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5.2.2 Lightening the load
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Now, imagine a slightly different shopping experience. Rather than pushing around a shopping cart and intermittently adding items over time, let s say we keep a shopping list offline instead. When we re ready to purchase the items on our list, we submit it to the e-commerce server, which performs the checkout process. We ve made a subtle, but important, change to the way we communicate with the server. From the server s perspective, the conversation is now stateless. This alleviates the need for the server to keep track of our shopping cart contents. Consequently, the server can make do with much less memory. Designing the checkout process as a stateless service offers another advantage. Since the process holds no state, nothing can be lost. If the service fails, the shopping list can be submitted to another identical service. Again, there s no free lunch; something must store the shopping list. In this case, the shopping list could be stored in the client itself. A number of advantages to shifting this responsibility may exist, including the distribution of memory requirements across a wider range of resources.
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Stateless Bean Instance Pool A
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Figure 5.3 Stateless session beans are pooled and swapped between clients to achieve higher levels of scalability. In this example, bean instance A is busy servicing Client A. When Client B invokes a method on a stateless session bean, bean instance B is plucked from the pool to service the request. Once the request is complete, bean instance B is returned to the pool and later reused by Client C.
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Stateless conversations are best handled by the other type of session bean the stateless session bean. Unlike stateful session beans, stateless session beans don t hold onto conversational state between client requests. The client is responsible for passing any relevant state each time it communicates with the bean. For example, a stateless session bean modeling a checkout process could provide a checkout() method that takes a shopping list as a parameter. The upshot is that stateless session beans have no allegiance to any given client and can therefore be shared among many clients. The EJB container simply creates a pool of identical stateless session beans and meters them out to handle client requests. As soon as the checkout() method returns from processing one client s shopping list, it can be reused immediately to service a different client. Figure 5.3 illustrates how a stateless session bean can be reused by multiple clients. By reusing stateless session beans in a pool, a small number of beans can handle a large number of concurrent clients. Consequently, stateless session beans generally scale better than stateful session beans. That s not to say that one type of bean is always better than the other, but it should nudge us toward making services stateless where possible.
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5.2.3 Solution 1: Strive for statelessness
Although certain situations do justify creating stateful services (e.g., e-commerce), you should avoid creating stateful services without close consideration of your situation s particular needs. Shopping cart applications specifically rely on a relatively large amount of conversational state to ensure a robust shopping experience. Indeed, we re willing to pay the price of managing shopping cart resources in return for lots of satisfied customers in the checkout line. However, if you do begin by building stateful services, you may add unnecessary complexity to other services added later. And, if you begin by creating stateless services, you can
A pivotal antipattern: Conversational Baggage
avoid overburdening future services. When no real benefit to managing session state exists, using stateful session beans should be avoided. In other words, stateful services should be the exception, not the rule not because they don t scale, but because, in the absence of session state, they re the wrong tools for the job. Just as you wouldn t go to the trouble of pushing around a shopping cart just to buy one item, you shouldn t use stateful session beans for stateless conversations. If the conversation is stateless, then stateless session beans are a better choice. Indeed, stateless session beans scale better because they re not burdened by state. For this reason, you should strive to design business methods that take clientspecific information as parameters when convenient. An example might be a credit card processing service that takes a DTO representing the card number, expiration date, and transaction amount as a parameter. It could then return the results in a separate DTO. Figure 5.4 illustrates a stateless credit card processing service. By all means, use stateless session beans to the maximum extent possible. Then use stateful services when necessary.
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