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Entity beans a closer look
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the session bean performs on the address list. So the extra time consumed by the transactional and security code would grow with the number of operations performed, making each access to the entity beans slower (figure 2.3). In a distributed component system, this extra overhead is negligible since invocations of the remote public component APIs are typically coarse-grained in nature. However, if an object model is represented via entity beans, this overhead can quickly add up. Because each field access is part of the entity bean API, it is therefore subject to this additional overhead. So if your data model includes
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Employee Fa ade Employee Manager Employee Entity Emp EJB Address Entity Address EJB
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Useful services Wasted services
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Figure 2.3 This simplified object interaction diagram shows the inherent overhead for EJB persistence. The problem is that EJB uses a coarse-grained framework for a fine-grained problem persistence. You can see the wasted services in dark gray.
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The bitter cost
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objects with or many objects with an average number of fields you can see that merely accessing all the data necessary to perform a business operation would be a heavyweight task. Now, let s imagine that the EJB container did not couple the coarse-grained services mentioned in figure 2.2 with the persistence service we are using in this example. Then, the object interactions would be more along the lines of what we would expect, as in figure 2.4. The extra overhead and complexity of checking and re-checking transactional status and security parameters would be eliminated, and the necessary execution time for a single operation would decrease. This means that the overhead of using the EJB specification would be less, and our application would be able to run faster on a given hardware configuration or process more user requests at the same speed in that hardware configuration. However, this imaginary EJB container would not be compliant with the EJB specification as it currently stands, because the Employee and Address entity beans would not be performing many services required of an Enterprise JavaBean, namely, the orthogonal services mentioned earlier: transaction control, security, and distribution. As we ve seen, the EJB specification team revised the specification to permit using beans without the distribution service. Perhaps future versions of the specification will decouple the rest of the services provided by the application server, making transactional and security services optional. A developer would then be able then to pick and choose among the services provided by the EJB specification, incurring the performance and design penalties implied by each service only when that service is needed. That the EJB specification team has decoupled remote access from the other services indicates progress may appear in this area in future specification updates. In fact, JBoss 4 will allow just this type of service decoupling and, hopefully, other application server vendors will follow its lead.
Entity beans a closer look
Employee Fa ade Employee Manager Employee EJB Address EJB
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Figure 2.4 This simplified object interaction diagram shows a hypothetical modified EJB implementation of the EmployeeManager.resetHomeAddress() session bean method. The entity beans need not use the transactional or security services of the EJB specification, saving significant overhead and making a much cleaner design.
The bitter cost
2.6 Summary
We began Bitter EJB with a premise: you may not want to use EJB at all. In fact, you should carefully justify EJB each time you choose to use it. We have seen that the EJB specification has a number of costs some related to performance, others to developer productivity. We weighed the constant tension between cost and value considering factors for choosing EJB and determining which projects are truly enterprise applications from the standpoint of the EJB specification. We concluded that you should not dive into EJB as if it were a lifeboat. Instead, you should thoughtfully weigh the pros and cons of using EJB. Plenty of both exist. In other words, carefully consider your application requirements and make sure that the benefits you ll see from EJB will offset the additional complexity that you re bound to encounter. In addition to considering application requirements, we considered the entity a possible hidden cost of EJB . Shoehorned into a role for which it was not designed, the entity bean specification contains services that may be unnecessary baggage for certain implementations and, consequently, may cause problems that certainly would add to the cost of using EJB. In subsequent chapters, we ll assume that you ve concluded that EJB is a framework that will work for you. We ll look at global EJB issues in chapter 3. From there we ll drill into each major EJB component, including session beans, message-driven beans, and entity beans. Throughout our discussions, remember to keep the tradeoff between value and cost in the back of your mind.
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