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EJB timers provide a lightweight scheduling service that you can use in your applications.
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You ll find these advanced features useful in moderate-sized, real-life applications. The only two features common to session and MDBs that we did not cover in this chapter are transaction and security management. You ll learn more about these features in the next chapter.
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Transactions and security
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This chapter covers
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Overview of transaction concepts Container and bean-managed transactions Security concepts Declarative and programmatic security
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Understanding transactions
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Transaction and security management are important aspects of any serious enterprise development effort. By the same token, both are system-level concerns rather than true business application development concerns, which is why they often become an afterthought. In the worst-case scenario, these critical aspects of application development are overlooked altogether. Given these facts, you ll be glad to know that EJB 3 provides functionality in both realms that is robust enough for the most demanding environments, and yet simple enough for those who prefer to focus on developing business logic. Although we have briefly mentioned these features in previous chapters, we haven t dealt with them in any detail until this chapter. The first part of this chapter is devoted to exploring the rich transaction management features of EJB 3. We ll briefly discuss transactions and explore container-managed and bean-managed transactions support in EJB. The remainder of the chapter deals with EJB security features, and you ll learn about declarative and programmatic security support.
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6.1 Understanding transactions
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We engage in transactions almost every day when withdrawing money from an ATM or paying a phone bill, for example. Transactions in computing are a closely related concept but differ slightly and are a little harder to define. In the most basic terms, a transaction is a grouping of tasks that must be processed as an inseparable unit. This means every task that is part of the transaction must succeed in order for the transaction to succeed. If any of the tasks fail, the transaction fails as well. You can think of a transaction as a three-legged wooden stool. All three legs must hold for the stool to stand. If any of them break, the stool collapses. In addition to this all-or-nothing value proposition, transactions must guarantee a degree of reliability and robustness. We will come back to what this last statement means when we describe what are called the ACID (atomicity, consistency, isolation, and durability) properties of transactions. A successful transaction is committed, meaning its results are made permanent, whereas a failed transaction is rolled back, as if it never happened. To explore transaction concepts further, let s take a look at a sample problem in the ActionBazaar application. Before exploring transaction support in EJB, we ll briefly discuss ACID properties, transaction management concepts such as resource and transaction managers, and two-phase commits.
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6.1.1 A transactional solution in ActionBazaar
Some items on ActionBazaar have a Snag-It ordering option. This option allows a user to purchase an item on bid at a set price before anyone else bids on it. As soon as the first bid is placed on an item, the Snag-It option disappears. This feature has become popular because neither the buyer nor the seller needs to wait for bidding to finish as long as they both like the initial Snag-It price tag. As soon as the user clicks the Snag-It button, the ActionBazaar application makes sure no bids have been placed on the item, validates the buyer s credit card, charges the buyer, and removes the item from bidding. Imagine what would happen if one of these four actions failed due to a system error, but the rest of the actions were allowed to succeed. For example, assume that we validate and charge the customer s credit card successfully. However, the order itself fails because the operation to remove the item from bid fails due to a sudden network outage and the user receives an error message. Since the credit card charge was already finalized, the customer is billed for a failed order! To make matters worse, the item remains available for bidding. Another user can put a bid on the item before anyone can fix the problem, creating an interesting situation for the poor customer support folks to sort out. We can see this situation in figure 6.1. While creating ad hoc application logic to automatically credit the customer back in case of an error is a Band-Aid for the problem, transactions are ideally suited to handle such situations. A transaction covering all of the ordering steps ensures that no actual ordering operation changes are finalized until the entire operation finishes successfully. If any errors occur, all pending data changes, including the credit card charge, are aborted. On the other hand, if all the operations succeed the transaction is marked successful and all ordering changes are made permanent. Although this all-or-nothing value proposition is a central theme of transactional systems, it is not the only attribute. A number of properties apply to transactional systems; we ll discuss them next.
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