectivity (JDBC) framework

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A Java Web application, meanwhile, is said to combine a subset of Java enterprise application components and technologies, namely:                     
                     

• WAR files
• Java Servlets
• JavaServer Faces or JavaServer Pages
• Java Database Connectivity (JDBC) framework

                     Popularframeworks such as Apache Struts, Spring, Hibernate, and others haveobscured the traditional distinctions between Java enterprise and JavaWeb applications. Each new framework offers a slightly different viewof how to accomplish the same tasks. Each framework attempts to solve agiven set of challenges in a more efficient manner, possibly using adifferent set of underlying technologies.
                     Perhaps, this obscurity might be cleared up by observing the behavior of components deployed within different scenarios for                        a typical Java-based enterprise/Web application. These scenarios are discussed next.

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Java EE application scenarios                     In atypical Java EE Web application, an HTML client posts a request to aserver where the request is handled by the Web container of theapplication server. The Web container invokes the servlet that isconfigured to handle the specific context of the request.
                     The initial HTML client request is illustrated in Figure 3.
                     
                     Figure 3. Initial HTML client request

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Once the servlet has received the initial request, some form of request dispatching ensues in order to perform the necessary                        business logic for completing the request.                     
                     One or more business services or components are then invoked to perform business logic, as illustrated in Figure 4.
                     
                     Figure 4. Business services or components in the JEE architecture                     Mostbusiness services or components require access to some form of datastorage or information system. Oftentimes an abstraction layer betweenthe business service and the data store is provided in order to protectagainst future changes in the data store. DAOs (data access objects)are often employed as data abstraction components in this situation, asshown in Figure 5.

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Figure 5.                                          
                     Figure 5. DAOs are invoked to perform business logic (click to enlarge)                     Whenthe DAO invocation step is complete, the response data is passed backup the chain of command, usually as one or more Java beans. The Javabeans are then passed to some type of state machine and/or view managerin order to organize and format the markup response.

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When processing is complete for a given request, a formatted response is passed back to the HTML client, as illustrated in                        Figure 6.                     
                     
                     Figure 6. A response is passed back to the HTML client (click to enlarge)                     Thescenario depicted in Figure 6 can be implemented using a very limitedset of Java EE technologies, including servlets, JDBC, and JSP oranother view technology. Tomcat or any Web server providing a servletengine and JSP engine could be used as the app server in this scenario.

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Scaling into complexity                     Now, supposewe add a requirement to the application for asynchronous messagingbetween business service components. In a Java-based system, this wouldtypically be handled using the Java Message Service (JMS), as shown inFigure 7.
                     
                     Figure 7. JMS handles asynchronous messaging between business service components (click to enlarge)                     MostWeb servers do not offer JMS as a standard feature, but it is simpleenough to add a JMS implementation to a Web server environment. So,once again, the application scenario depicted in Figure 7 could behandled quite easily with just a Web server providing a servlet engineand JSP engine.

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Next, suppose we add the requirement for connectivity betweenbusiness services and disparate enterprise information systems. Java EEoffers the Java Connector Architecture as a common standard to meetthis challenge. Using the Java Connector Architecture, the applicationarchitecture evolves to something similar to what you see in Figure 8.
                     
                     Figure 8. JCA handles connectivity between business services and EISs (click to enlarge)                     Thearchitecture is now approaching a complexity that is arguably bettersuited for a Java EE application server. A Web server such as Tomcatcould possibly be used in combination with other frameworks to meet therequirements, but system management and monitoring complications mightmake the server/framework mix impractical.

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Finally, Figure 9 presents a moderately complex, Java-based, service-oriented architecture employing all of the technologies                        discussed so far, along with communication between multiple WAR deployments, EJBs, and Web services.                     
                     
                     Figure 9. A moderately complex service-oriented architecture (click to enlarge)                     The architecture in Figure 9 has entered the realm of complexity that requires a tested, scalable, manageable Java EE enterprise                        application server. Once again, a development team with the proper skill level coulduse Tomcat for the Web tier and piece together technologies andframeworks to support the business and data tiers. Whether that effortwould be well spent is questionable, particularly given the option touse a full-fledged application server.